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SubjectRe: [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush
On Fri, Aug 27 2010 at  5:47am -0400,
Kiyoshi Ueda <> wrote:

> Hi Mike,
> On 08/26/2010 12:28 AM +0900, Mike Snitzer wrote:
> > Kiyoshi Ueda <> wrote:
> >> Anyway, as you said, the flush error handling of dm-mpath is already
> >> broken if data loss really happens on any storage used by dm-mpath.
> >> Although it's a serious issue and quick fix is required, I think
> >> you may leave the old behavior in your patch-set, since it's
> >> a separate issue.
> >
> > I'm not seeing where anything is broken with current mpath. If a
> > multipathed LUN is WCE=1 then it should be fair to assume the cache is
> > mirrored or shared across ports. Therefore retrying the SYNCHRONIZE
> > CACHE is needed.
> >
> > Do we still have fear that SYNCHRONIZE CACHE can silently drop data?
> > Seems unlikely especially given what Tejun shared from SBC.
> Do we have any proof to wipe that fear?
> If retrying on flush failure is safe on all storages used with multipath
> (e.g. SCSI, CCISS, DASD, etc), then current dm-mpath should be fine in
> the real world.
> But I'm afraid if there is a storage where something like below can happen:
> - a flush command is returned as error to mpath because a part of
> cache has physically broken at the time or so, then that part of
> data loses and the size of the cache is shrunk by the storage.
> - mpath retries the flush command using other path.
> - the flush command is returned as success to mpath.
> - mpath passes the result, success, to upper layer, but some of
> the data already lost.

That does seem like a valid concern. But I'm not seeing why its unique
to SYNCHRONIZE CACHE. Any IO that fails on the target side should be
passed up once the error gets to DM.


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