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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-7720) Add a more consistent snapshot mechanism
Date Fri, 08 Aug 2014 03:34:11 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7720?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14090259#comment-14090259
] 

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.



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