On Fri, Jan 23, 2015 at 2:18 PM, Sage Weil <sweil@xxxxxxxxxx> wrote: > On Fri, 23 Jan 2015, Gregory Farnum wrote: >> On Fri, Jan 23, 2015 at 1:43 PM, Sage Weil <sweil@xxxxxxxxxx> 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? Maybe? I'm not super-familiar with the pg log code and the bit where we actually fill in extra_reqids that I'm concerned about...*refreshes page* wait, you just did that. Yeah, that looks okay to me. Sam? -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
