Virtual File System

Thunder provides a sandboxed, in-memory Virtual File System (VFS) so that code using node:fs works without access to the host file system. The VFS is scoped to each isolate and is completely ephemeral — all data is lost when the isolate is recycled.

Mount points

Path	Mode	Source	Purpose
/bundle	Read-only	Deployed function bundle	Application code, static assets, configuration files packaged with the function.
/tmp	Read-write	Empty at startup	Scratch space for caches, temporary files, intermediate data. Quota-enforced.
/dev/null	Write-only	Sink	Discards all writes. Reads return EOF.

Any path outside these mount points returns ENOENT.

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Quotas

Writable space under /tmp is constrained by two quotas to prevent a single function from exhausting isolate memory.

Parameter	Default	CLI flag	Env var
Total writable bytes	10 MiB	--vfs-total-quota-bytes	EDGE_RUNTIME_VFS_TOTAL_QUOTA_BYTES
Max single file bytes	5 MiB	--vfs-max-file-bytes	EDGE_RUNTIME_VFS_MAX_FILE_BYTES

When a write would exceed either limit, the operation fails with ENOSPC.

import { writeFileSync } from "node:fs";

// This succeeds if within quota
writeFileSync("/tmp/data.json", JSON.stringify(payload));

// This throws ENOSPC if the file exceeds vfsMaxFileBytes
try {
  writeFileSync("/tmp/large.bin", hugeBuffer);
} catch (err) {
  console.error(err.code); // "ENOSPC"
}

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Supported operations

Read operations

Function	Sync	Async	Notes
readFileSync / readFile	Yes	Yes	Returns Buffer or string depending on encoding option.
readdirSync / readdir	Yes	Yes	Returns file and directory names. Supports withFileTypes.
statSync / stat	Yes	Yes	Returns size, mtime, isFile, isDirectory.
lstatSync / lstat	Yes	Yes	Same as stat (no symlinks in VFS).
existsSync	Yes	—	Returns boolean.
accessSync / access	Yes	Yes	Checks existence and permissions.
readlinkSync / readlink	Yes	Yes	Always throws EOPNOTSUPP (no symlinks).

Write operations (only under /tmp)

Function	Sync	Async	Notes
writeFileSync / writeFile	Yes	Yes	Creates or overwrites a file. Enforces quotas.
appendFileSync / appendFile	Yes	Yes	Appends to a file. Enforces quotas.
mkdirSync / mkdir	Yes	Yes	Creates directories. Supports recursive.
unlinkSync / unlink	Yes	Yes	Removes a file. Frees quota.
rmdirSync / rmdir	Yes	Yes	Removes an empty directory.
rmSync / rm	Yes	Yes	Supports recursive and force.
renameSync / rename	Yes	Yes	Moves/renames within /tmp.
copyFileSync / copyFile	Yes	Yes	Copies within or into /tmp. Enforces quotas.

Not supported

These operations are intentionally not available:

• Symlinks (symlink, link)
• File permissions (chmod, chown)
• File watchers (watch, watchFile)
• File descriptors (open, read, write, close — low-level fd API)
• Streams (createReadStream, createWriteStream)

Calling unsupported operations throws with code EOPNOTSUPP.

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Error model

The VFS uses standard Node.js SystemError codes:

Code	Meaning
ENOENT	File or directory not found.
ENOTDIR	A component of the path is not a directory.
EISDIR	Expected a file but found a directory.
EROFS	Write attempted on a read-only mount (/bundle).
ENOSPC	Write would exceed VFS quota.
EOPNOTSUPP	Operation is not supported by the VFS.

Error objects include code, syscall, path, and message fields, matching the Node.js convention.

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Configuration

Bundle resources

Files are mounted under /bundle from the function manifest. When deploying a function, include static assets in the bundle directory and they will be available at /bundle/<relative-path>.

// Read a config file shipped with the function
import { readFileSync } from "node:fs";
const config = JSON.parse(readFileSync("/bundle/config.json", "utf-8"));

CLI flags

--vfs-total-quota-bytes <bytes>    Total writable bytes across /tmp (default: 10485760)
--vfs-max-file-bytes <bytes>       Maximum size of a single file in /tmp (default: 5242880)

Environment variables

EDGE_RUNTIME_VFS_TOTAL_QUOTA_BYTES=10485760
EDGE_RUNTIME_VFS_MAX_FILE_BYTES=5242880

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Lifecycle

1. Isolate creation — The VFS is initialised. /bundle is populated from the function manifest. /tmp is empty.
2. Request handling — Code may read from /bundle and read/write to /tmp. Quota is tracked.
3. Isolate recycle — All VFS state is discarded. No data persists across isolate lifetimes.

Because /tmp is purely in-memory and ephemeral, it should only be used for transient data such as caches or intermediate computation artifacts. Never rely on /tmp for durable storage.