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From "Richard Low (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-9753) LOCAL_QUORUM reads can block cross-DC if there is a digest mismatch
Date Mon, 27 Jul 2015 15:01:06 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-9753?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14642825#comment-14642825
] 

Richard Low commented on CASSANDRA-9753:
----------------------------------------

I agree with Sankalp. This is with read_repair_chance = 0.

> LOCAL_QUORUM reads can block cross-DC if there is a digest mismatch
> -------------------------------------------------------------------
>
>                 Key: CASSANDRA-9753
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-9753
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>            Reporter: Richard Low
>
> When there is a digest mismatch during the initial read, a data read request is sent
to all replicas involved in the initial read. This can be more than the initial blockFor if
read repair was done and if speculative retry kicked in. E.g. for RF 3 in two DCs, the number
of reads could be 4: 2 for LOCAL_QUORUM, 1 for read repair and 1 for speculative read if one
replica was slow. If there is then a digest mismatch, Cassandra will issue the data read to
all 4 and set blockFor=4. Now the read query is blocked on cross-DC latency. The digest mismatch
read blockFor should be capped at RF for the local DC when using CL.LOCAL_*.
> You can reproduce this behaviour by creating a keyspace with NetworkTopologyStrategy,
RF 3 per DC, dc_local_read_repair=1.0 and ALWAYS for speculative read. If you force a digest
mismatch (e.g. by deleting a replicas SSTables and restarting) you can see in tracing that
it is blocking for 4 responses.



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