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From "Jeremiah Jordan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-8729) Commitlog causes read before write when overwriting
Date Mon, 20 Apr 2015 20:07:02 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-8729?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14503527#comment-14503527
] 

Jeremiah Jordan commented on CASSANDRA-8729:
--------------------------------------------

His fix is actually to make the segment size = commit log size, aka 2 GB, which means you
get one giant segment.  That seems a little extreme, and makes commit log archiving much harder.
 If this really causes such a big performance degradation, can we just turn off segment recycle
in 2.1?  Seems like that isn't too invasive of a change, since we don't always recycle anyway?

> Commitlog causes read before write when overwriting
> ---------------------------------------------------
>
>                 Key: CASSANDRA-8729
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8729
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Ariel Weisberg
>              Labels: commitlog
>             Fix For: 3.0
>
>
> The memory mapped commit log implementation writes directly to the page cache. If a page
is not in the cache the kernel will read it in even though we are going to overwrite.
> The way to avoid this is to write to private memory, and then pad the write with 0s at
the end so it is page (4k) aligned before writing to a file.
> The commit log would benefit from being refactored into something that looks more like
a pipeline with incoming requests receiving private memory to write in, completed buffers
being submitted to a  parallelized compression/checksum step, followed by submission to another
thread for writing to a file that preserves the order.



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