On Fri, Jan 10, 2020 at 2:43 AM Xuehan Xu <xxhdx1985126@xxxxxxxxx> wrote: > > On Fri, 10 Jan 2020 at 02:00, Sam Just <sjust@xxxxxxxxxx> wrote: > > > > On Wed, Jan 8, 2020 at 7:35 PM Xuehan Xu <xxhdx1985126@xxxxxxxxx> wrote: > > > > > > > I added a few comments, my high level perspective is that it looks > > > > like an approach for dealing with multiversioned extents which might > > > > be a component of rados pool level point-in-time globally consistent > > > > snapshots for purposes like rados pool level cross-cluster > > > > replication. However, that sort of thing would require a great deal > > > > of higher level support, so I'd consider the disk layout portion to be > > > > out of scope for now. Is there another use case you are hoping to > > > > address with this? > > > > > > Hi, sam. Thanks for reviewing the doc:-) > > > > > > The main focus of this initiative is about doing efficient > > > replication/backup. Specifically, we intended to provide higher level > > > modules, especially to rbd and cephfs, the ability to do very high > > > rate snapshots (like one snapshot every 5 seconds or even multiple > > > snapshots within a second) and efficient snapshot diff and > > > export-diff. > > > > I didn't mention it explicitly, but the refcounts in the seastore doc > > lba tree are intended to permit extent sharing to support the existing > > snapshot machinery via clone(). > > > > > > > > We thought, with this ability, upper level applications can achieve > > > near real-time replication that can be compared to the common op-by-op > > > replication, but with less overhead. Because it doesn't involve any > > > extra replication-dedicated journal operations. And as multiple write > > > operations' targeting extents may overlap with each other, even the > > > op-by-op replication can also avoid extra journal operations, they > > > inevitably replicate overlapped extents multiple times, while, in > > > snapshot diff export, only the latest version of the overlapped > > > extents need to be replicated. > > > > It's not clear to me how this versioning scheme changes journaling or > > pg logging. For recovery, we already track overlapping extents > > between versions and use cloning appropriately. Can you expand on > > this portion? > > Oh, sorry that I caused this miss understanding. By op-by-op > replication, I meant mechanisms which, to do cross cluster > replication, do an extra journaling operation for every rbd image > write operation, like rbd mirroring. The upper layer applications did > this because RADOS doesn't provide cross cluster replication, which > leaves them no other choice. And even if we implement op-by-op cross > cluster replication within RADOS, there still seems to be some > drawbacks, because multiple write operations' target extents could > overlap with each other and op-by-op replication will replicate each > one of those write ops many of which may already been outdated, while > on the contrary, lightweight snapshots based cross cluster replication > won't have these overheads. The op-by-op scheme has a really specific advantage: interrupting replication scheme at any point results in a point-in-time consistent state which I believe is why rbd uses it for maintaining near-realtime replication. RBD does have a different replication mode based on looking at deltas between snapshots which does simply ship only the new version of the changed extents, right? Something like this might well be a useful primitive for doing cross-cluster rados level replication, but I'd want to start with a sketch of how it fits into a larger replication scheme and work down to a disk format rather than going in the other direction. > > > > > > > > > We thought maybe we can let upper layer applications to choose whether > > > to replicate their data instead of doing the replication forcibly at > > > the whole rados pool scale. > > > > The existing self-managed snapshot scheme already gives rbd image > > granularity snapshots and cephfs recursive, subtree granularity > > snapshots. The difference is that the versioning lives in the > > hobject_t tuple -- each version is a different object with shared > > extents. > > Yes, but, if I'm understanding correctly, in the current snapshot > mechanism, we have to create clone objects, copy attrs and omaps of > the cloned ones, store them on the disk and calculate and record > clone_overlaps for every write when doing writes on a snapshoted > objects. And in the upper layer applications, snapshot-creating > clients need to request a snapshot id from MONs and cooperate with > other clients who are writing to the same set of objects to finish the > snapshot creation. All of these may produce non-negligible impact on > the performance of normal read/write operations when doing high rate > snapshots. The "create clone objects" etc is just osd record keeping. You'd have similar overheads once you attempt to build cross-cluster replication on