On Tue, 2012-08-14 at 22:30 +0800, Peng Tao wrote:
> On Tue, Aug 14, 2012 at 9:45 PM, Myklebust, Trond
> <Trond.Myklebust@xxxxxxxxxx> wrote:
> > On Tue, 2012-08-14 at 10:48 +0300, Benny Halevy wrote:
> >> On 2012-08-09 18:39, Myklebust, Trond wrote:
> >> > On Thu, 2012-08-09 at 23:01 +0800, Peng Tao wrote:
> >> >> On Thu, Aug 9, 2012 at 10:36 PM, Myklebust, Trond
> >> >> <Trond.Myklebust@xxxxxxxxxx> wrote:
> >> >>> On Thu, 2012-08-09 at 22:30 +0800, Peng Tao wrote:
> >> >>>> On Thu, Aug 9, 2012 at 4:21 AM, Trond Myklebust
> >> >>>> <Trond.Myklebust@xxxxxxxxxx> wrote:
> >> >>>>> Ever since commit 0a57cdac3f (NFSv4.1 send layoutreturn to fence
> >> >>>>> disconnected data server) we've been sending layoutreturn calls
> >> >>>>> while there is potentially still outstanding I/O to the data
> >> >>>>> servers. The reason we do this is to avoid races between replayed
> >> >>>>> writes to the MDS and the original writes to the DS.
> >> >>>>>
> >> >>>>> When this happens, the BUG_ON() in nfs4_layoutreturn_done can
> >> >>>>> be triggered because it assumes that we would never call
> >> >>>>> layoutreturn without knowing that all I/O to the DS is
> >> >>>>> finished. The fix is to remove the BUG_ON() now that the
> >> >>>>> assumptions behind the test are obsolete.
> >> >>>>>
> >> >>>> Isn't MDS supposed to recall the layout if races are possible between
> >> >>>> outstanding write-to-DS and write-through-MDS?
> >> >>>
> >> >>> Where do you read that in RFC5661?
> >> >>>
> >> >> That's my (maybe mis-)understanding of how server works... But looking
> >> >> at rfc5661 section 18.44.3. layoutreturn implementation.
> >> >> "
> >> >> After this call,
> >> >> the client MUST NOT use the returned layout(s) and the associated
> >> >> storage protocol to access the file data.
> >> >> "
> >> >> And given commit 0a57cdac3f, client is using the layout even after
> >> >> layoutreturn, which IMHO is a violation of rfc5661.
> >> >
> >> > No. It is using the layoutreturn to tell the MDS to fence off I/O to a
> >> > data server that is not responding. It isn't attempting to use the
> >> > layout after the layoutreturn: the whole point is that we are attempting
> >> > write-through-MDS after the attempt to write through the DS timed out.
> >> >
> >>
> >> I hear you, but this use case is valid after a time out / disconnect
> >> (which will translate to PNFS_OSD_ERR_UNREACHABLE for the objects layout)
> >> In other cases, I/Os to the DS might obviously be in flight and the BUG_ON
> >> indicates that.
> >>
> >> IMO, the right way to implement that is to initially mark the lsegs invalid
> >> and increment plh_block_lgets, as we do today in _pnfs_return_layout
> >> but actually send the layout return only when the last segment is dereferenced.
> >
> > This is what we do for object and block layout types, so your
> > objects-specific objection is unfounded.
> >
> object layout is also doing layout return on IO error (commit
> fe0fe83585f8). And it doesn't take care of draining concurrent
> in-flight IO. I guess that's why Boaz saw the same BUG_ON.
Yes. I did notice that code when I was looking into this. However that's
Boaz's own patch, and it _only_ applies to the objects layout type. I
assumed that he had tested it when I applied it...
One way to fix that would be to keep a count of "outstanding
read/writes" in the layout, so that when the error occurs, and we want
to fall back to MDS, we just increment plh_block_lgets, invalidate the
layout, and then let the outstanding read/writes fall to zero before
sending the layoutreturn.
If the objects layout wants to do that, then I have no objection. As
I've said multiple times, though, I'm not convinced we want to do that
for the files layout.
--
Trond Myklebust
Linux NFS client maintainer
NetApp
Trond.Myklebust@xxxxxxxxxx
www.netapp.com
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