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From "Zhihong Ted Yu (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-6351) IO impact reduction for compaction
Date Sun, 08 Jul 2012 15:20:34 GMT

    [ https://issues.apache.org/jira/browse/HBASE-6351?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13408941#comment-13408941
] 

Zhihong Ted Yu commented on HBASE-6351:
---------------------------------------

The FSDirectory above is specific to Lucene. It employs RateLimiter to to rate limit IO.
RateLimiter.pause() calculates how much to sleep based on instantaneous rate. See javadoc
below:
{code}
    // TODO: this is purely instantaneous rate; maybe we
    // should also offer decayed recent history one?
{code}
                
> IO impact reduction for compaction
> ----------------------------------
>
>                 Key: HBASE-6351
>                 URL: https://issues.apache.org/jira/browse/HBASE-6351
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Zhihong Ted Yu
>
> The following came from Otis:
> Lucene 4.0.0-Alpha was recently released.  Mike McCandless, sne of the Lucene developers,
wrote a really nice post about new things in this version of Lucene.  The part that I think
is interesting for HBase, and that HBase devs may want to look at (and borrow to use with
compactions) is this:
> Reducing merge IO impact 
> Merging (consolidating many small segments into a single big one) is a very IO and CPU
intensive operation which can easily interfere with ongoing searches. In 4.0.0 we now have
two ways to reduct this impact:
>         * Rate-limit the IO caused by ongoing merging, by calling FSDirectory.setMaxMergeWriteMBPerSec.

>         * Use the new NativeUnixDirectory which bypasses the OS's IO cache for all merge
IO, by using direct IO. This ensures that a merge won't evict hot pages used by searches.
(Note that there is also a native WindowsDirectory, but it does not yet use direct IO during
merging... patches welcome!). 
> Remember to also set swappiness to 0 on Linux if you want to maximize search responsiveness.

> More generally, the APIs that open an input or output file (Directory.openInput andDirectory.createOutput)
now take an IOContext describing what's being done (e.g., flush vs merge), so you can create
a custom Directory that changes its behavior depending on the context. 

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