On Fri, Jan 28, 2022 at 8:17 AM Andres Freund <and...@anarazel.de> wrote: > On 2022-01-28 08:01:01 +1300, Thomas Munro wrote: > > It might be possible to avoid that on xfs or pretty much any other > > file system. I wasn't following this closely, but even with ext4's > > recent fast commit changes, its fsync implementation still > > deliberately synchronises data for other file descriptors as a side > > effect as summarised in [1], unlike xfs and other systems. > > Not data, just metadata, right? Well, and volatile write caches (by virtue of > doing an otherwise unnecessary REQ_PREFLUSH). With data=writeback file > contents for file a are not flushed to disk (rather than from disk write > cache) when file b is fsynced. Before/After the fast commit feature.
Pass. [/me suppresses the urge to go and test]. I'm just going by words in that article and T'so's linked email about file entanglement and the global barrier nature of fsync. I'm not studying the code. > > So they've caught up with xfs's concurrent writes (and gone further than xfs > > by doing it also for buffered I/O giving up even page-level atomicity, as > > discussed in a couple of other threads), but not yet decided to pull the > > trigger on just-fsync-what-I-asked-for. > > I don't think the page level locking and the changes above / fsyncing are > pretty much independent? Right, the concurrency thing is completely unrelated and an old change (maybe so old that I'm actually thinking of ext3?). I was just commenting on two historical reasons why people used to switch to xfs (especially other databases using DIO). I see that Noah's results showed "atomicity" (for some definition) for read/write in 3.10, but not for pread/pwrite, with the difference gone in 4.9, so I guess perhaps somewhere between those releases is where our unlocked control file access became problematic, but perhaps it wasn't fundamental, just a quirk of the implementation of read/write with implicit position, and if we'd used pread/pwrite for the control file we'd have seen it sooner. *shrug*
