OPFS

Per-origin sandboxed file system, rooted at navigator.storage.getDirectory().

Not visible in the user’s real filesystem, wiped when site data is cleared.
Main thread only gets async handles: getFileHandle, createWritable
Workers get createSyncAccessHandle(): synchronous read/write/truncate/flush at a byte offset
- Exclusive lock, one open handle per file
- Effectively pread/pwrite, which is what C compiled to WASM already assumes it has
IndexedDB can’t back a storage engine: async, and keyed on whole values rather than byte ranges, so every page read needs a shim
SQLite and DuckDB both ship an OPFS VFS for this reason
Evictable under storage pressure unless navigator.storage.persist() is granted

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