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From "John Carrino (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6764) Using Batch commitlog_sync is slow and doesn't actually batch writes
Date Wed, 26 Feb 2014 21:44:19 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6764?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13913547#comment-13913547
] 

John Carrino commented on CASSANDRA-6764:
-----------------------------------------

If you have batch turned on the assumption is that the writes are durable
if you respond successfully. Anything less is a violation of the trust
clients have in the data store. At that point the system has become a
logging framework and that's not what people want or expect. If people are
running with quorum reads and writes and batch enabled there really should
be an fsync protecting the writes. People expect reads following write
success to return their data.  If this is not the case I must know because
I'm kinda betting my customer's data on this being the case.



> Using Batch commitlog_sync is slow and doesn't actually batch writes
> --------------------------------------------------------------------
>
>                 Key: CASSANDRA-6764
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6764
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: John Carrino
>             Fix For: 1.2.16
>
>         Attachments: cassandra_6764_v2.patch, cassandra_6764_v3.patch
>
>
> The assumption behind batch commit mode is that the client does it's own batching and
wants to wait until the write is durable before returning.  The problem is that the queue
that cassandra uses under the covers only allows for a single ROW (RowMutation) per thread
(concurrent_writes).  This means that commitlog_sync_batch_window_in_ms should really be called
sleep_between each_concurrent_writes_rows_in_ms.
> I assume the reason this slipped by for so long is that no one uses batch mode, probably
because people say "it's slow".  We need durability so this isn't an option.
> However it doesn't need to be this slow.
> Also, if you write a row that is larger than the commit log size it silently (warn) fails
to put it in the commit log.  This is not ideal for batch mode.



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