On Wed, Jun 12, 2024 at 10:46:48AM -0700, Darrick J. Wong wrote: > From: Christoph Hellwig <hch@xxxxxx> > > The XFS XFS_DIFLAG_APPEND maps to the VFS S_APPEND flag, which forbids > writes that don't append at the current EOF. > > But the commit originally adding XFS_DIFLAG_APPEND support (commit > a23321e766d in xfs xfs-import repository) also checked it to skip > releasing speculative preallocations, which doesn't make any sense. I disagree, there was a very good reason for this behaviour: preventing append-only log files from getting excessively fragmented because speculative prealloc would get removed on close(). i.e. applications that slowly log messages to append only files with the pattern open(O_APPEND); write(a single line to the log); close(); caused worst case file fragmentation because the close() always removed the speculative prealloc beyond EOF. The fix for this pessimisitic XFS behaviour is for the application to use chattr +A (like they would for ext3/4) hence triggering the existence of XFS_DIFLAG_APPEND and that avoided the removal speculative delalloc removed when the file is closed. hence the fragmentation problems went away. Note that fragmentation issue didn't affect the log writes - it badly affected log reads because it turned them into worse case random read workloads instead of sequential reads. As such, I think the justification for this change is wrong and that it removes a longstanding feature that prevents severe fragmentation of append only log files. I think we should be leaving this code as it currently stands. -Dave. -- Dave Chinner david@xxxxxxxxxxxxx
