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From "Hadoop QA (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3510) Fix FSEditLog pre-allocation
Date Tue, 12 Jun 2012 17:06:43 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3510?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13293774#comment-13293774
] 

Hadoop QA commented on HDFS-3510:
---------------------------------

-1 overall.  Here are the results of testing the latest attachment 
  http://issues.apache.org/jira/secure/attachment/12531849/HDFS-3510.004.patch
  against trunk revision .

    +1 @author.  The patch does not contain any @author tags.

    +1 tests included.  The patch appears to include 1 new or modified test files.

    -1 patch.  The patch command could not apply the patch.

Console output: https://builds.apache.org/job/PreCommit-HDFS-Build/2647//console

This message is automatically generated.
                
> Fix FSEditLog pre-allocation
> ----------------------------
>
>                 Key: HDFS-3510
>                 URL: https://issues.apache.org/jira/browse/HDFS-3510
>             Project: Hadoop HDFS
>          Issue Type: Bug
>    Affects Versions: 1.0.0, 2.0.0-alpha
>            Reporter: Colin Patrick McCabe
>            Assignee: Colin Patrick McCabe
>             Fix For: 1.0.0, 2.0.1-alpha
>
>         Attachments: HDFS-3510-b1.001.patch, HDFS-3510-b1.002.patch, HDFS-3510.001.patch,
HDFS-3510.003.patch, HDFS-3510.004.patch
>
>
> In the FSEditLog, we want to avoid running out of space in the middle of writing an edit
log operation to the disk. We do this by a process called "preallocation"-- reserving space
on the disk for the upcoming edit log entries before beginning to write them.
> The idea is that if we're going to encounter an out-of-disk-space condition, we don't
want it to happen in the middle of writing valid data.  Instead, we want it to happen in the
middle of writing padding bytes.  The edit log uses bytes with the value 0xff (in decimal,
-1) as padding.  These bytes correspond to FSEditLogOp.OP_INVALID.
> The current preallocation strategy is flawed.  Although we preallocate a very large chunk
at a time-- 1 megabyte, in fact-- we only do this preallocation when we are more than 4096
bytes away from the end of the file.  This means that the effective preallocation length is
only 4096 bytes.  A batch of edit log entries could easily be more than this.  There is evidence
that this has caused problems in the field for end-users.
> Here is a visual illustration of the old preallocation strategy:
> {code}
> first write
> |
> V <----- 1 MB ----->
> +--+---------------+
> |__|FFFFFFFFFFFFFFF|
> +--+---------------+
>     second write
>     |
>     V
> +--+------+--------+
> |__|______|FFFFFFFF|
> +--+------+--------+
>            third write
>            |
>            V
> +--+------+------+-+
> |__|______|______|_|
> +--+------+------+-+
>                   fourth write
>                   | (NOT preallocated)
>                   V
> +--+------+------+-+
> |__|______|______|________
> +--+------+------+-+
>                           fifth write
>                           |
>                           V<--- 1 MB -->
> +--+------+------+--------+---+--------+
> |__|______|______|________|___|FFFFFFFF|
> +--+------+------+--------+---+--------+
> {code}
> And here is the new preallocation strategy:
> {code}
> first write
> |
> V
> +--+
> |__|
> +--+
>     second write
>     |
>     V
> +--+------+
> |__|______|
> +--+------+
>            third write
>            |
>            V
> +--+------+------+
> |__|______|______|
> +--+------+------+
>                   fourth write
>                   |
>                   V
> +--+------+------+--------+
> |__|______|______|________|
> +--+------+------+--------+
>                            fifth write
>                            |
>                            V
> +--+------+------+--------+---+
> |__|______|______|________|___|
> +--+------+------+--------+---+
> {code}

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