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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3672) Expose disk-location information for blocks to enable better scheduling
Date Fri, 20 Jul 2012 21:55:34 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3672?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13419604#comment-13419604

Todd Lipcon commented on HDFS-3672:

A few comments on the initial patch:

- I definitely think we need to separate the API for getting disk locations so that you can
pass a list of LocatedBlocks. For some of the above-mentioned use cases (eg MR scheduler),
you need to get the locations for many files, and you don't want to have to do a fan-out round
for each of the files separately.
- Per above, I agree that we should make the disk IDs opaque. But a single byte seems short-sighted.
Let's expose them as an interface "DiskId" which can be entirely devoid of getters for now
-- its only contract would be that it properly implements comparison, equals, and hashcode,
so users can use them to aggregate stats by disk, etc. Internally we can implement it with
a wrapper around a byte[].
- In the protobuf response, given the above, I think we should do something like:
message Response {
  repeated bytes diskIds;
  repeated uint32 diskIndexes; // for each block, pointers into above diskId array, or MAX_INT
to indicate blocks not found
- Per above, need to figure out what you're doing for blocks that aren't found on a given
DN. We also need to specify in the JavaDoc what happens in the response for DNs which don't
respond. I think it's OK that the result would have some "unknown" - it's likely if any of
the DNs are down.
- Doing the fan-out RPC does seem important. Unfortunately it might be tricky, so I agree
we should do it in a separate follow-up optimization.
> Expose disk-location information for blocks to enable better scheduling
> -----------------------------------------------------------------------
>                 Key: HDFS-3672
>                 URL: https://issues.apache.org/jira/browse/HDFS-3672
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>    Affects Versions: 2.0.0-alpha
>            Reporter: Andrew Wang
>            Assignee: Andrew Wang
>         Attachments: hdfs-3672-1.patch
> Currently, HDFS exposes on which datanodes a block resides, which allows clients to make
scheduling decisions for locality and load balancing. Extending this to also expose on which
disk on a datanode a block resides would enable even better scheduling, on a per-disk rather
than coarse per-datanode basis.
> This API would likely look similar to Filesystem#getFileBlockLocations, but also involve
a series of RPCs to the responsible datanodes to determine disk ids.

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