On Tue, Dec 01, 2015 at 04:10:45PM -0500, Glauber Costa wrote: > On Tue, Dec 1, 2015 at 4:04 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > On Tue, Dec 01, 2015 at 05:22:38PM +0200, Avi Kivity wrote: > >> On 12/01/2015 04:56 PM, Brian Foster wrote: > >> mount -o discard. And yes, overwrites are supposedly more expensive > >> than trim old data + allocate new data, but maybe if you compare it > >> with the work XFS has to do, perhaps the tradeoff is bad. > > > > Oh, you do realise that using "-o discard" causes significant delays > > in journal commit processing? i.e. the journal commit completion > > blocks until all the discards have been submitted and waited on > > *synchronously*. This is a problem with the linux block layer in > > that blkdev_issue_discard() is a synchronous operation..... > > > > Hence if you are seeing delays in transactions (e.g. timestamp updates) > > it's entirely possible that things will get much better if you > > remove the discard mount option. It's much better from a performance > > perspective to use the fstrim command every so often - fstrim issues > > discard operations in the context of the fstrim process - it does > > not interact with the transaction subsystem at all. > > Hi Dave, > > This is news to me. > > However, in the disk that we have used during the acquisition of this > trace, discard doesn't seem to be supported: > $ sudo fstrim /data/ > fstrim: /data/: the discard operation is not supported > > In that case, if I understand correctly the discard mount option > should be a noop, no? XFS still makes the blkdev_issue_discard() calls, though, because the block device can turn discard support on and off dynamically. e.g. raid devices where a faulty drive is replaced temporarily with a drive that doesn't have discard support. The block device suddenly starts returning -EOPNOTSUPP to the filesystem from blkdev_issue_discard() calls. However, the admin then replaces that drive with a new one that des have discard support, and now blkdev_issue_discard() works as exepected. IOWs, if you set the mount option, XFS will always attempt to issue discards... > That recommendation is great for our general case, though. For the moment. Given lots of time, reworking this code could greatly reduce the impact/overhead of it and so make it practical to enable. There's a lot of work to get to that point, though... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
