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? -Sam On Wed, Jan 8, 2020 at 12:47 AM Xuehan Xu <xxhdx1985126(a)gmail.com> wrote:

On Thu, 9 Jan 2020 at 02:45, Matt Benjamin <mbenjami(a)redhat.com> wrote: Thanks for reviewing, Matt, I totally agree:-) > > Matt

Just a note re time sync standards: In my previous job, we had been playing with the TSN standards, trying to achieve synchronization in the 10s of microseconds realm. The specific TSN standard (802.1AS) is a specific PTP profile (more or less 1588 v3, if I remember correctly). To achieve sub-millisecond accuracy one should better have hardware timestamping, but we did achieve very good results with just software times. Ronen On Thu, Jan 9, 2020 at 5:09 PM Brett Niver <bniver(a)redhat.com> wrote:

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. 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. 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. 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:-) Thanks.

On Fri, 10 Jan 2020 at 02:00, Sam Just <sjust(a)redhat.com> wrote: 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. 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. 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. 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:-) Thanks. > -Sam > > > > > Thanks. > > >