diff --git a/cloud/agent/BUILD.bazel b/cloud/agent/BUILD.bazel
index 7e7614d..3d342ae 100644
--- a/cloud/agent/BUILD.bazel
+++ b/cloud/agent/BUILD.bazel
@@ -27,6 +27,7 @@
         "//osbase/pki",
         "//osbase/scsi",
         "//osbase/smbios",
+        "//osbase/structfs",
         "//osbase/supervisor",
         "@com_github_cenkalti_backoff_v4//:backoff",
         "@com_github_mdlayher_ethtool//:ethtool",
diff --git a/cloud/agent/install.go b/cloud/agent/install.go
index cdc8fa5..914b0be 100644
--- a/cloud/agent/install.go
+++ b/cloud/agent/install.go
@@ -9,7 +9,6 @@
 	_ "embed"
 	"errors"
 	"fmt"
-	"io/fs"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -23,24 +22,28 @@
 	"source.monogon.dev/osbase/build/mkimage/osimage"
 	"source.monogon.dev/osbase/efivarfs"
 	npb "source.monogon.dev/osbase/net/proto"
+	"source.monogon.dev/osbase/structfs"
 )
 
 //go:embed metropolis/node/core/abloader/abloader.efi
 var abloader []byte
 
-// FileSizedReader is a small adapter from fs.File to fs.SizedReader
-// Panics on Stat() failure, so should only be used with sources where Stat()
-// cannot fail.
-type FileSizedReader struct {
-	fs.File
+// zipBlob looks up a file in a [zip.Reader] and adapts it to [structfs.Blob].
+func zipBlob(reader *zip.Reader, name string) (zipFileBlob, error) {
+	for _, file := range reader.File {
+		if file.Name == name {
+			return zipFileBlob{file}, nil
+		}
+	}
+	return zipFileBlob{}, fmt.Errorf("file %q not found", name)
 }
 
-func (f FileSizedReader) Size() int64 {
-	stat, err := f.Stat()
-	if err != nil {
-		panic(err)
-	}
-	return stat.Size()
+type zipFileBlob struct {
+	*zip.File
+}
+
+func (f zipFileBlob) Size() int64 {
+	return int64(f.File.UncompressedSize64)
 }
 
 // install dispatches OSInstallationRequests to the appropriate installer
@@ -115,16 +118,14 @@
 	if err != nil {
 		return fmt.Errorf("failed to open node bundle: %w", err)
 	}
-	efiPayload, err := bundle.Open("kernel_efi.efi")
+	efiPayload, err := zipBlob(bundle, "kernel_efi.efi")
 	if err != nil {
 		return fmt.Errorf("invalid bundle: %w", err)
 	}
-	defer efiPayload.Close()
-	systemImage, err := bundle.Open("verity_rootfs.img")
+	systemImage, err := zipBlob(bundle, "verity_rootfs.img")
 	if err != nil {
 		return fmt.Errorf("invalid bundle: %w", err)
 	}
-	defer systemImage.Close()
 
 	nodeParamsRaw, err := proto.Marshal(req.NodeParameters)
 	if err != nil {
@@ -143,9 +144,9 @@
 			Data:   128,
 		},
 		SystemImage:    systemImage,
-		EFIPayload:     FileSizedReader{efiPayload},
-		ABLoader:       bytes.NewReader(abloader),
-		NodeParameters: bytes.NewReader(nodeParamsRaw),
+		EFIPayload:     efiPayload,
+		ABLoader:       structfs.Bytes(abloader),
+		NodeParameters: structfs.Bytes(nodeParamsRaw),
 		Output:         rootDev,
 	}
 
diff --git a/cloud/agent/takeover/e2e/BUILD.bazel b/cloud/agent/takeover/e2e/BUILD.bazel
index bfa7baa..b83b131 100644
--- a/cloud/agent/takeover/e2e/BUILD.bazel
+++ b/cloud/agent/takeover/e2e/BUILD.bazel
@@ -20,6 +20,7 @@
         "//cloud/agent/api",
         "//osbase/fat32",
         "//osbase/freeport",
+        "//osbase/structfs",
         "@com_github_pkg_sftp//:sftp",
         "@io_bazel_rules_go//go/runfiles",
         "@org_golang_google_protobuf//proto",
diff --git a/cloud/agent/takeover/e2e/main_test.go b/cloud/agent/takeover/e2e/main_test.go
index 9ab6769..756eb40 100644
--- a/cloud/agent/takeover/e2e/main_test.go
+++ b/cloud/agent/takeover/e2e/main_test.go
@@ -27,6 +27,7 @@
 
 	"source.monogon.dev/osbase/fat32"
 	"source.monogon.dev/osbase/freeport"
+	"source.monogon.dev/osbase/structfs"
 )
 
 var (
@@ -79,25 +80,16 @@
 		t.Fatal(err)
 	}
 
-	rootInode := fat32.Inode{
-		Attrs: fat32.AttrDirectory,
-		Children: []*fat32.Inode{
-			{
-				Name:    "user-data",
-				Content: strings.NewReader("#cloud-config\n" + string(userData)),
-			},
-			{
-				Name:    "meta-data",
-				Content: strings.NewReader(""),
-			},
-		},
+	root := structfs.Tree{
+		structfs.File("user-data", structfs.Bytes("#cloud-config\n"+string(userData))),
+		structfs.File("meta-data", structfs.Bytes("")),
 	}
 	cloudInitDataFile, err := os.CreateTemp("", "cidata*.img")
 	if err != nil {
 		t.Fatal(err)
 	}
 	defer os.Remove(cloudInitDataFile.Name())
-	if err := fat32.WriteFS(cloudInitDataFile, rootInode, fat32.Options{Label: "cidata"}); err != nil {
+	if err := fat32.WriteFS(cloudInitDataFile, root, fat32.Options{Label: "cidata"}); err != nil {
 		t.Fatal(err)
 	}
 
diff --git a/metropolis/cli/metroctl/BUILD.bazel b/metropolis/cli/metroctl/BUILD.bazel
index f4c5c15..bd7ab70 100644
--- a/metropolis/cli/metroctl/BUILD.bazel
+++ b/metropolis/cli/metroctl/BUILD.bazel
@@ -48,10 +48,9 @@
         "//metropolis/node/core/rpc/resolver",
         "//metropolis/proto/api",
         "//metropolis/proto/common",
-        "//osbase/blkio",
-        "//osbase/fat32",
         "//osbase/logtree",
         "//osbase/logtree/proto",
+        "//osbase/structfs",
         "//version",
         "@com_github_adrg_xdg//:xdg",
         "@com_github_schollz_progressbar_v3//:progressbar",
diff --git a/metropolis/cli/metroctl/cmd_install.go b/metropolis/cli/metroctl/cmd_install.go
index a34e470..1b9b8cb 100644
--- a/metropolis/cli/metroctl/cmd_install.go
+++ b/metropolis/cli/metroctl/cmd_install.go
@@ -4,7 +4,6 @@
 package main
 
 import (
-	"bytes"
 	"context"
 	"crypto/ed25519"
 	_ "embed"
@@ -22,8 +21,7 @@
 	"source.monogon.dev/metropolis/cli/flagdefs"
 	"source.monogon.dev/metropolis/cli/metroctl/core"
 	common "source.monogon.dev/metropolis/node"
-	"source.monogon.dev/osbase/blkio"
-	"source.monogon.dev/osbase/fat32"
+	"source.monogon.dev/osbase/structfs"
 )
 
 var installCmd = &cobra.Command{
@@ -111,20 +109,15 @@
 	return params, nil
 }
 
-func external(name, datafilePath string, flag *string) (fat32.SizedReader, error) {
+func external(name, datafilePath string, flag *string) (structfs.Blob, error) {
 	if flag == nil || *flag == "" {
 		rPath, err := runfiles.Rlocation(datafilePath)
 		if err != nil {
 			return nil, fmt.Errorf("no %s specified", name)
 		}
-		df, err := os.ReadFile(rPath)
-		if err != nil {
-			return nil, fmt.Errorf("can't read file: %w", err)
-		}
-		return bytes.NewReader(df), nil
+		return structfs.OSPathBlob(rPath)
 	}
-
-	f, err := blkio.NewFileReader(*flag)
+	f, err := structfs.OSPathBlob(*flag)
 	if err != nil {
 		return nil, fmt.Errorf("failed to open specified %s: %w", name, err)
 	}
diff --git a/metropolis/cli/metroctl/cmd_install_ssh.go b/metropolis/cli/metroctl/cmd_install_ssh.go
index efd1fd7..f0a5379 100644
--- a/metropolis/cli/metroctl/cmd_install_ssh.go
+++ b/metropolis/cli/metroctl/cmd_install_ssh.go
@@ -24,7 +24,7 @@
 	"google.golang.org/protobuf/proto"
 
