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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6764) Using Batch commitlog_sync is slow and doesn't actually batch writes
Date Fri, 28 Mar 2014 22:38:16 GMT


Benedict commented on CASSANDRA-6764:

bq. That was definitely the intention, and that's what it did unless the math was off.

Right, agreed after re-reading, but the name testExceedSegmentSize... understandably threw
me :)

bq. How does this differ batch vs periodic?

I thought we had agreed to only change batch behaviour here? I must admit I didn't check the
code, I was going by the conversation.

> Using Batch commitlog_sync is slow and doesn't actually batch writes
> --------------------------------------------------------------------
>                 Key: CASSANDRA-6764
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: John Carrino
>            Assignee: John Carrino
>             Fix For: 2.1 beta2
>         Attachments: 6764.fix.txt, 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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