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https://issues.apache.org/jira/browse/HDFS-8791?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15014082#comment-15014082
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Chris Trezzo commented on HDFS-8791:
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Thanks [~cmccabe] for the response. I can add upgrade path unit tests to the
patch.
Also please see the [attached
document|https://issues.apache.org/jira/secure/attachment/12772917/32x32DatanodeLayoutTesting-v1.pdf]
for some performance numbers around data node startup time. I performed 4
tests with setups varying the number of namespaces and block density. Hopefully
these tests give a feeling for the startup performance improvement with the
32x32 layout in the worst case scenario where the dentries for the directory
structure have fallen out of the cache. This could happen, for example, after a
hard reboot of the data node (i.e. rolling restart due to OS upgrade). During
the tests I also used iostat to verify that there was indeed a large amount of
disk I/O (reads) happening during the long startup times with the 256x256
layout. This is presumably the persisted information needed for all of the
dentries.
I have also set up a test to verify the scenario where scans over the finalized
directory structure (i.e. du, find or other commands) could impact the IO
performance of user level containers running on the machine. I do not see a
meaningful performance impact to user level jobs on our test clusters with the
32x32 layout. I am continuing to monitor this closely as we roll this out to
more and more clusters.
Let me know if there is any other specific performance data that you would like
to see.
> block ID-based DN storage layout can be very slow for datanode on ext4
> ----------------------------------------------------------------------
>
> Key: HDFS-8791
> URL: https://issues.apache.org/jira/browse/HDFS-8791
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: datanode
> Affects Versions: 2.6.0, 2.8.0, 2.7.1
> Reporter: Nathan Roberts
> Assignee: Chris Trezzo
> Priority: Critical
> Attachments: 32x32DatanodeLayoutTesting-v1.pdf,
> HDFS-8791-trunk-v1.patch
>
>
> We are seeing cases where the new directory layout causes the datanode to
> basically cause the disks to seek for 10s of minutes. This can be when the
> datanode is running du, and it can also be when it is performing a
> checkDirs(). Both of these operations currently scan all directories in the
> block pool and that's very expensive in the new layout.
> The new layout creates 256 subdirs, each with 256 subdirs. Essentially 64K
> leaf directories where block files are placed.
> So, what we have on disk is:
> - 256 inodes for the first level directories
> - 256 directory blocks for the first level directories
> - 256*256 inodes for the second level directories
> - 256*256 directory blocks for the second level directories
> - Then the inodes and blocks to store the the HDFS blocks themselves.
> The main problem is the 256*256 directory blocks.
> inodes and dentries will be cached by linux and one can configure how likely
> the system is to prune those entries (vfs_cache_pressure). However, ext4
> relies on the buffer cache to cache the directory blocks and I'm not aware of
> any way to tell linux to favor buffer cache pages (even if it did I'm not
> sure I would want it to in general).
> Also, ext4 tries hard to spread directories evenly across the entire volume,
> this basically means the 64K directory blocks are probably randomly spread
> across the entire disk. A du type scan will look at directories one at a
> time, so the ioscheduler can't optimize the corresponding seeks, meaning the
> seeks will be random and far.
> In a system I was using to diagnose this, I had 60K blocks. A DU when things
> are hot is less than 1 second. When things are cold, about 20 minutes.
> How do things get cold?
> - A large set of tasks run on the node. This pushes almost all of the buffer
> cache out, causing the next DU to hit this situation. We are seeing cases
> where a large job can cause a seek storm across the entire cluster.
> Why didn't the previous layout see this?
> - It might have but it wasn't nearly as pronounced. The previous layout would
> be a few hundred directory blocks. Even when completely cold, these would
> only take a few a hundred seeks which would mean single digit seconds.
> - With only a few hundred directories, the odds of the directory blocks
> getting modified is quite high, this keeps those blocks hot and much less
> likely to be evicted.
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