 	"source.monogon.dev/go/net/ssh"
-	"source.monogon.dev/osbase/fat32"
+	"source.monogon.dev/osbase/structfs"
 )
 
 var sshCmd = &cobra.Command{
@@ -130,14 +130,20 @@
 			return err
 		}
 
-		barUploader := func(r fat32.SizedReader, targetPath string) {
+		barUploader := func(blob structfs.Blob, targetPath string) {
+			content, err := blob.Open()
+			if err != nil {
+				log.Fatalf("error while uploading %q: %v", targetPath, err)
+			}
+			defer content.Close()
+
 			bar := progressbar.DefaultBytes(
-				r.Size(),
+				blob.Size(),
 				targetPath,
 			)
 			defer bar.Close()
 
-			proxyReader := progressbar.NewReader(r, bar)
+			proxyReader := progressbar.NewReader(content, bar)
 			defer proxyReader.Close()
 
 			if err := conn.Upload(ctx, targetPath, &proxyReader); err != nil {
diff --git a/metropolis/cli/metroctl/core/BUILD.bazel b/metropolis/cli/metroctl/core/BUILD.bazel
index 3a599a7..73527d5 100644
--- a/metropolis/cli/metroctl/core/BUILD.bazel
+++ b/metropolis/cli/metroctl/core/BUILD.bazel
@@ -21,6 +21,7 @@
         "//osbase/blockdev",
         "//osbase/fat32",
         "//osbase/gpt",
+        "//osbase/structfs",
         "@io_k8s_client_go//pkg/apis/clientauthentication/v1:clientauthentication",
         "@io_k8s_client_go//tools/clientcmd",
         "@io_k8s_client_go//tools/clientcmd/api",
diff --git a/metropolis/cli/metroctl/core/install.go b/metropolis/cli/metroctl/core/install.go
index 190f66a..5d43a89 100644
--- a/metropolis/cli/metroctl/core/install.go
+++ b/metropolis/cli/metroctl/core/install.go
@@ -4,7 +4,6 @@
 package core
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
 	"math"
@@ -16,6 +15,7 @@
 	"source.monogon.dev/osbase/blockdev"
 	"source.monogon.dev/osbase/fat32"
 	"source.monogon.dev/osbase/gpt"
+	"source.monogon.dev/osbase/structfs"
 )
 
 type MakeInstallerImageArgs struct {
@@ -23,13 +23,13 @@
 	TargetPath string
 
 	// Reader for the installer EFI executable. Mandatory.
-	Installer fat32.SizedReader
+	Installer structfs.Blob
 
 	// Optional NodeParameters to be embedded for use by the installer.
 	NodeParams *api.NodeParameters
 
 	// Optional Reader for a Metropolis bundle for use by the installer.
-	Bundle fat32.SizedReader
+	Bundle structfs.Blob
 }
 
 // MakeInstallerImage generates an installer disk image containing a Table
@@ -40,7 +40,7 @@
 		return errors.New("installer is mandatory")
 	}
 
-	espRoot := fat32.Inode{Attrs: fat32.AttrDirectory}
+	var espRoot structfs.Tree
 
 	// This needs to be a "Removable Media" according to the UEFI Specification
 	// V2.9 Section 3.5.1.1. This file is booted by any compliant UEFI firmware
@@ -54,7 +54,7 @@
 		if err != nil {
 			return fmt.Errorf("failed to marshal node params: %w", err)
 		}
-		if err := espRoot.PlaceFile("metropolis-installer/nodeparams.pb", bytes.NewReader(nodeParamsRaw)); err != nil {
+		if err := espRoot.PlaceFile("metropolis-installer/nodeparams.pb", structfs.Bytes(nodeParamsRaw)); err != nil {
 			return err
 		}
 	}
diff --git a/metropolis/cli/takeover/BUILD.bazel b/metropolis/cli/takeover/BUILD.bazel
index fe58518..c262164 100644
--- a/metropolis/cli/takeover/BUILD.bazel
+++ b/metropolis/cli/takeover/BUILD.bazel
@@ -40,6 +40,7 @@
         "//osbase/kexec",
         "//osbase/net/dump",
         "//osbase/net/proto",
+        "//osbase/structfs",
         "//osbase/supervisor",
         "@com_github_cavaliergopher_cpio//:cpio",
         "@com_github_klauspost_compress//zstd",
diff --git a/metropolis/cli/takeover/e2e/BUILD.bazel b/metropolis/cli/takeover/e2e/BUILD.bazel
index b085947..ae4810b 100644
--- a/metropolis/cli/takeover/e2e/BUILD.bazel
+++ b/metropolis/cli/takeover/e2e/BUILD.bazel
@@ -22,6 +22,7 @@
     deps = [
         "//osbase/fat32",
         "//osbase/freeport",
+        "//osbase/structfs",
         "@io_bazel_rules_go//go/runfiles",
         "@org_golang_x_crypto//ssh",
         "@org_golang_x_crypto//ssh/agent",
diff --git a/metropolis/cli/takeover/e2e/main_test.go b/metropolis/cli/takeover/e2e/main_test.go
index 1ba5354..afce515 100644
--- a/metropolis/cli/takeover/e2e/main_test.go
+++ b/metropolis/cli/takeover/e2e/main_test.go
@@ -25,6 +25,7 @@
 
 	"source.monogon.dev/osbase/fat32"
 	"source.monogon.dev/osbase/freeport"
+	"source.monogon.dev/osbase/structfs"
 )
 
 var (
@@ -112,25 +113,16 @@
 		t.Fatal(err)
 	}
 
-	rootInode := fat32.Inode{
-		Attrs: fat32.AttrDirectory,
-		Children: []*fat32.Inode{
-			{
-				Name:    "user-data",
-				Content: strings.NewReader("#cloud-config\n" + string(userData)),
-			},
-			{
-				Name:    "meta-data",
-				Content: strings.NewReader(""),
-			},
-		},
+	root := structfs.Tree{
+		structfs.File("user-data", structfs.Bytes("#cloud-config\n"+string(userData))),
+		structfs.File("meta-data", structfs.Bytes("")),
 	}
 	cloudInitDataFile, err := os.CreateTemp("", "cidata*.img")
 	if err != nil {
 		t.Fatal(err)
 	}
 	defer os.Remove(cloudInitDataFile.Name())
-	if err := fat32.WriteFS(cloudInitDataFile, rootInode, fat32.Options{Label: "cidata"}); err != nil {
+	if err := fat32.WriteFS(cloudInitDataFile, root, fat32.Options{Label: "cidata"}); err != nil {
 		t.Fatal(err)
 	}
 
diff --git a/metropolis/cli/takeover/install.go b/metropolis/cli/takeover/install.go
index 005b590..2b76095 100644
--- a/metropolis/cli/takeover/install.go
+++ b/metropolis/cli/takeover/install.go
@@ -5,10 +5,8 @@
 
 import (
 	"archive/zip"
-	"bytes"
 	_ "embed"
 	"fmt"
-	"io/fs"
 	"os"
 	"path/filepath"
 
@@ -16,24 +14,28 @@
 	"source.monogon.dev/osbase/blockdev"
 	"source.monogon.dev/osbase/build/mkimage/osimage"
 	"source.monogon.dev/osbase/efivarfs"
+	"source.monogon.dev/osbase/structfs"
 )
 
 //go:embed metropolis/node/core/abloader/abloader.efi
 var abloader []byte
 
-// FileSizedReader is a small adapter from fs.File to fs.SizedReader
-// Panics on Stat() failure, so should only be used with sources where Stat()
-// cannot fail.
-type FileSizedReader struct {
-	fs.File
+// zipBlob looks up a file in a [zip.Reader] and adapts it to [structfs.Blob].
+func zipBlob(reader *zip.Reader, name string) (zipFileBlob, error) {
+	for _, file := range reader.File {
+		if file.Name == name {
+			return zipFileBlob{file}, nil
+		}
+	}
+	return zipFileBlob{}, fmt.Errorf("file %q not found", name)
 }
 
-func (f FileSizedReader) Size() int64 {
-	stat, err := f.Stat()
-	if err != nil {
-		panic(err)
-	}
-	return stat.Size()
+type zipFileBlob struct {
+	*zip.File
+}
+
+func (f zipFileBlob) Size() int64 {
+	return int64(f.File.UncompressedSize64)
 }
 
