On Tue, 27 Jan 2015, Wang, Zhiqiang wrote:
> Not sure if it is correct, but below is my understanding. Correct me if
> I'm wrong.
>
> Yes, in the current code, a hash lookup on each IO is used to check dup
> op. But when adding extra_reqids as a log entry in the pg_log, 2 hash
> lookups may be not sufficient. The extra_reqids is organized by object
> id as in your code. We may have other ops on an object just promoted. If
> a dup op comes in after these ops, then we have to search for log
> entries of this object in pg_log.
Not sure I follow... we have a second hash_map for that?
https://github.com/liewegas/ceph/commit/fd66f515953e1af5af107cae2cc9bb01d1fab011#diff-d0421fbca7045e2a15a4ff980f3eb00cR60
Perhaps that should be a unordered_multimap in case a reqid shows up in
two entries (e.g., object is modified, promoted, then flushed). But
either way it will be a single lookup to determine if it's a dup...?
sage
>
> -----Original Message-----
> From: ceph-devel-ow...@vger.kernel.org
> [mailto:ceph-devel-ow...@vger.kernel.org] On Behalf Of Sage Weil
> Sent: Tuesday, January 27, 2015 1:21 AM
> To: Wang, Zhiqiang
> Cc: ceph-devel@vger.kernel.org; Gregory Farnum
> Subject: RE: idempotent op (esp delete)
>
> On Mon, 26 Jan 2015, Wang, Zhiqiang wrote:
> > The downside of this approach is that we may need to search the pg_log
> > for a specific object in every write io?
>
> Not quite. IndexedLog maintains a hash_map of all of the request ids in the
> log, so it's just a hash lookup on each IO. (Well, now 2 hash lookups,
> because I put the additional request IDs in a second auxilliary map to handle
> dups properly. I think we can avoid that lookup if we use the request flags
> carefully, though.. the RETRY and REDIRECTED flags I think? Need to check
> carefully.)
>
> > Maybe we can combine this
> > approach and the changes in PR 3447. For the flush case when the
> > object is deleted in the base, we search the pg_log for dup op. This
> > should be rare cases. Otherwise the object exists, we check the reqid
> > list in the object_info_t for dup op.
>
> We could do a hybrid approach, but there is some cost to the per-object
> tracking: a tiny bit more memory, and an O(n) search of the items in that
> list (~10 or 20?) for the dup check. I suspect the hash lookup is cheaper?
> And simpler.
>
> sage
>
>
> >
> > -----Original Message-----
> > From: ceph-devel-ow...@vger.kernel.org
> > [mailto:ceph-devel-ow...@vger.kernel.org] On Behalf Of Wang, Zhiqiang
> > Sent: Monday, January 26, 2015 10:35 AM
> > To: Sage Weil; Gregory Farnum
> > Cc: ceph-devel@vger.kernel.org
> > Subject: RE: idempotent op (esp delete)
> >
> > This method puts the reqid list in the pg_log instead of the object_info_t,
> > so that it's preserved even in the delete case, which sounds more
> > reasonable.
> >
> > -----Original Message-----
> > From: ceph-devel-ow...@vger.kernel.org
> > [mailto:ceph-devel-ow...@vger.kernel.org] On Behalf Of Sage Weil
> > Sent: Saturday, January 24, 2015 6:19 AM
> > To: Gregory Farnum
> > Cc: ceph-devel@vger.kernel.org
> > Subject: Re: idempotent op (esp delete)
> >
> > On Fri, 23 Jan 2015, Gregory Farnum wrote:
> > > On Fri, Jan 23, 2015 at 1:43 PM, Sage Weil <sw...@redhat.com> wrote:
> > > > Background:
> > > >
> > > > 1) Way back when we made a task that would thrash the cache modes
> > > > by adding and removing the cache tier while ceph_test_rados was running.
> > > > This mostly worked, but would occasionally fail because we would
> > > >
> > > > - delete an object from the cache tier
> > > > - a network failure injection would lose the reply
> > > > - we'd disable the cache
> > > > - the delete would resend to the base tier, not get recognized as
> > > > a dup (different pool, different pg log)
> > > > -> -ENOENT instead of 0
> > > >
> > > > 2) The proxy write code hits a similar problem:
> > > >
> > > > - delete gets proxied
> > > > - we initiate async promote
> > > > - a network failure injection loses the delete reply
> > > > - delete resends and blocks on promote (or arrives after it
> > > > finishes)
> > > > - promote finishes
> > > > - delete is handled
> > > > -> -ENOENT instead of 0
> > > >
> > > > The ticket is http://tracker.ceph.com/issues/8935
> > > >
> > > > The problem is partially addressed by
> > > >
> > > > https://github.com/ceph/ceph/pull/3447
> > > >
> > > > by logging a few request ids on every object_info_t and preserving
> > > > that on promote and flush.
> > > >
> > > > However, it doesn't solve the problem for delete because we throw
> > > > out object_info_t so that reqid_t is lost.
> > > >
> > > > I think we have two options, not necessarily mutually exclusive:
> > > >
> > > > 1) When promoting an object that doesn't exist (to create a
> > > > whiteout), pull reqids out of the base tier's pg log so that the
> > > > whiteout is primed with request ids.
> > > >
> > > > 1.5) When flushing... well, that is harder because we have nowhere
> > > > to put the reqids. Unless we make a way to cram a list of reqid's
> > > > into a single PG log entry...? In that case, we wouldn't strictly
> > > > need the per-object list since we could pile the base tier's
> > > > reqids into the promote log entry in the cache tier.
> > > >
> > > > 2) Make delete idempotent (0 instead of ENOENT if the object
> > > > doesn't exist). This will require a delicate compat transition
> > > > (let's ignore that a moment) but you can preserve the old behavior
> > > > for callers that care by preceding the delete with an assert_exists op.
> > > > Most callers don't care, but a handful do. This simplifies the
> > > > semantics we need to support going forward.
> > > >
> > > > Of course, it's all a bit delicate. The idempotent op semantics
> > > > have a time horizon so it's all a bit wishy-washy... :/
> > > >
> > > > Thoughts?
> > >
> > > Do we have other cases that we're worried about which would be
> > > improved by maintaining reqids across pool cache transitions? I'm
> > > not a big fan of maintaining those per-op lists (they sound really
> > > expensive?), but if we need them for something else that's a point
> > > in their favor.
> >
> > I don't think they're *too* expensive (say, vector of 20 per
> > object_info_t?). But the only thing I can think of beyond the cache
> > tiering stuff would be cases where the pg log isnt long enough for a very
> > laggy client. In general ops will be distributed across ops so it will be
> > catch the dup from another angle.
> >
> > However.. I just hacked up a patch that lets us cram lots of reqids into a
> > single pg_log_entry_t and I think that may be a simpler solution. We can
> > cram all reqids (for the last N of them) for promote and flush into the
> > single log entry and the delete is no longer special.. it'd work equally
> > well for other dups and for class methods that do who knows what. The
> > patch is here:
> >
> > https://github.com/liewegas/ceph/commit/wip-pg-reqids
> >
> > What do you think?
> >
> > > We could make delete idempotent instead and that's what I initially
> > > favor, but it also seems a bit scary (it's not like our operations
> > > can be made idempotent; lots of them invoke classes that will differ
> > > or
> > > whatever!) and I can't think of which callers might care so I'm
> > > having trouble formulating the bounds of this solution.
> >
> > Yeah, it seems like an easier endpoint but a dangerous path to get there...
> >
> > sage
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