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https://issues.apache.org/jira/browse/HDFS-8564?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15184895#comment-15184895
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Wei-Chiu Chuang commented on HDFS-8564:
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[~esteban] there are a few disk checkers in addition to
{{BlockPoolSlice#checkDirs()}}.
VolumeScanner is one (which is disabled by default though), and DU is another
one which runs every 10 minutes by default. Do you think that could be the
potential source of I/O spikes?
> 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
> 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 algorithm:
> {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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