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Yuki Morishita commented on CASSANDRA-8177:
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Sequential repair first asks every replica to take snapshot, and then ask each
replica to calculate Merkle Tree on snapshot one by one. Parallel repair
There is no difference in I/O operation.
Calculating Merkle Tree ('validation') and streaming SSTables are the two most
time consuming part of repair.
Sequential repair does validation one node at a time, so it takes time compared
to parallel.
Which is 'write' I/O line in the graph? Is it possible that the node is
streaming SSTables each other for sequential repair, and there are much less
out of sync SSTables in parallel repair?
> sequential repair is much more expensive than parallel repair
> -------------------------------------------------------------
>
> Key: CASSANDRA-8177
> URL: https://issues.apache.org/jira/browse/CASSANDRA-8177
> Project: Cassandra
> Issue Type: Bug
> Reporter: Sean Bridges
> Assignee: Yuki Morishita
> Attachments: iostats.png
>
>
> This is with 2.0.10
> The attached graph shows io read/write throughput (as measured with iostat)
> when doing repairs.
> The large hump on the left is a sequential repair of one node. The two much
> smaller peaks on the right are parallel repairs.
> This is a 3 node cluster using vnodes (I know vnodes on small clusters isn't
> recommended). Cassandra reports load of 40 gigs.
> We noticed a similar problem with a larger cluster.
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