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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8564) BlockPoolSlice.checkDirs() will trigger excessive IO while traversing all sub-directories under finalizedDir
Date Thu, 11 Jun 2015 21:06:03 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8564?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14582517#comment-14582517

Colin Patrick McCabe commented on HDFS-8564:

Another possibility would be to rate-limit these scans so that we never do more than 1 per
half hour (or whatever).  It seems like if we hit a lot of I/O errors on a failing drive (a
common scenario for a failing drive), we'll be constantly doing these scans and discovering
nothing interesting.  I also don't understand why the scan happens on all drives rather than
just the failing one-- does anyone have some background on that?

> BlockPoolSlice.checkDirs() will trigger excessive IO while traversing all sub-directories
under finalizedDir
> ------------------------------------------------------------------------------------------------------------
>                 Key: HDFS-8564
>                 URL: https://issues.apache.org/jira/browse/HDFS-8564
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: datanode, HDFS
>    Affects Versions: 3.0.0
>            Reporter: Esteban Gutierrez
>            Assignee: Esteban Gutierrez
>            Priority: Critical
> DataNodes continuously call checkDiskErrorAsync() for multiple operations in the DN in
order to verify if a volume hasn't experienced any failure. When DN.startCheckDiskErrorThread()
is invoked we need to traverse all configured data volumes on a DN to see which volumes need
to be removed (see FsVolumeList.checkDir(s)) however that means that for each directory on
BlockPoolSlice  we need to call DiskChecker.checkDirs() which will recursively will look into
the rbw, tmp and finalized directories:
> {code}
> void checkDirs() throws DiskErrorException {
>     DiskChecker.checkDirs(finalizedDir);
>     DiskChecker.checkDir(tmpDir);
>     DiskChecker.checkDir(rbwDir);
>   }
> {code}
> Unfortunately after HDFS-6482, the subdirectory structure is created with the following
> {code}
> public static File idToBlockDir(File root, long blockId) {
>     int d1 = (int)((blockId >> 16) & 0xff);
>     int d2 = (int)((blockId >> 8) & 0xff);
>     String path = DataStorage.BLOCK_SUBDIR_PREFIX + d1 + SEP +
>         DataStorage.BLOCK_SUBDIR_PREFIX + d2;
>     return new File(root, path);
>   }
> {code}
> Which leaves each data volume with  64K directories (256 directories x 256 subdirectories)
A side effect of this is that if the dentries haven't been cached by the OS, then the DN needs
to recursively scan up to 64k directories x the number of configured data volumes (x number
of files) impacting IO for other operations while DiskChecker.checkDirs(finalizedDir) is running.
> There are few possibilities to address this problem:
> 1. Do not scan at all finalizedDir
> 2. Limit to one level the number of sub directories to scan recursively. (256)
> 3. Remove a subdirectory immediately it doesn't have any block under it. 

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