Fsync Errors

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Here is a summary of what we have learned so far about the behaviour of the fsync() system call in the presence of write-back errors on various operating systems of interest to PostgreSQL users (if our build farm is a reliable survey).

What we want to know is: when can write-back errors be forgotten and go unreported to userspace? Arbitrarily, if errors are detected during asynchronous write-back? What about errors that occurred before you opened the file and got a new file descriptor and called fsync()? If fsync() reports failure and then you call fsync() again, can it falsely report success? PostgreSQL believes that a successful call to fsync() means that *all* data for a file is on disk, as part of its checkpointing protocol. Apparently that is not the case on some operating systems, leading to the potential for unreported data loss. Triggered by fsyncgate 2018.

If you see a mistake or know something I don't, please update this document with supporting references, or ping thomas.munro@gmail.com!

Update: As of this commit, PostgreSQL will now PANIC on fsync() failure. (Similar changes were made in InnoDB/MySQL, WiredTiger/MongoDB and no doubt other software as a result of the PR around this.)

Open source kernels:

Closed source kernels:

  • AIX: unknown
  • HPUX: unknown
  • Solaris: maybe the same as Illumos, but there was apparently a great VM allocator rewrite after Solaris reverted to closed source
  • Windows: unknown

Note that ZFS is likely to be a special case even on Linux, because it doesn't use the regular page cache and has special handling for failures. More information needed.

Archeological notes: All BSD-derived systems probably inherited that brelse() logic from their common ancestor, but FreeBSD changed it in 1999 and DragonflyBSD forked from FreeBSD in 2003 but apparently rewrote the bio code significantly. Darwin inherited code directly from ancient BSD via NeXT, and later took more code from FreeBSD but apparently not the behaviour discussed above. Ancient Bell UNIX was conceptually had the same problem but since it didn't have fsync(), that's somewhat moot. According to various man pages, fsync() was introduced by 4.2BSD (1983, not sure if fsync was added a bit later), developed around the same time and same place as POSTGRES (1986), and said in its man page it for making transactional facilities. Also fsync(1) appeared in FreeBSD 4.3 (2001), a command line tool that lets you sync a named file, which probably only makes sense if you have a certain model of how I/O errors and buffering work.

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