 // EnvInstallTarget environment variable which tells the takeover binary where
@@ -93,12 +95,12 @@
 		return nil, fmt.Errorf("failed to open root device: %w", err)
 	}
 
-	efiPayload, err := bundle.Open("kernel_efi.efi")
+	efiPayload, err := zipBlob(bundle, "kernel_efi.efi")
 	if err != nil {
 		return nil, fmt.Errorf("invalid bundle: %w", err)
 	}
 
-	systemImage, err := bundle.Open("verity_rootfs.img")
+	systemImage, err := zipBlob(bundle, "verity_rootfs.img")
 	if err != nil {
 		return nil, fmt.Errorf("invalid bundle: %w", err)
 	}
@@ -110,9 +112,9 @@
 			Data:   128,
 		},
 		SystemImage:    systemImage,
-		EFIPayload:     FileSizedReader{efiPayload},
-		ABLoader:       bytes.NewReader(abloader),
-		NodeParameters: bytes.NewReader(metropolisSpecRaw),
+		EFIPayload:     efiPayload,
+		ABLoader:       structfs.Bytes(abloader),
+		NodeParameters: structfs.Bytes(metropolisSpecRaw),
 		Output:         rootDev,
 	}, nil
 }
diff --git a/metropolis/installer/BUILD.bazel b/metropolis/installer/BUILD.bazel
index 5792326..fe80669 100644
--- a/metropolis/installer/BUILD.bazel
+++ b/metropolis/installer/BUILD.bazel
@@ -16,6 +16,7 @@
         "//osbase/bringup",
         "//osbase/build/mkimage/osimage",
         "//osbase/efivarfs",
+        "//osbase/structfs",
         "//osbase/supervisor",
         "//osbase/sysfs",
         "@org_golang_x_sys//unix",
diff --git a/metropolis/installer/main.go b/metropolis/installer/main.go
index e9956fd..216f99c 100644
--- a/metropolis/installer/main.go
+++ b/metropolis/installer/main.go
@@ -8,12 +8,10 @@
 
 import (
 	"archive/zip"
-	"bytes"
 	"context"
 	_ "embed"
 	"errors"
 	"fmt"
-	"io/fs"
 	"os"
 	"path/filepath"
 	"strings"
@@ -25,6 +23,7 @@
 	"source.monogon.dev/osbase/bringup"
 	"source.monogon.dev/osbase/build/mkimage/osimage"
 	"source.monogon.dev/osbase/efivarfs"
+	"source.monogon.dev/osbase/structfs"
 	"source.monogon.dev/osbase/supervisor"
 	"source.monogon.dev/osbase/sysfs"
 )
@@ -104,19 +103,22 @@
 	return suitable, nil
 }
 
-// FileSizedReader is a small adapter from fs.File to fs.SizedReader
-// Panics on Stat() failure, so should only be used with sources where Stat()
-// cannot fail.
-type FileSizedReader struct {
-	fs.File
+// zipBlob looks up a file in a [zip.Reader] and adapts it to [structfs.Blob].
+func zipBlob(reader *zip.Reader, name string) (zipFileBlob, error) {
+	for _, file := range reader.File {
+		if file.Name == name {
+			return zipFileBlob{file}, nil
+		}
+	}
+	return zipFileBlob{}, fmt.Errorf("file %q not found", name)
 }
 
-func (f FileSizedReader) Size() int64 {
-	stat, err := f.Stat()
-	if err != nil {
-		panic(err)
-	}
-	return stat.Size()
+type zipFileBlob struct {
+	*zip.File
+}
+
+func (f zipFileBlob) Size() int64 {
+	return int64(f.File.UncompressedSize64)
 }
 
 func main() {
@@ -162,7 +164,7 @@
 		return fmt.Errorf("while mounting the installer ESP: %w", err)
 	}
 
-	nodeParameters, err := os.Open("/installer/metropolis-installer/nodeparams.pb")
+	nodeParameters, err := structfs.OSPathBlob("/installer/metropolis-installer/nodeparams.pb")
 	if err != nil {
 		return fmt.Errorf("failed to open node parameters from ESP: %w", err)
 	}
@@ -173,16 +175,14 @@
 		return fmt.Errorf("failed to open node bundle from ESP: %w", err)
 	}
 	defer bundle.Close()
-	efiPayload, err := bundle.Open("kernel_efi.efi")
+	efiPayload, err := zipBlob(&bundle.Reader, "kernel_efi.efi")
 	if err != nil {
 		return fmt.Errorf("cannot open EFI payload in bundle: %w", err)
 	}
-	defer efiPayload.Close()
-	systemImage, err := bundle.Open("verity_rootfs.img")
+	systemImage, err := zipBlob(&bundle.Reader, "verity_rootfs.img")
 	if err != nil {
 		return fmt.Errorf("cannot open system image in bundle: %w", err)
 	}
-	defer systemImage.Close()
 
 	// Build the osimage parameters.
 	installParams := osimage.Params{
@@ -198,9 +198,9 @@
 			Data: 128,
 		},
 		SystemImage:    systemImage,
-		EFIPayload:     FileSizedReader{efiPayload},
-		ABLoader:       bytes.NewReader(abloader),
-		NodeParameters: FileSizedReader{nodeParameters},
+		EFIPayload:     efiPayload,
+		ABLoader:       structfs.Bytes(abloader),
+		NodeParameters: nodeParameters,
 	}
 	// Calculate the minimum target size based on the installation parameters.
 	minSize := uint64((installParams.PartitionSize.ESP +
diff --git a/metropolis/installer/test/BUILD.bazel b/metropolis/installer/test/BUILD.bazel
index 7f7ad54..7b7828d 100644
--- a/metropolis/installer/test/BUILD.bazel
+++ b/metropolis/installer/test/BUILD.bazel
@@ -24,6 +24,7 @@
         "//metropolis/proto/api",
         "//osbase/build/mkimage/osimage",
         "//osbase/cmd",
+        "//osbase/structfs",
         "@com_github_diskfs_go_diskfs//:go-diskfs",
         "@com_github_diskfs_go_diskfs//disk",
         "@com_github_diskfs_go_diskfs//partition/gpt",
diff --git a/metropolis/installer/test/run_test.go b/metropolis/installer/test/run_test.go
index 766c839..cd87852 100644
--- a/metropolis/installer/test/run_test.go
+++ b/metropolis/installer/test/run_test.go
@@ -7,7 +7,6 @@
 package installer
 
 import (
-	"bytes"
 	"context"
 	"fmt"
 	"log"
@@ -27,6 +26,7 @@
 	mctl "source.monogon.dev/metropolis/cli/metroctl/core"
 	"source.monogon.dev/osbase/build/mkimage/osimage"
 	"source.monogon.dev/osbase/cmd"
+	"source.monogon.dev/osbase/structfs"
 )
 
 var (
@@ -134,21 +134,21 @@
 func TestMain(m *testing.M) {
 	installerImage = filepath.Join(os.Getenv("TEST_TMPDIR"), "installer.img")
 
-	installer, err := os.ReadFile(xInstallerPath)
+	installer, err := structfs.OSPathBlob(xInstallerPath)
 	if err != nil {
 		log.Fatal(err)
 	}
 
-	bundle, err := os.ReadFile(xBundlePath)
+	bundle, err := structfs.OSPathBlob(xBundlePath)
 	if err != nil {
 		log.Fatal(err)
 	}
 
 	iargs := mctl.MakeInstallerImageArgs{
-		Installer:  bytes.NewReader(installer),
+		Installer:  installer,
 		TargetPath: installerImage,
 		NodeParams: &api.NodeParameters{},
-		Bundle:     bytes.NewReader(bundle),
+		Bundle:     bundle,
 	}
 	if err := mctl.MakeInstallerImage(iargs); err != nil {
 		log.Fatalf("Couldn't create the installer image at %q: %v", installerImage, err)
diff --git a/metropolis/node/core/update/e2e/BUILD.bazel b/metropolis/node/core/update/e2e/BUILD.bazel
index 3f6aa66..83716d1 100644
--- a/metropolis/node/core/update/e2e/BUILD.bazel
+++ b/metropolis/node/core/update/e2e/BUILD.bazel
@@ -25,9 +25,9 @@
         "xAbloaderPath": "$(rlocationpath //metropolis/node/core/abloader )",
     },
     deps = [
-        "//osbase/blkio",
         "//osbase/blockdev",
         "//osbase/build/mkimage/osimage",
+        "//osbase/structfs",
         "@io_bazel_rules_go//go/runfiles",
     ],
 )
diff --git a/metropolis/node/core/update/e2e/e2e_test.go b/metropolis/node/core/update/e2e/e2e_test.go
index f7d0e36..4a1c6a2 100644
--- a/metropolis/node/core/update/e2e/e2e_test.go
+++ b/metropolis/node/core/update/e2e/e2e_test.go
@@ -21,9 +21,9 @@
 