top of this scheme since I think you need to remember a lot of the same stuff. At the least, all osds need to agree on which versions they are choosing not to gc and need an efficient index for reclaiming obsolete extents. Moreover, needing to keep around prior versions until async gc clears them constrains online segment cleaning and would require additional space to be kept free. You also need to maintain the R tree, which increases metadata overhead in both space and write amp. Recovery would need a way to query and duplicate the versioned state of an object across osds during backfill and log-based recovery just as it currently needs the clone_overlap metadata to preserve sharing. > > On the other hand, HLC gives systems a method to query any system > state in the past by physical clock as long as those states are > remembered and it promises point-in-time consistency within the scope > of the system just as what Lamport clock promises. So if write ops is > tagged with a HLC timestamp and recorded somewhere (in the journal, > for example), they are already snapshots, and all we need to do when > creating a snapshot is just remembering which snapshot is needed and > tell OSDs not to clear that snapshot, right? So, for the upper layer > applications to do snapshots, they just need to record the time at > which they want to do the snapshot and tell OSDs not to clear the > corresponding write ops. There's no need to cooperate with other > clients through some kind of mutual locking or request snapshot ids. > With R-tree, we can a range search to easily read any data at any time > out of the journal or calculate a snapshot diff, so there's no need to > do those objects clone work. So, generally speaking, with HLC and > R-tree, the system don't need to do any normal R/W performance > influencing job when doing snapshot related work, which makes the > snapshots really lightweight. And as a side-effect, since we can read > data out of journal with the help of r-tree, there's no need for OSDs > to flush dirty data blocks to the underlying disk because those data > are already recorded in the journal, which I think could simplify the > design of OSDs. What I'm not really sure of is why we need to track arbitrary prior versions. Normally, schemes like this are handy because if you track N minutes of prior updates, you can consistently respond to arbitrary read queries with time stamps up to N minutes in the past permitting efficient read snapshot isolation across partitions without explicit snapshots. However, if we're going to tell osds up front what snapshots to remember and we aren't serving transactional reads, they need only take note of when they are crossing such a snapshot point and explicitly remember a snapshot of any such objects -- as we already do with the self-managed snapshot mechanism. We could likely do that with the current snapshot machinery if we change the way snapids are generated. > > > > > > > > > Whether this approach can really achieve that goal and whether to do > > > it is to be discussed, as we also realised that it may not be > > > cost-effective with respect to the amount of development work:-) > > > > I guess I'm not sure what this approach gets us that the existing > > cloning scheme does not. The main problem with high snapshot rates > > currently isn't that the ondisk structure doesn't support it, but > > rather that snapshot stamps are mediated through the monitor. There > > are reasons for doing it that way -- an rbd client need only issue a > > single monitor command to get rid of a snapshot and all involved osds > > will remove the now unnecessary clones asynchronously without > > requiring the client to track them down. Similarly, the mds needn't > > find every clone within a subtree -- a potentially expensive > > operation. > > > > I think what I'm missing is how this structure fits into some higher > > level snapshot scheme you are proposing. > > Um...Actually I didn't thought much about how the higher level > snapshot mechanism should be to keep the advantages of both the > current snapshot mechanism and the OSD structure I'm proposing. > Because I thought the higher level snapshot mechanism would be > relatively easy once we have a clear in-OSD snapshot mechanism, which > has obviously been proved wrong now.... > > I think I can take some time to try to figure out one such higher > level snapshot mechanism:-) I think something like this does have the potential to both simplify the existing snapshot machinery within rados and permit interesting cross-cluster replication capabilities. I think the next step would be to sketch out a not detailed view of a scheme up through the osd and clients which can accomplish that and continue discussing from there. -Sam > > Thanks. > > > -Sam > > > > > > > > Thanks. > > > > > > _______________________________________________ Dev mailing list -- dev@xxxxxxx To unsubscribe send an email to dev-leave@xxxxxxx
