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From "Jakob Homan (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-5795) Add a bulk FIleSystem.getFileBlockLocations
Date Tue, 16 Jun 2009 17:39:07 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-5795?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12720254#action_12720254
] 

Jakob Homan commented on HADOOP-5795:
-------------------------------------

bq. That is more general, but what is the use case?
The original motivation was Arun and Owen noticing during the terasort work that there were
a large number of rpc calls were made during the task scheduling and that a bulk method could
ameliorate that.  That seems reasonable to me.  I'll let Arun lobby further.

One point that came up in discussions is that it would be a good idea to have a maximum number
of files that can be returned at once in order to not overwhelm the namenode.  Whether this
is hard-coded or configurable was not decided.

> 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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