 	"github.com/bazelbuild/rules_go/go/runfiles"
 
-	"source.monogon.dev/osbase/blkio"
 	"source.monogon.dev/osbase/blockdev"
 	"source.monogon.dev/osbase/build/mkimage/osimage"
+	"source.monogon.dev/osbase/structfs"
 )
 
 var (
@@ -197,18 +197,16 @@
 	t.Cleanup(func() { os.Remove(rootDevPath) })
 	defer rootDisk.Close()
 
-	boot, err := blkio.NewFileReader(xBootPath)
+	boot, err := structfs.OSPathBlob(xBootPath)
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer boot.Close()
-	system, err := os.Open(xSystemXPath)
+	system, err := structfs.OSPathBlob(xSystemXPath)
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer system.Close()
 
-	loader, err := blkio.NewFileReader(xAbloaderPath)
+	loader, err := structfs.OSPathBlob(xAbloaderPath)
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/osbase/blkio/BUILD.bazel b/osbase/blkio/BUILD.bazel
deleted file mode 100644
index 8b23a34..0000000
--- a/osbase/blkio/BUILD.bazel
+++ /dev/null
@@ -1,8 +0,0 @@
-load("@io_bazel_rules_go//go:def.bzl", "go_library")
-
-go_library(
-    name = "blkio",
-    srcs = ["blkio.go"],
-    importpath = "source.monogon.dev/osbase/blkio",
-    visibility = ["//visibility:public"],
-)
diff --git a/osbase/blkio/blkio.go b/osbase/blkio/blkio.go
deleted file mode 100644
index a4669ab..0000000
--- a/osbase/blkio/blkio.go
+++ /dev/null
@@ -1,86 +0,0 @@
-// Copyright The Monogon Project Authors.
-// SPDX-License-Identifier: Apache-2.0
-
-package blkio
-
-import (
-	"fmt"
-	"io"
-	"os"
-)
-
-type ReaderWithSize struct {
-	io.Reader
-	size int64
-}
-
-// SizedReader is an io.Reader with a known size
-type SizedReader interface {
-	io.Reader
-	Size() int64
-}
-
-// NewSizedReader returns a SizedReader given a reader and a size.
-// The returned SizedReader is a ReaderWithSize.
-func NewSizedReader(r io.Reader, size int64) SizedReader {
-	return &ReaderWithSize{r, size}
-}
-
-func (r *ReaderWithSize) Size() int64 {
-	return r.size
-}
-
-// LazyFileReader implements a SizedReader which opens a file on first read
-// and closes it again after the reader has reached EOF.
-type LazyFileReader struct {
-	name string
-	size int64
-	f    *os.File
-	done bool
-}
-
-func (r *LazyFileReader) init() error {
-	f, err := os.Open(r.name)
-	if err != nil {
-		return fmt.Errorf("failed to open file for reading: %w", err)
-	}
-	r.f = f
-	return nil
-}
-
-func (r *LazyFileReader) Size() int64 {
-	return r.size
-}
-
-func (r *LazyFileReader) Read(b []byte) (n int, err error) {
-	if r.done {
-		return 0, io.EOF
-	}
-	if r.f == nil {
-		if err = r.init(); err != nil {
-			return
-		}
-	}
-	n, err = r.f.Read(b)
-	if err == io.EOF {
-		r.done = true
-		r.f.Close()
-	}
-	return
-}
-
-func (r *LazyFileReader) Close() {
-	r.done = true
-	r.f.Close()
-}
-
-func NewFileReader(name string) (*LazyFileReader, error) {
-	info, err := os.Stat(name)
-	if err != nil {
-		return nil, fmt.Errorf("failed to stat: %w", err)
-	}
-	return &LazyFileReader{
-		size: info.Size(),
-		name: name,
-	}, nil
-}
diff --git a/osbase/build/mkimage/BUILD.bazel b/osbase/build/mkimage/BUILD.bazel
index a822c62..aba561e 100644
--- a/osbase/build/mkimage/BUILD.bazel
+++ b/osbase/build/mkimage/BUILD.bazel
@@ -6,9 +6,9 @@
     importpath = "source.monogon.dev/osbase/build/mkimage",
     visibility = ["//visibility:private"],
     deps = [
-        "//osbase/blkio",
         "//osbase/blockdev",
         "//osbase/build/mkimage/osimage",
+        "//osbase/structfs",
     ],
 )
 
diff --git a/osbase/build/mkimage/main.go b/osbase/build/mkimage/main.go
index 79e2920..bef7517 100644
--- a/osbase/build/mkimage/main.go
+++ b/osbase/build/mkimage/main.go
@@ -19,9 +19,9 @@
 	"log"
 	"os"
 
-	"source.monogon.dev/osbase/blkio"
 	"source.monogon.dev/osbase/blockdev"
 	"source.monogon.dev/osbase/build/mkimage/osimage"
+	"source.monogon.dev/osbase/structfs"
 )
 
 func main() {
@@ -51,33 +51,30 @@
 	// Open the input files for osimage.Create, fill in reader objects and
 	// metadata in osimage.Params.
 	// Start with the EFI Payload the OS will boot from.
-	p, err := blkio.NewFileReader(efiPayload)
+	var err error
+	cfg.EFIPayload, err = structfs.OSPathBlob(efiPayload)
 	if err != nil {
 		log.Fatalf("while opening the EFI payload at %q: %v", efiPayload, err)
 	}
-	cfg.EFIPayload = p
 
-	ab, err := blkio.NewFileReader(abLoaderPayload)
+	cfg.ABLoader, err = structfs.OSPathBlob(abLoaderPayload)
 	if err != nil {
 		log.Fatalf("while opening the abloader payload at %q: %v", abLoaderPayload, err)
 	}
-	cfg.ABLoader = ab
 
 	// Attempt to open the system image if its path is set. In case the path
 	// isn't set, the system partition will still be created, but no
 	// contents will be written into it.
 	if systemImage != "" {
-		img, err := os.Open(systemImage)
+		cfg.SystemImage, err = structfs.OSPathBlob(systemImage)
 		if err != nil {
 			log.Fatalf("while opening the system image at %q: %v", systemImage, err)
 		}
-		defer img.Close()
-		cfg.SystemImage = img
 	}
 
 	// Attempt to open the node parameters file if its path is set.
 	if nodeParams != "" {
-		np, err := blkio.NewFileReader(nodeParams)
+		np, err := structfs.OSPathBlob(nodeParams)
 		if err != nil {
 			log.Fatalf("while opening node parameters at %q: %v", nodeParams, err)
 		}
diff --git a/osbase/build/mkimage/osimage/BUILD.bazel b/osbase/build/mkimage/osimage/BUILD.bazel
index cfcf096..f85fa20 100644
--- a/osbase/build/mkimage/osimage/BUILD.bazel
+++ b/osbase/build/mkimage/osimage/BUILD.bazel
@@ -10,6 +10,7 @@
         "//osbase/efivarfs",
         "//osbase/fat32",
         "//osbase/gpt",
+        "//osbase/structfs",
         "@com_github_google_uuid//:uuid",
     ],
 )
diff --git a/osbase/build/mkimage/osimage/osimage.go b/osbase/build/mkimage/osimage/osimage.go
index 71b5499..4cd3952 100644
--- a/osbase/build/mkimage/osimage/osimage.go
+++ b/osbase/build/mkimage/osimage/osimage.go
@@ -16,6 +16,7 @@
 	"source.monogon.dev/osbase/efivarfs"
 	"source.monogon.dev/osbase/fat32"
 	"source.monogon.dev/osbase/gpt"
+	"source.monogon.dev/osbase/structfs"
 )
 
 var (
@@ -60,17 +61,17 @@
 	Output blockdev.BlockDev
 	// ABLoader provides the A/B loader which then loads the EFI loader for the
 	// correct slot.
-	ABLoader fat32.SizedReader
+	ABLoader structfs.Blob
 	// EFIPayload provides contents of the EFI payload file. It must not be
 	// nil. This gets put into boot slot A.
-	EFIPayload fat32.SizedReader
+	EFIPayload structfs.Blob
 	// SystemImage provides contents of the Metropolis system partition.
 	// If nil, no contents will be copied into the partition.
-	SystemImage io.Reader
+	SystemImage structfs.Blob
 	// NodeParameters provides contents of the node parameters file. If nil,
 	// the node parameters file won't be created in the target ESP
 	// filesystem.
-	NodeParameters fat32.SizedReader
+	NodeParameters structfs.Blob
 	// DiskGUID is a unique identifier of the image and a part of Table
 	// header. It's optional and can be left blank if the identifier is
 	// to be randomly generated. Setting it to a predetermined value can
@@ -86,7 +87,7 @@
 
 type plan struct {
 	*Params
-	rootInode        fat32.Inode
+	efiRoot          structfs.Tree
 	tbl              *gpt.Table
 	efiPartition     *gpt.Partition
 	systemPartitionA *gpt.Partition
@@ -100,7 +101,7 @@
 	// Ignore errors, this is only advisory.
 	i.Output.Discard(0, i.Output.BlockCount()*i.Output.BlockSize())
 
