On Mon, Mar 25, 2019 at 11:10:44AM +1100, Dave Chinner wrote: > That's one part of it. The other is POSIX atomic write semantics. > > https://pubs.opengroup.org/onlinepubs/009695399/functions/read.html > > "I/O is intended to be atomic to ordinary files and pipes and FIFOs. > Atomic means that all the bytes from a single operation that started > out together end up together, without interleaving from other I/O > operations." > > i.e. that independent read()s should see a write() as a single > atomic change. hence if you do a read() concurrently with a write(), > the read should either run to completion before the write, or the > write run to completion before the read(). > > XFS is the only linux filesystem that provides this behaviour. I don't think that is entirely true. ocfs2 and gfs2 also took a cluster wide shared lock in read last time I looked, and of course we do the right thing when using DAX for all supported file systems.
