std::fs::File::sync_all durability guarantee for backup restoration verification

The Durability Guarantee of sync_all

std::fs::File::sync_all does not provide a portable guarantee that data has reached non-volatile physical media. It guarantees that the operating system has handed off the data and metadata to the storage device driver. Whether that data survives a sudden power loss depends entirely on the hardware's write-cache configuration and the filesystem's implementation.

Platform-Specific Behavior

The Rust standard library maps sync_all to the following system calls, each with different durability semantics:

• Linux: Maps to fsync(2). This flushes the file's dirty pages to the disk, but many consumer SSDs/HDDs ignore the flush command or keep data in a volatile write-back cache unless the drive is configured for write-through.
• Windows: Maps to FlushFileBuffers. This requests the device driver to flush its internal caches, though hardware-level volatile caches may still persist.
• macOS: Attempts to use fcntl(F_FULLFSYNC). This is stronger than fsync on macOS, as it specifically asks the drive to flush its hardware cache to the platter or flash.

Verification Strategy for Backup Restoration

Because sync_all is a "best-effort" operation regarding physical persistence, backup workflows requiring survival against power loss should implement the following strategy:

1. Call sync_all() and handle the Result: Never ignore the return value. An Err indicates the OS failed to initiate the flush.
2. Flush the Parent Directory: On Linux and macOS, calling sync_all on a file does not guarantee the directory entry (the file's existence/name) is persisted. You must open the parent directory as a file and call sync_all on it to ensure the file is visible after a reboot.
3. Hardware Verification: For critical systems, use tools like hdparm -W0 (Linux) to disable write-caching on the destination drive, forcing a synchronous write-through.
4. Application-Level Checksumming: Read the restored data back from the disk and verify it against a known hash before marking the restoration as "complete." This verifies the data is readable but does not guarantee it will survive a power pull five seconds later.

Assumptions and Constraints

This analysis assumes the use of standard Rust std::fs on stable targets. It assumes the presence of consumer-grade hardware where write-back caching is the default. If you are using Copy-on-Write (CoW) filesystems like ZFS or Btrfs, sync_all may trigger a full transaction commit, which significantly impacts performance but provides stronger consistency guarantees than traditional filesystems.

Missing Diagnostic Detail: Are you targeting enterprise-grade storage with Power Loss Protection (PLP) capacitors? If so, sync_all is effectively a durability guarantee, as the hardware ensures the volatile cache is flushed during power failure.