On Wed, 16 Feb 2011 17:15:55 +1100 NeilBrown <neilb@xxxxxxx> wrote: > > Hi Trond, > I wonder if I might get your help/advice on an issue with NFS. > > It seems that NFS_DATA_SYNC is hardly used at all currently. It is used for > O_DIRECT writes and for writes 'for_reclaim', and for handling some error > conditions, but that is about it. > > This appears to be a regression. > > Back in 2005, commit ab0a3dbedc5 in 2.6.13 says: > > [PATCH] NFS: Write optimization for short files and small O_SYNC writes. > > Use stable writes if we can see that we are only going to put a single > write on the wire. > > which seems like a sensible optimisation, and we have a customer which > values it. Very roughly, they have an NFS server which optimises 'unstable' > writes for throughput and 'stable' writes for latency - these seems like a > reasonable approach. > With a 2.6.16 kernel an application which generates many small sync writes > gets adequate performance. In 2.6.32 they see unstable writes followed by > commits, which cannot be (or at least aren't) optimised as well. > > It seems this was changed by commit c63c7b0513953 > > NFS: Fix a race when doing NFS write coalescing > > in 2.6.22. > > Is it possible/easy/desirable to get this behaviour back. i.e. to use > NFS_DATA_SYNC at least on sub-page writes triggered by a write to an > O_SYNC file. > > My (possibly naive) attempt is as follows. It appears to work as I expect > (though it still uses SYNC for 1-page writes) but I'm not confident that it > is "right". > > Thanks, > > NeilBrown > > diff --git a/fs/nfs/write.c b/fs/nfs/write.c > index 10d648e..392bfa8 100644 > --- a/fs/nfs/write.c > +++ b/fs/nfs/write.c > @@ -178,6 +178,9 @@ static int wb_priority(struct writeback_control *wbc) > return FLUSH_HIGHPRI | FLUSH_STABLE; > if (wbc->for_kupdate || wbc->for_background) > return FLUSH_LOWPRI; > + if (wbc->sync_mode == WB_SYNC_ALL && > + (wbc->range_end - wbc->range_start) < PAGE_SIZE) > + return FLUSH_STABLE; > return 0; > } > I'm not so sure about this change. wb_priority is called from nfs_wb_page. The comments there say: /* * Write back all requests on one page - we do this before reading it. */ ...do we really need those writes to be NFS_FILE_SYNC? I think that the difficulty here is determining when we really are going to just be doing a single write. In that case, then clearly a FILE_SYNC write is better than an unstable + COMMIT. This is very workload dependent though. It's hard to know beforehand whether a page that we intend to write will be redirtied soon afterward. If it is, then FILE_SYNC writes may be worse than letting the server cache the writes until a COMMIT comes in. -- Jeff Layton <jlayton@xxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
