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https://issues.apache.org/jira/browse/HDFS-8791?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14631703#comment-14631703
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Colin Patrick McCabe commented on HDFS-8791:
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We could certainly make the sharding configurable. Newer filesystems such as
ext4 can handle quite a large number of files per directory efficiently, so we
could bump up that number. People using ext2 or ext3 would probably want to
stick with the current configuration, though. This would also be a somewhat
complex change since we'd have to put the sharding into the VERSION file so
that the DN could figure out where to look for blocks. Also, this would
obviously be a DN layout version change.
In the long term, I think doing a full du is going to get more and more
expensive over time as our data volumes continue to increase and the amount of
cold data increases. We shouldn't be assuming this is an operation we can do
synchronously or quickly. We should be rate-limiting our calls to checkDirs so
that we don't swamp the actual workload on the cluster. Currently, we also
kick off a du on all hard drives if we spot an error on just one hard drive--
this is a behavior we should fix, because it doesn't really make sense. I
think fixing the directory scanner to be rate-limited and to do less
unnecessary work would be an easier fix than another layout version change.
> 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: 2.6.1
> Reporter: Nathan Roberts
> Priority: Critical
>
> 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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