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+---
+title: "`fsync(2)` Pitfalls"
+date: 2025-07-11
+---
+
+# `fsync` Pitfalls
+
+This is a non-comprehensive list of the pitfalls of the `fsync` syscall.
+
+<details>
+<summary>Linux `man 2 fsync`</summary>
+
+> `fsync()` transfers ("flushes") all modified in-core data of (i.e., modified
+> buffer cache pages for) the file referred to by the file descriptor fd to the
+> disk device (or other permanent storage device) so that all changed
+> information can be retrieved even if the system crashes or is rebooted. This
+> includes writing through or flushing a disk cache if present. The call blocks
+> until the device reports that the transfer has completed.
+>
+> As well as flushing the file data, `fsync()` also flushes the metadata
+> information associated with the file (see inode(7)).
+>
+> Calling `fsync()` does not necessarily ensure that the entry in the directory
+> containing the file has also reached disk. For that an explicit `fsync()` on a
+> file descriptor for the directory is also needed.
+>
+> `fdatasync()` is similar to `fsync()`, but does not flush modified metadata
+> unless that metadata is needed in order to allow a subsequent data retrieval
+> to be correctly handled. For example, changes to st_atime or st_mtime
+> (respectively, time of last access and time of last modification; see
+> inode(7)) do not require flushing because they are not necessary for a
+> subsequent data read to be handled correctly. On the other hand, a change to
+> the file size (st_size, as made by say ftruncate(2)), would require a metadata
+> flush.
+>
+> The aim of `fdatasync()` is to reduce disk activity for applications that do
+> not require all metadata to be synchronized with the disk.
+
+</details>
+
+I will expand this list as I have more questions about all the questionable
+filesystems used and created by operating system enthusiasts.
+
+---
+
+## `fsync` does not ensure that a `fsync`'d file is visible in its parent directory
+
+From the manpage:
+
+> Calling `fsync()` does not necessarily ensure that the entry in the directory
+> containing the file has also reached disk. For that an explicit `fsync()` on a
+> file descriptor for the directory is also needed.
+
+This means that that you cannot rely on a file being in the directory after
+`fsync`ing the file itself. You have to `fsync` the directory too.
+
+Speaking about `fsync`ing a directory:
+
+---
+
+## `fsync` on a directory does not ensure children are `fsync`'d
+
+From the manpage:
+
+> Calling `fsync()` does not necessarily ensure that the entry in the directory
+> containing the file has also reached disk. For that an explicit `fsync()` on a
+> file descriptor for the directory is also needed.
+
+The assumption that `fsync` a directory will fsync the files themselves is also
+wrong. You can imagine a directory as a file containing a list of children, and
+the list is just pointers to inodes. So `fsync`ing a directory will just write
+the list of pointers to disk.
+
+---
+
+## More reading on `fsync`
+
+- [(danluu) Fsyncgate: Errors on `fsync` are unrecoverable](https://danluu.com/fsyncgate/)
+- [(puzpuzpuz) The secret life of `fsync`](https://puzpuzpuz.dev/the-secret-life-of-fsync)
+- [(stackoverflow) Difference between `syncfs` (Linux only) and `fsync` (POSIX)](https://stackoverflow.com/questions/48171855/what-is-the-difference-between-fsync-and-syncfs)
+  (TL;DR: `syncfs` is "pretty please" fsync and doesn't block until the
+  operation is done)
+- [(LWN) Feathersticth: Killing `fsync` softly](https://lwn.net/Articles/354861/)
+- [(stackoverflow) Your Program ---~~`fflush`~~---> Your OS ---~~`fsync`~~---> Your Disk](https://stackoverflow.com/questions/2340610/difference-between-fflush-and-fsync)
+- [(despairlabs) `fsync()` after `open()` is an elaborate no-op](https://despairlabs.com/blog/posts/2025-03-13-fsync-after-open-is-an-elaborate-no-op/)
+- [(Postgres Wiki) `fsync` errors](https://wiki.postgresql.org/wiki/Fsync_Errors)