What about the commitlogs? Are you saving those off anywhere in between the 
snapshot and the crash?


Sean Durity

From: Anuj Wadehra [mailto:anujw_2...@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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