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https://issues.apache.org/jira/browse/HDFS-2379?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13116304#comment-13116304
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Todd Lipcon commented on HDFS-2379:
-----------------------------------
As discussed in the above-referenced JIRA, I think we can do something like the
following pseudocode:
{code}
Set<Block> blocksFoundByScan = inconsistentScanVolume(); // ignore any
file-not-founds we get due to concurrent FS modifications
synchronized (volume) {
Set<Block> missingFromScan = Sets.difference(volumeMap.keySet(),
blocksFoundByScan);
Set<Block> missingFromMem = Sets.difference(blocksFoundByScan,
volumeMap.keySet());
for (Block b : missingFromScan) { // block is in memory but not in scan
if (b exists on disk) {
// it got added after we scanned that part of the tree!
add it to block report
}
}
for (Block b : missingFromMem) { // block was on disk but not in memory
if (b no longer exists on disk) {
// remove from block report - it was deleted after we scanned that part
}
}
}
{code}
Anyone see a reason why this wouldn't work? Basically the idea is to do a
"rough sketch" scan first, then anywhere we detect inconsistency, we touch it
up, while holding the lock.
> 0.20: Allow block reports to proceed without holding FSDataset lock
> -------------------------------------------------------------------
>
> Key: HDFS-2379
> URL: https://issues.apache.org/jira/browse/HDFS-2379
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: data-node
> Affects Versions: 0.20.206.0
> Reporter: Todd Lipcon
> Priority: Critical
>
> As disks are getting larger and more plentiful, we're seeing DNs with
> multiple millions of blocks on a single machine. When page cache space is
> tight, block reports can take multiple minutes to generate. Currently, during
> the scanning of the data directories to generate a report, the FSVolumeSet
> lock is held. This causes writes and reads to block, timeout, etc, causing
> big problems especially for clients like HBase.
> This JIRA is to explore some of the ideas originally discussed in HADOOP-4584
> for the 0.20.20x series.
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