-	if err := fat32.WriteFS(blockdev.NewRWS(i.efiPartition), i.rootInode, fat32.Options{
+	if err := fat32.WriteFS(blockdev.NewRWS(i.efiPartition), i.efiRoot, fat32.Options{
 		BlockSize:  uint16(i.efiPartition.BlockSize()),
 		BlockCount: uint32(i.efiPartition.BlockCount()),
 		Label:      "MNGN_BOOT",
@@ -108,9 +109,15 @@
 		return nil, fmt.Errorf("failed to write FAT32: %w", err)
 	}
 
-	if _, err := io.Copy(blockdev.NewRWS(i.systemPartitionA), i.SystemImage); err != nil {
+	systemImage, err := i.SystemImage.Open()
+	if err != nil {
+		return nil, fmt.Errorf("failed to open system image: %w", err)
+	}
+	if _, err := io.Copy(blockdev.NewRWS(i.systemPartitionA), systemImage); err != nil {
+		systemImage.Close()
 		return nil, fmt.Errorf("failed to write system partition A: %w", err)
 	}
+	systemImage.Close()
 
 	if err := i.tbl.Write(); err != nil {
 		return nil, fmt.Errorf("failed to write Table: %w", err)
@@ -155,25 +162,22 @@
 		return nil, fmt.Errorf("failed to allocate ESP: %w", err)
 	}
 
-	params.rootInode = fat32.Inode{
-		Attrs: fat32.AttrDirectory,
-	}
-	if err := params.rootInode.PlaceFile(strings.TrimPrefix(EFIBootAPath, "/"), params.EFIPayload); err != nil {
+	if err := params.efiRoot.PlaceFile(strings.TrimPrefix(EFIBootAPath, "/"), params.EFIPayload); err != nil {
 		return nil, err
 	}
 	// Place the A/B loader at the EFI bootloader autodiscovery path.
-	if err := params.rootInode.PlaceFile(strings.TrimPrefix(EFIPayloadPath, "/"), params.ABLoader); err != nil {
+	if err := params.efiRoot.PlaceFile(strings.TrimPrefix(EFIPayloadPath, "/"), params.ABLoader); err != nil {
 		return nil, err
 	}
 	if params.NodeParameters != nil {
-		if err := params.rootInode.PlaceFile(nodeParamsPath, params.NodeParameters); err != nil {
+		if err := params.efiRoot.PlaceFile(nodeParamsPath, params.NodeParameters); err != nil {
 			return nil, err
 		}
 	}
 
 	// Try to layout the fat32 partition. If it detects that the disk is too
 	// small, an error will be returned.
-	if _, err := fat32.SizeFS(params.rootInode, fat32.Options{
+	if _, err := fat32.SizeFS(params.efiRoot, fat32.Options{
 		BlockSize:  uint16(params.efiPartition.BlockSize()),
 		BlockCount: uint32(params.efiPartition.BlockCount()),
 		Label:      "MNGN_BOOT",
diff --git a/osbase/fat32/BUILD.bazel b/osbase/fat32/BUILD.bazel
index 1e0e909..ac3eb96 100644
--- a/osbase/fat32/BUILD.bazel
+++ b/osbase/fat32/BUILD.bazel
@@ -11,6 +11,7 @@
     ],
     importpath = "source.monogon.dev/osbase/fat32",
     visibility = ["//visibility:public"],
+    deps = ["//osbase/structfs"],
 )
 
 go_test(
@@ -28,6 +29,7 @@
         "xFsckPath": "$(rlocationpath @com_github_dosfstools_dosfstools//:fsck )",
     },
     deps = [
+        "//osbase/structfs",
         "@com_github_stretchr_testify//assert",
         "@com_github_stretchr_testify//require",
         "@io_bazel_rules_go//go/runfiles",
diff --git a/osbase/fat32/dos83.go b/osbase/fat32/dos83.go
index 1d988be..b8d219d 100644
--- a/osbase/fat32/dos83.go
+++ b/osbase/fat32/dos83.go
@@ -28,12 +28,12 @@
 // well as valid ASCII
 var validDOSName = regexp.MustCompile(`^^([A-Z0-9!#$%&'()@^_\x60{}~-]{0,8})(\.[A-Z0-9!#$%&'()-@^_\x60{}~-]{1,3})?$`)
 
-func makeUniqueDOSNames(inodes []*Inode) error {
+func makeUniqueDOSNames(nodes []*node) error {
 	taken := make(map[[11]byte]bool)
-	var lossyNameInodes []*Inode
+	var lossyNameNodes []*node
 	// Make two passes to ensure that names can always be passed through even
 	// if they would conflict with a generated name.
-	for _, i := range inodes {
+	for _, i := range nodes {
 		for j := range i.dosName {
 			i.dosName[j] = ' '
 		}
@@ -54,7 +54,7 @@
 			taken[i.dosName] = true
 			continue
 		}
-		lossyNameInodes = append(lossyNameInodes, i)
+		lossyNameNodes = append(lossyNameNodes, i)
 	}
 	// Willfully ignore the recommended short name generation algorithm as it
 	// requires tons of bookkeeping and doesn't result in stable names so
@@ -63,7 +63,7 @@
 	// of that because of long file name entries), so 4 hex characters
 	// guarantee uniqueness, regardless of the rest of name.
 	var nameIdx int
-	for _, i := range lossyNameInodes {
+	for _, i := range lossyNameNodes {
 		nameUpper := strings.ToUpper(i.Name)
 		dotParts := strings.Split(nameUpper, ".")
 		for j := range dotParts {
diff --git a/osbase/fat32/fat32.go b/osbase/fat32/fat32.go
index e5c2e58..340bb45 100644
--- a/osbase/fat32/fat32.go
+++ b/osbase/fat32/fat32.go
@@ -13,9 +13,10 @@
 	"io/fs"
 	"math"
 	"math/bits"
-	"strings"
 	"time"
 	"unicode/utf16"
+
+	"source.monogon.dev/osbase/structfs"
 )
 
 // This package contains multiple references to the FAT32 specification, called
@@ -33,7 +34,7 @@
 	// as large as it needs to be.
 	BlockCount uint32
 
-	// Human-readable filesystem label. Maximum 10 bytes (gets cut off), should
+	// Human-readable filesystem label. Maximum 11 bytes (gets cut off), should
 	// be uppercase alphanumeric.
 	Label string
 
@@ -42,13 +43,7 @@
 	ID uint32
 }
 
-// SizedReader is an io.Reader with a known size
-type SizedReader interface {
-	io.Reader
-	Size() int64
-}
-
-// Attribute is a bitset of flags set on an inode.
+// Attribute is a bitset of flags set on a directory entry.
 // See also the spec page 24
 type Attribute uint8
 
@@ -59,39 +54,51 @@
 	AttrHidden Attribute = 0x02
 	// AttrSystem indicates that this is an operating system file.
 	AttrSystem Attribute = 0x04
-	// AttrDirectory indicates that this is a directory and not a file.
-	AttrDirectory Attribute = 0x10
+	// attrVolumeID indicates that this is a special directory entry which
+	// contains the volume label.
+	attrVolumeID Attribute = 0x08
+	// attrDirectory indicates that this is a directory and not a file.
+	attrDirectory Attribute = 0x10
 	// AttrArchive canonically indicates that a file has been created/modified
 	// since the last backup. Its use in practice is inconsistent.
 	AttrArchive Attribute = 0x20
 )
 
