The pg_log_t variant does seem to be cleaner. -Sam
On Mon, Jan 26, 2015 at 9:21 AM, Sage Weil <[email protected]> wrote: > 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: [email protected] >> [mailto:[email protected]] On Behalf Of Wang, Zhiqiang >> Sent: Monday, January 26, 2015 10:35 AM >> To: Sage Weil; Gregory Farnum >> Cc: [email protected] >> 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: [email protected] >> [mailto:[email protected]] On Behalf Of Sage Weil >> Sent: Saturday, January 24, 2015 6:19 AM >> To: Gregory Farnum >> Cc: [email protected] >> 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 <[email protected]> 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 >> -- >> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the >> body of a message to [email protected] More majordomo info at >> http://vger.kernel.org/majordomo-info.html >> -- >> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the >> body of a message to [email protected] More majordomo info at >> http://vger.kernel.org/majordomo-info.html >> >> > -- > To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > the body of a message to [email protected] > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
