On Mon, 2013-03-18 at 18:22 +0200, Benny Halevy wrote:
> On 2013-03-18 17:55, Myklebust, Trond wrote:
> > On Mon, 2013-03-18 at 16:38 +0200, Benny Halevy wrote:
> >> We're seeing roughly 20% of the I/Os going to the MDS
> >> when installing a VM over KVM in "none" caching mode (O_DIRECT).
> >> Instrumenting the client reveled that this is caused by buffer
> >> alignment vs. file offset alignment.
> >> Besides being a performance problem, when the MDS caches data
> >> this is also manifested as data corruption when data is written
> >> first via the MDS, then via the DS, eventually the stale data is
> >> read back from the MDS.
> >
> > That's why we should return the layout.
>
> We are not in this case.
Doh! I was thinking it was a case where we need to fence...
Actually, it shouldn't be needed: we will always do a _stable_ write of
the data before we try to read it back in from the server, so MDS
caching shouldn't be a problem.
> >> Note that this check exists also for the file layout specific
> >> pg_init_* functions. The objects (ORE) and block
> >> (bl_{read,write}_pagelist) layouts seem to deal correctly with
> >> splitting IOs in the case where req->wb_offset != req->wb_pgbase
> >> though this hasn't been tested wen submitting this patch.
> >>
> > NACK. I see no evidence that we've addressed the issues that were raised
> > by Fred in commit 1825a0d08f22463e5a8f4b1636473efd057a3479 (NFS: prepare
> > coalesce testing for directio).
> > If you think that his concerns about the coalescing assumptions are no
> > longer true, then please point to why this is the case. AFAICR that
> > patch was added to fix corruption issues.
> >
>
> We see no problems with this patch with the workloads we're testing.
> Do you have a test that reproduces the original problem that we can try running?
I suspect it was one of the nfstests. (see
git://git.linux-nfs.org/projects/mora/nfstest.git ) since Fred was
working with Jorge to do the O_DIRECT testing.
Fred, Jorge?
--
Trond Myklebust
Linux NFS client maintainer
NetApp
Trond.Myklebust@xxxxxxxxxx
www.netapp.com
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