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https://issues.apache.org/jira/browse/HDFS-2379?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13117073#comment-13117073
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Todd Lipcon commented on HDFS-2379:
-----------------------------------
We have some customers who have lots of small blocks (unfortunately they don't
make good use of HAR). So, a single drive may have 400k+ blocks. When there's a
lot of page cache pressure and the dentry/inode caches get pushed out, we're
seeing it take several minutes per drive to do the scan. I've been
experimenting with tuning /proc/sys/vm/vfs_cache_pressure which seems to help
some, but even still it's taking many seconds when under lots of load. (eg in
the middle of a terasort)
It was a little tricky to get right, but this patch includes a "sanity check"
mode which I used to catch several bugs. I think given that, today, we don't
even properly synchronize it, the chance that this introduces more bugs is low.
Still, I'm running some continuous cluster tests with this patch -- HBase write
workloads with block report interval 90s. This shuffles through a lot of blocks
quickly and helped me find some issues while working on the patch.
> 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
> Attachments: hdfs-2379.txt, hdfs-2379.txt
>
>
> 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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