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From "Andrzej Bialecki (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-50) dfs datanode should store blocks in multiple directories
Date Wed, 29 Mar 2006 09:23:19 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-50?page=comments#action_12372204 ] 

Andrzej Bialecki  commented on HADOOP-50:
-----------------------------------------

That would be very useful. I've seen similar solutions in many places (e.g. squid, or Mozilla
cache dir).

Currently, each time a block report is sent we need to list this huge dir. That's still ok,
it's infrequent enough. However, each time we need to access a block, a correct file needs
to be open. Inside the native code JVM uses an open(2) call, which causes the OS to perform
a name-to-inode lookup. Even though OS is caching partial results of this lookup (in Linux
this is known as dcache/dentries), still depending on the size of this LRU cache and the FS
implementation details, doing real lookups for e.g. new blocks or newly requested blocks may
take a long time.

Having said that, I'm not sure what would be the real performance benefit of this change,
perhaps you could come up with a simpler test first...?

> dfs datanode should store blocks in multiple directories
> --------------------------------------------------------
>
>          Key: HADOOP-50
>          URL: http://issues.apache.org/jira/browse/HADOOP-50
>      Project: Hadoop
>         Type: Bug
>   Components: dfs
>     Versions: 0.2
>     Reporter: Doug Cutting
>     Assignee: Mike Cafarella
>      Fix For: 0.2

>
> The datanode currently stores all file blocks in a single directory.  With 32MB blocks
and terabyte filesystems, this will create too many files in a single directory for many filesystems.
 Thus blocks should be stored in multiple directories, perhaps even a shallow hierarchy.

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