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Jonathan Ellis commented on CASSANDRA-7720: ------------------------------------------- But even with your proposal, if the A write is delayed for any other reason you still get a snapshot with B but no A. (In the scenario where snapshot "now" is later than both A and B.) That is what I mean by not really changing the scenarios you have to tolerate. > Add a more consistent snapshot mechanism > ---------------------------------------- > > Key: CASSANDRA-7720 > URL: https://issues.apache.org/jira/browse/CASSANDRA-7720 > Project: Cassandra > Issue Type: Improvement > Components: Core > Reporter: Mike Schrag > > We’ve hit an interesting issue with snapshotting, which makes sense in > hindsight, but presents an interesting challenge for consistent restores: > * initiate snapshot > * snapshotting flushes table A and takes the snapshot > * insert into table A > * insert into table B > * snapshotting flushes table B and takes the snapshot > * snapshot finishes > So what happens here is that we end up having a B, but NOT having an A, even > though B was chronologically inserted after A. > It makes sense when I think about what snapshot is doing, but I wonder if > snapshots actually should get a little fancier to behave a little more like > what I think most people would expect. What I think should happen is > something along the lines of the following: > For each node: > * pass a client timestamp in the snapshot call corresponding to "now" > * snapshot the tables using the existing procedure > * walk backwards through the linked snapshot sstables in that snapshot > * if the earliest update in that sstable is after the client's timestamp, > delete the sstable in the snapshot > * if the earliest update in the sstable is before the client's timestamp, > then look at the last update. Walk backwards through that sstable. > * if any updates fall after the timestamp, make a copy of that sstable in > the snapshot folder only up to the point of the timestamp and then delete the > original sstable in the snapshot (we need to copy because we're likely > holding a shared hard linked sstable) > I think this would guarantee that you have a chronologically consistent view > of your snapshot across all machines and columnfamilies within a given > snapshot. -- This message was sent by Atlassian JIRA (v6.2#6252)