-// Inode is file or directory on the FAT32 filesystem. Note that the concept
-// of an inode doesn't really exist on FAT32, its directories are just special
-// files.
-type Inode struct {
-	// Name of the file or directory (not including its path)
-	Name string
-	// Time the file or directory was last modified
-	ModTime time.Time
+// DirEntrySys contains additional directory entry fields which are specific to
+// FAT32. To set these fields, the Sys field of a [structfs.Node] can be set to
+// a pointer to this or to a struct which embeds it.
+type DirEntrySys struct {
 	// Time the file or directory was created
 	CreateTime time.Time
 	// Attributes
 	Attrs Attribute
-	// Children of this directory (only valid when Attrs has AttrDirectory set)
-	Children []*Inode
-	// Content of this file
-	// Only valid when Attrs doesn't have AttrDirectory set.
-	Content SizedReader
+}
 
-	// Filled out on placement and write-out
-	startCluster int
-	parent       *Inode
+func (d *DirEntrySys) FAT32() *DirEntrySys {
+	return d
+}
+
+// DirEntrySysAccessor is used to access [DirEntrySys] instead of directly type
+// asserting the struct, to allow for embedding.
+type DirEntrySysAccessor interface {
+	FAT32() *DirEntrySys
+}
+
+// node is a file or directory on the FAT32 filesystem. It wraps a
+// [structfs.Node] and holds additional fields which are filled during planning.
+type node struct {
+	*structfs.Node
 	dosName      [11]byte
+	createTime   time.Time
+	attrs        Attribute
+	parent       *node
+	children     []*node
+	size         uint32
+	startCluster int
 }
 
 // Number of LFN entries + normal entry (all 32 bytes)
-func (i Inode) metaSize() (int64, error) {
+func (i node) metaSize() (int64, error) {
 	fileNameUTF16 := utf16.Encode([]rune(i.Name))
 	// VFAT file names are null-terminated
 	fileNameUTF16 = append(fileNameUTF16, 0x00)
@@ -112,9 +119,9 @@
 	return sum
 }
 
-// writeMeta writes information about this inode into the contents of the parent
-// inode.
-func (i Inode) writeMeta(w io.Writer) error {
+// writeMeta writes information about this node into the contents of the parent
+// node.
+func (i node) writeMeta(w io.Writer) error {
 	fileNameUTF16 := utf16.Encode([]rune(i.Name))
 	// VFAT file names are null-terminated
 	fileNameUTF16 = append(fileNameUTF16, 0x00)
@@ -153,21 +160,15 @@
 			return err
 		}
 	}
-	selfSize, err := i.dataSize()
-	if err != nil {
-		return err
-	}
-	if selfSize >= 4*1024*1024*1024 {
-		return errors.New("single file size exceeds 4GiB which is prohibited in FAT32")
-	}
-	if i.Attrs&AttrDirectory != 0 {
+	selfSize := i.size
+	if i.attrs&attrDirectory != 0 {
 		selfSize = 0 // Directories don't have an explicit size
 	}
 	date, t, _ := timeToMsDosTime(i.ModTime)
-	cdate, ctime, ctens := timeToMsDosTime(i.CreateTime)
+	cdate, ctime, ctens := timeToMsDosTime(i.createTime)
 	if err := binary.Write(w, binary.LittleEndian, &dirEntry{
 		DOSName:           i.dosName,
-		Attributes:        uint8(i.Attrs),
+		Attributes:        uint8(i.attrs),
 		CreationTenMilli:  ctens,
 		CreationTime:      ctime,
 		CreationDate:      cdate,
@@ -175,27 +176,27 @@
 		LastWrittenToTime: t,
 		LastWrittenToDate: date,
 		FirstClusterLow:   uint16(i.startCluster & 0xffff),
-		FileSize:          uint32(selfSize),
+		FileSize:          selfSize,
 	}); err != nil {
 		return err
 	}
 	return nil
 }
 
-// writeData writes the contents of this inode (including possible metadata
+// writeData writes the contents of this node (including possible metadata
 // of its children, but not its children's data)
-func (i Inode) writeData(w io.Writer, volumeLabel [11]byte) error {
-	if i.Attrs&AttrDirectory != 0 {
+func (i node) writeData(w io.Writer, volumeLabel [11]byte) error {
+	if i.attrs&attrDirectory != 0 {
 		if i.parent == nil {
 			if err := binary.Write(w, binary.LittleEndian, &dirEntry{
 				DOSName:    volumeLabel,
-				Attributes: 0x08, // Volume ID, internal use only
+				Attributes: uint8(attrVolumeID),
 			}); err != nil {
 				return err
 			}
 		} else {
 			date, t, _ := timeToMsDosTime(i.ModTime)
-			cdate, ctime, ctens := timeToMsDosTime(i.CreateTime)
+			cdate, ctime, ctens := timeToMsDosTime(i.createTime)
 			if err := binary.Write(w, binary.LittleEndian, &dirEntry{
 				DOSName:           [11]byte{'.', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' '},
 				CreationDate:      cdate,
@@ -203,7 +204,7 @@
 				CreationTenMilli:  ctens,
 				LastWrittenToTime: t,
 				LastWrittenToDate: date,
-				Attributes:        uint8(i.Attrs),
+				Attributes:        uint8(i.attrs),
 				FirstClusterHigh:  uint16(i.startCluster >> 16),
 				FirstClusterLow:   uint16(i.startCluster & 0xffff),
 			}); err != nil {
@@ -224,93 +225,56 @@
 				CreationTenMilli:  ctens,
 				LastWrittenToTime: t,
 				LastWrittenToDate: date,
-				Attributes:        uint8(AttrDirectory),
+				Attributes:        uint8(attrDirectory),
 				FirstClusterHigh:  uint16(startCluster >> 16),
 				FirstClusterLow:   uint16(startCluster & 0xffff),
 			}); err != nil {
 				return err
 			}
 		}
-		err := makeUniqueDOSNames(i.Children)
-		if err != nil {
-			return err
-		}
-		for _, c := range i.Children {
+		for _, c := range i.children {
 			if err := c.writeMeta(w); err != nil {
 				return err
 			}
 		}
 	} else {
-		if _, err := io.CopyN(w, i.Content, i.Content.Size()); err != nil {
+		content, err := i.Content.Open()
+		if err != nil {
+			return err
+		}
+		defer content.Close()
+		if _, err := io.CopyN(w, content, int64(i.size)); err != nil {
 			return err
 		}
 	}
 	return nil
 }
 
-func (i Inode) dataSize() (int64, error) {
-	if i.Attrs&AttrDirectory != 0 {
-		var size int64
-		if i.parent != nil {
-			// Dot and dotdot directories
-			size += 2 * 32
-		} else {
-			// Volume ID
-			size += 1 * 32
-		}
-		for _, c := range i.Children {
-			cs, err := c.metaSize()
-			if err != nil {
-				return 0, err
-			}
-			size += cs
-		}
-		if size > 2*1024*1024 {
-			return 0, errors.New("directory contains > 2MiB of metadata which is prohibited in FAT32")
-		}
-		return size, nil
+func (i node) dirSize() (uint32, error) {
+	var size int64
+	if i.parent != nil {
+		// Dot and dotdot directories
+		size += 2 * 32
 	} else {
-		return i.Content.Size(), nil
+		// Volume ID
+		size += 1 * 32
 	}
-}
-
-func (i *Inode) PlaceFile(path string, reader SizedReader) error {
-	pathParts := strings.Split(path, "/")
-	inodeRef := i
-	for j, part := range pathParts {
-		var childExists bool
-		for _, child := range inodeRef.Children {
-			if strings.EqualFold(child.Name, part) {
-				inodeRef = child
-				childExists = true
-				break
-			}
+	for _, c := range i.children {
+		cs, err := c.metaSize()
+		if err != nil {
+			return 0, err
 		}
-		if j == len(pathParts)-1 { // Is last path part (i.e. file name)
-			if childExists {
-				return &fs.PathError{Path: path, Err: fs.ErrExist, Op: "create"}
-			}
-			newInode := &Inode{
-				Name:    part,
-				Content: reader,
-			}
-			inodeRef.Children = append(inodeRef.Children, newInode)
-			return nil
-		} else if !childExists {
-			newInode := &Inode{
-				Name:  part,
-				Attrs: AttrDirectory,
-			}
-			inodeRef.Children = append(inodeRef.Children, newInode)
-			inodeRef = newInode
-		}
+		size += cs
 	}
-	panic("unreachable")
+	if size > 2*1024*1024 {
+		return 0, errors.New("directory contains > 2MiB of metadata which is prohibited in FAT32")
+	}
+	return uint32(size), nil
 }
 
 type planningState struct {
-	// List of inodes in filesystem layout order
-	orderedInodes []*Inode
+	// List of nodes in filesystem layout order
+	orderedNodes []*node
 	// File Allocation Table
 	fat []uint32
 	// Size of a single cluster in the FAT in bytes
@@ -335,16 +299,54 @@
 	return allocStartCluster
 }
 
