On Mon, Mar 16, 2009 at 2:50 PM, Les Mikesell <lesmikesell@gmail.com> wrote: > Stuart D. Gathman wrote: >> >>> No, it's more than that. PostgreSQL gurus say LVM doesn't honor fsync(), >> >> That is clearly wrong - since fsync() isn't LVM's responsibility. >> I think they mean that fsync() can't garrantee that any writes are >> actually on the platter. >> >>> that data doesn't even get to the controller, and it doesn't matter >>> if the disks have write caches enabled or not. Or if they have battery >>> backed >>> caches. Please read the thread I linked. If what they say it's true, >> >> That is clearly wrong. If writes don't work, nothing works. >> >>> you can't use LVM for anything that needs fsync(), including mail queues >>> (sendmail), mail storage (imapd), as such. So I'd really like to know. >> >> fsync() is a file system call that writes dirty buffers, and then waits >> for the physical writes to complete. It is only the waiting part that >> is broken. > > It's a yes or no question... Fsync() either guarantees that the write is > committed to physical media so the application can continue knowing that > it's own transactional expectations are met (i.e. you can crash and recover > that piece of data), or it is broken. If it doesn't wait for completion, it > can't possibly report the correct status. > This discussion seems a bit bizarre to me. Many apps require data get to stable memory in a well defined way. Barriers is certainly one way to do that, but I don't think barriers are supported by LVM, mdraid, or drbd. Those are some very significant subsystems. I have to believe filesystems have another way to implement fsync if barriers are not supported in the stack of block susbsystems. Maybe this discussion needs to move to a filesystem list, since it is the filesystem that is responsible for making fsync() work even in the absence of barriers. Greg -- Greg Freemyer Head of EDD Tape Extraction and Processing team Litigation Triage Solutions Specialist http://www.linkedin.com/in/gregfreemyer First 99 Days Litigation White Paper - http://www.norcrossgroup.com/forms/whitepapers/99%20Days%20whitepaper.pdf The Norcross Group The Intersection of Evidence & Technology http://www.norcrossgroup.com _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
