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From "Konstantin Shvachko (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-5795) Add a bulk FIleSystem.getFileBlockLocations
Date Tue, 12 May 2009 01:18:45 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-5795?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12708282#action_12708282
] 

Konstantin Shvachko commented on HADOOP-5795:
---------------------------------------------

> (non-recursively?)

I think the rpc call itself should not be recursive. It is like with ls: the getListing()
call is non-recursive, but the client recursively calls getListing() on sub-directories.
The idea is to prevent people from making a mistake to call getBlockLocation("/") on large
directory trees recursively, which may freeze the name-node for a long period of time.
Non-recursive variant should be sufficient to cover Arun's use case.

> Add a bulk FIleSystem.getFileBlockLocations
> -------------------------------------------
>
>                 Key: HADOOP-5795
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5795
>             Project: Hadoop Core
>          Issue Type: New Feature
>          Components: dfs
>    Affects Versions: 0.20.0
>            Reporter: Arun C Murthy
>            Assignee: Jakob Homan
>             Fix For: 0.21.0
>
>
> Currently map-reduce applications (specifically file-based input-formats) use FileSystem.getFileBlockLocations
to compute splits. However they are forced to call it once per file.
> The downsides are multiple:
>    # Even with a few thousand files to process the number of RPCs quickly starts getting
noticeable
>    # The current implementation of getFileBlockLocations is too slow since each call
results in 'search' in the namesystem. Assuming a few thousand input files it results in that
many RPCs and 'searches'.
> It would be nice to have a FileSystem.getFileBlockLocations which can take in a directory,
and return the block-locations for all files in that directory. We could eliminate both the
per-file RPC and also the 'search' by a 'scan'.
> When I tested this for terasort, a moderate job with 8000 input files the runtime halved
from the current 8s to 4s. Clearly this is much more important for latency-sensitive applications...

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