-func (i *Inode) placeRecursively(p *planningState) error {
-	selfDataSize, err := i.dataSize()
-	if err != nil {
-		return fmt.Errorf("%s: %w", i.Name, err)
+func (i *node) placeRecursively(p *planningState) error {
+	if i.Mode.IsDir() {
+		for _, c := range i.Node.Children {
+			node := &node{
+				Node:       c,
+				createTime: c.ModTime,
+				parent:     i,
+			}
+			if sys, ok := c.Sys.(DirEntrySysAccessor); ok {
+				sys := sys.FAT32()
+				node.attrs = sys.Attrs & (AttrReadOnly | AttrHidden | AttrSystem | AttrArchive)
+				if !sys.CreateTime.IsZero() {
+					node.createTime = sys.CreateTime
+				}
+			}
+			switch {
+			case c.Mode.IsRegular():
+				size := c.Content.Size()
+				if size < 0 {
+					return fmt.Errorf("%s: negative file size", c.Name)
+				}
+				if size >= 4*1024*1024*1024 {
+					return fmt.Errorf("%s: single file size exceeds 4GiB which is prohibited in FAT32", c.Name)
+				}
+				node.size = uint32(size)
+				if len(c.Children) != 0 {
+					return fmt.Errorf("%s: file cannot have children", c.Name)
+				}
+			case c.Mode.IsDir():
+				node.attrs |= attrDirectory
+			default:
+				return fmt.Errorf("%s: unsupported file type %s", c.Name, c.Mode.Type().String())
+			}
+			i.children = append(i.children, node)
+		}
+		err := makeUniqueDOSNames(i.children)
+		if err != nil {
+			return err
+		}
+		i.size, err = i.dirSize()
+		if err != nil {
+			return fmt.Errorf("%s: %w", i.Name, err)
+		}
 	}
-	i.startCluster = p.allocBytes(selfDataSize)
-	p.orderedInodes = append(p.orderedInodes, i)
-	for _, c := range i.Children {
-		c.parent = i
-		err = c.placeRecursively(p)
+	i.startCluster = p.allocBytes(int64(i.size))
+	p.orderedNodes = append(p.orderedNodes, i)
+	for _, c := range i.children {
+		err := c.placeRecursively(p)
 		if err != nil {
 			return fmt.Errorf("%s/%w", i.Name, err)
 		}
@@ -352,10 +354,9 @@
 	return nil
 }
 
-// WriteFS writes a filesystem described by a root inode and its children to a
-// given io.Writer.
-func WriteFS(w io.Writer, rootInode Inode, opts Options) error {
-	bs, fsi, p, err := prepareFS(&opts, rootInode)
+// WriteFS writes a filesystem described by a tree to a given io.Writer.
+func WriteFS(w io.Writer, root structfs.Tree, opts Options) error {
+	bs, fsi, p, err := prepareFS(&opts, root)
 	if err != nil {
 		return err
 	}
@@ -405,9 +406,9 @@
 		}
 	}
 
-	for _, i := range p.orderedInodes {
+	for _, i := range p.orderedNodes {
 		if err := i.writeData(wb, bs.Label); err != nil {
-			return fmt.Errorf("failed to write inode %q: %w", i.Name, err)
+			return fmt.Errorf("failed to write contents of %q: %w", i.Name, err)
 		}
 		if err := wb.FinishBlock(int64(opts.BlockSize)*int64(bs.BlocksPerCluster), true); err != nil {
 			return err
@@ -420,7 +421,7 @@
 	return nil
 }
 
-func prepareFS(opts *Options, rootInode Inode) (*bootSector, *fsinfo, *planningState, error) {
+func prepareFS(opts *Options, root structfs.Tree) (*bootSector, *fsinfo, *planningState, error) {
 	if opts.BlockSize == 0 {
 		opts.BlockSize = 512
 	}
@@ -437,9 +438,6 @@
 		}
 		opts.ID = binary.BigEndian.Uint32(buf[:])
 	}
-	if rootInode.Attrs&AttrDirectory == 0 {
-		return nil, nil, nil, errors.New("root inode must be a directory (i.e. have AttrDirectory set)")
-	}
 	bs := bootSector{
 		// Assembled x86_32 machine code corresponding to
 		// jmp $
@@ -499,7 +497,14 @@
 	}
 	// First two clusters are special
 	p.fat = append(p.fat, 0x0fffff00|uint32(bs.MediaCode), 0x0fffffff)
-	err := rootInode.placeRecursively(&p)
+	rootNode := &node{
+		Node: &structfs.Node{
+			Mode:     fs.ModeDir,
+			Children: root,
+		},
+		attrs: attrDirectory,
+	}
+	err := rootNode.placeRecursively(&p)
 	if err != nil {
 		return nil, nil, nil, err
 	}
@@ -514,7 +519,7 @@
 		p.fat = append(p.fat, fatFree)
 	}
 
-	bs.RootClusterNumber = uint32(rootInode.startCluster)
+	bs.RootClusterNumber = uint32(rootNode.startCluster)
 
 	bs.BlocksPerFAT = uint32(binary.Size(p.fat)+int(opts.BlockSize)-1) / uint32(opts.BlockSize)
 	occupiedBlocks := uint32(bs.ReservedBlocks) + (uint32(len(p.fat)-2) * uint32(bs.BlocksPerCluster)) + bs.BlocksPerFAT*uint32(bs.NumFATs)
@@ -552,10 +557,10 @@
 }
 
 // SizeFS returns the number of blocks required to hold the filesystem defined
-// by rootInode and opts. This can be used for sizing calculations before
-// calling WriteFS.
-func SizeFS(rootInode Inode, opts Options) (int64, error) {
-	bs, _, _, err := prepareFS(&opts, rootInode)
+// by root and opts. This can be used for sizing calculations before calling
+// WriteFS.
+func SizeFS(root structfs.Tree, opts Options) (int64, error) {
+	bs, _, _, err := prepareFS(&opts, root)
 	if err != nil {
 		return 0, err
 	}
diff --git a/osbase/fat32/fsck_test.go b/osbase/fat32/fsck_test.go
index d4908be..5043003 100644
--- a/osbase/fat32/fsck_test.go
+++ b/osbase/fat32/fsck_test.go
@@ -14,6 +14,8 @@
 	"time"
 
 	"github.com/bazelbuild/rules_go/go/runfiles"
+
+	"source.monogon.dev/osbase/structfs"
 )
 
 var (
@@ -39,14 +41,14 @@
 	}
 }
 
-func testWithFsck(t *testing.T, rootInode Inode, opts Options) {
+func testWithFsck(t *testing.T, root structfs.Tree, opts Options) {
 	t.Helper()
 	testFile, err := os.CreateTemp("", "fat32-fsck-test")
 	if err != nil {
 		t.Fatal(err)
 	}
 	defer os.Remove(testFile.Name())
-	sizeBlocks, err := SizeFS(rootInode, opts)
+	sizeBlocks, err := SizeFS(root, opts)
 	if err != nil {
 		t.Fatalf("failed to calculate size: %v", err)
 	}
@@ -62,7 +64,7 @@
 		t.Fatalf("seek failed: %v", err)
 	}
 
