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From <SEAN_R_DUR...@homedepot.com>
Subject RE: Inconsistent Reads after Restoring Snapshot
Date Thu, 28 Apr 2016 15:54:15 GMT
https://docs.datastax.com/en/cassandra/2.0/cassandra/configuration/configLogArchive_t.html

Sean Durity

From: Anuj Wadehra [mailto:anujw_2003@yahoo.co.in]
Sent: Wednesday, April 27, 2016 10:44 PM
To: user@cassandra.apache.org
Subject: RE: Inconsistent Reads after Restoring Snapshot

No.We are not saving them.I have never read that in DataStax documentation.

Thanks
Anuj
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On Thu, 28 Apr, 2016 at 12:45 AM, SEAN_R_DURITY@homedepot.com<mailto:SEAN_R_DURITY@homedepot.com>
<SEAN_R_DURITY@homedepot.com<mailto:SEAN_R_DURITY@homedepot.com>> wrote:
What about the commitlogs? Are you saving those off anywhere in between the snapshot and the
crash?


Sean Durity

From: Anuj Wadehra [mailto:anujw_2003@yahoo.co.in]
Sent: Monday, April 25, 2016 10:26 PM
To: User
Subject: Inconsistent Reads after Restoring Snapshot

Hi,

We have 2.0.14. We use RF=3 and read/write at Quorum. Moreover, we dont use incremental backups.
As per the documentation at https://docs.datastax.com/en/cassandra/2.0/cassandra/operations/ops_backup_snapshot_restore_t.html
, if i need to restore a Snapshot on SINGLE node in a cluster, I would run repair at the end.
But while the repair is going on, reads may get inconsistent.


Consider following scenario:
10 AM Daily Snapshot taken of node A and moved to backup location
11 AM A record is inserted such that node A and B insert the record but there is a mutation
drop on node C.
1 PM Node A crashes and data is restored from latest 10 AM snapshot. Now, only Node B has
the record.

Now, my question is:

Till the repair is completed on node A,a read at Quorum may return inconsistent result based
on the nodes from which data is read.If data is read from node A and node C, nothing is returned
and if data is read from node A and node B, record is returned. This is a vital point which
is not highlighted anywhere.


Please confirm my understanding.If my understanding is right, how to make sure that my reads
are not inconsistent while a node is being repair after restoring a snapshot.

I think, autobootstrapping the node without joining the ring till the repair is completed,
is an alternative option. But snapshots save lot of streaming as compared to bootstrap.

Will incremental backups guarantee that

Thanks
Anuj


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