-	if err := WriteFS(testFile, rootInode, opts); err != nil {
+	if err := WriteFS(testFile, root, opts); err != nil {
 		t.Fatalf("failed to write test FS: %v", err)
 	}
 	// Run fsck non-interactively (-n), disallow spaces in short file names (-S)
@@ -89,11 +91,7 @@
 	for _, blockSize := range []uint16{512, 4096, 32768} {
 		for _, fixed := range []string{"", "Fixed"} {
 			t.Run(fmt.Sprintf("BlockSize%d%v", blockSize, fixed), func(t *testing.T) {
-				rootInode := Inode{
-					Attrs:      AttrDirectory,
-					ModTime:    time.Date(2022, 03, 04, 5, 6, 7, 8, time.UTC),
-					CreateTime: time.Date(2022, 03, 04, 5, 6, 7, 8, time.UTC),
-				}
+				var root structfs.Tree
 				files := []struct {
 					name    string
 					path    string
@@ -105,7 +103,7 @@
 					{"LargeFile", "test1/largefile.txt", largeString.String()},
 				}
 				for _, c := range files {
-					err := rootInode.PlaceFile(c.path, strings.NewReader(c.content))
+					err := root.PlaceFile(c.path, structfs.Bytes(c.content))
 					if err != nil {
 						t.Errorf("failed to place file: %v", err)
 					}
@@ -115,7 +113,7 @@
 					// Use a block count that is slightly higher than the minimum
 					opts.BlockCount = 67000
 				}
-				testWithFsck(t, rootInode, opts)
+				testWithFsck(t, root, opts)
 			})
 		}
 	}
@@ -125,19 +123,18 @@
 	if os.Getenv("IN_KTEST") == "true" {
 		t.Skip("In ktest")
 	}
-	rootInode := Inode{
-		Attrs:   AttrDirectory,
-		ModTime: time.Date(2022, 03, 04, 5, 6, 7, 8, time.UTC),
-	}
+	var root structfs.Tree
 	for i := 0; i < (32*1024)-2; i++ {
-		rootInode.Children = append(rootInode.Children, &Inode{
+		root = append(root, &structfs.Node{
 			Name:    fmt.Sprintf("test%d", i),
-			Content: strings.NewReader("random test content"),
-			// Add some random attributes
-			Attrs: AttrHidden | AttrSystem,
-			// And a random ModTime
+			Content: structfs.Bytes("random test content"),
+			// Add a random ModTime
 			ModTime: time.Date(2022, 03, 04, 5, 6, 7, 8, time.UTC),
+			Sys: &DirEntrySys{
+				// Add some random attributes
+				Attrs: AttrHidden | AttrSystem,
+			},
 		})
 	}
-	testWithFsck(t, rootInode, Options{ID: 1234, Label: "TEST"})
+	testWithFsck(t, root, Options{ID: 1234, Label: "TEST"})
 }
diff --git a/osbase/fat32/linux_test.go b/osbase/fat32/linux_test.go
index ffe7672..002ac2c 100644
--- a/osbase/fat32/linux_test.go
+++ b/osbase/fat32/linux_test.go
@@ -4,9 +4,9 @@
 package fat32
 
 import (
-	"bytes"
 	"fmt"
 	"io"
+	"io/fs"
 	"math/rand"
 	"os"
 	"strings"
@@ -16,6 +16,8 @@
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"golang.org/x/sys/unix"
+
+	"source.monogon.dev/osbase/structfs"
 )
 
 func TestKernelInterop(t *testing.T) {
@@ -25,7 +27,7 @@
 
 	type testCase struct {
 		name     string
-		setup    func(root *Inode) error
+		setup    func() structfs.Tree
 		validate func(t *testing.T) error
 	}
 
@@ -43,14 +45,15 @@
 	tests := []testCase{
 		{
 			name: "SimpleFolder",
-			setup: func(root *Inode) error {
-				root.Children = []*Inode{{
-					Name:       "testdir",
-					Attrs:      AttrDirectory,
-					CreateTime: testTimestamp1,
-					ModTime:    testTimestamp2,
+			setup: func() structfs.Tree {
+				return structfs.Tree{{
+					Name:    "testdir",
+					Mode:    fs.ModeDir,
+					ModTime: testTimestamp2,
+					Sys: &DirEntrySys{
+						CreateTime: testTimestamp1,
+					},
 				}}
-				return nil
 			},
 			validate: func(t *testing.T) error {
 				var stat unix.Statx_t
@@ -81,14 +84,15 @@
 		},
 		{
 			name: "SimpleFile",
-			setup: func(root *Inode) error {
-				root.Children = []*Inode{{
-					Name:       "testfile",
-					CreateTime: testTimestamp3,
-					ModTime:    testTimestamp4,
-					Content:    strings.NewReader(testContent1),
+			setup: func() structfs.Tree {
+				return structfs.Tree{{
+					Name:    "testfile",
+					ModTime: testTimestamp4,
+					Sys: &DirEntrySys{
+						CreateTime: testTimestamp3,
+					},
+					Content: structfs.Bytes(testContent1),
 				}}
-				return nil
 			},
 			validate: func(t *testing.T) error {
 				var stat unix.Statx_t
@@ -118,20 +122,23 @@
 		},
 		{
 			name: "FolderHierarchy",
-			setup: func(i *Inode) error {
-				i.Children = []*Inode{{
-					Name:       "l1",
-					Attrs:      AttrDirectory,
-					CreateTime: testTimestamp1,
-					ModTime:    testTimestamp2,
-					Children: []*Inode{{
-						Name:       "l2",
-						Attrs:      AttrDirectory,
+			setup: func() structfs.Tree {
+				return structfs.Tree{{
+					Name:    "l1",
+					Mode:    fs.ModeDir,
+					ModTime: testTimestamp2,
+					Sys: &DirEntrySys{
 						CreateTime: testTimestamp1,
-						ModTime:    testTimestamp2,
+					},
+					Children: structfs.Tree{{
+						Name:    "l2",
+						Mode:    fs.ModeDir,
+						ModTime: testTimestamp2,
+						Sys: &DirEntrySys{
+							CreateTime: testTimestamp1,
+						},
 					}},
 				}}
-				return nil
 			},
 			validate: func(t *testing.T) error {
 				dirInfo, err := os.ReadDir("/dut/l1")
@@ -149,14 +156,13 @@
 		},
 		{
 			name: "LargeFile",
-			setup: func(i *Inode) error {
+			setup: func() structfs.Tree {
 				content := make([]byte, 6500)
 				io.ReadFull(rand.New(rand.NewSource(1)), content)
-				i.Children = []*Inode{{
+				return structfs.Tree{{
 					Name:    "test.bin",
-					Content: bytes.NewReader(content),
+					Content: structfs.Bytes(content),
 				}}
-				return nil
 			},
 			validate: func(t *testing.T) error {
 				var stat unix.Stat_t
@@ -176,12 +182,11 @@
 		},
 		{
 			name: "Unicode",
-			setup: func(i *Inode) error {
-				i.Children = []*Inode{{
+			setup: func() structfs.Tree {
+				return structfs.Tree{{
 					Name:    "✨😂", // Really exercise that UTF-16 conversion
-					Content: strings.NewReader("😂"),
+					Content: structfs.Bytes("😂"),
 				}}
-				return nil
 			},
 			validate: func(t *testing.T) error {
 				file, err := os.Open("/dut/✨😂")
@@ -197,25 +202,26 @@
 					t.Fatalf("Failed to open unicode file: %v (available files: %v)", err, strings.Join(availableFileNames, ", "))
 				}
 				defer file.Close()
-				contents, err := io.ReadAll(file)
-				if err != nil {
-					t.Errorf("Wrong content: expected %x, got %x", []byte("😂"), contents)
-				}
+				expected := []byte("😂")
+				actual, err := io.ReadAll(file)
+				assert.NoError(t, err, "failed to read test file")
+				assert.Equal(t, expected, actual, "content not identical")
 				return nil
 			},
 		},
 		{
 			name: "MultipleMetaClusters",
-			setup: func(root *Inode) error {
+			setup: func() structfs.Tree {
 				// Only test up to 2048 files as Linux gets VERY slow if going
 				// up to the maximum of approximately 32K
+				var root structfs.Tree
 				for i := 0; i < 2048; i++ {
-					root.Children = append(root.Children, &Inode{
+					root = append(root, &structfs.Node{
 						Name:    fmt.Sprintf("verylongtestfilename%d", i),
-						Content: strings.NewReader("random test content"),
+						Content: structfs.Bytes("random test content"),
 					})
 				}
-				return nil
+				return root
 			},
 			validate: func(t *testing.T) error {
 				files, err := os.ReadDir("/dut")
@@ -245,13 +251,8 @@
 				t.Fatalf("failed to get ramdisk block size: %v", err)
 			}
 			defer file.Close()
-			rootInode := Inode{
-				Attrs: AttrDirectory,
-			}
-			if err := test.setup(&rootInode); err != nil {
-				t.Fatalf("setup failed: %v", err)
-			}
-			if err := WriteFS(file, rootInode, Options{
+			root := test.setup()
+			if err := WriteFS(file, root, Options{
 				ID:         1234,
 				Label:      "KTEST",
 				BlockSize:  uint16(blockSize),