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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8791) block ID-based DN storage layout can be very slow for datanode on ext4
Date Wed, 22 Jul 2015 18:58:06 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8791?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14637440#comment-14637440

Colin Patrick McCabe commented on HDFS-8791:

bq. Not a linux filesystem expert, but here's where I think the confusion is...

Thanks for the explanation.  It is great that you used blktrace as well... very good information.

bq. I'm a little confused about iterating to find the meta file. Don't we already keep track
of the genstamp we discovered during startup? If so, it seems like a simple stat is sufficient.

That's a fair point.  There are a lot of cases where we don't scan the directory because we
have cached the genstamp value.  This corresponds to calls to {{FsDatasetUtil#getMetaFile}}.
 However, there are a few other cases like {{DataNode#transferReplicaForPipelineRecovery}}
and {{VolumeScanner#scanBlock}} which do end up calling {{FsDatasetUtil#findMetaFile}}.  If
we moved to really big directories, we might need to somehow avoid all of those cases.

bq. I haven't tried xfs, but that would also be a REALLY heavy hammer in our case

I think most people would consider a layout version upgrade a "heavier hammer" than using
XFS... but maybe I'm wrong :)  I would actually really like to know if this problem affects
XFS too, or if it manages the cache in a different way.  I guess that information might be
tough to get, since you'd have to reformat everything.

If you want to experiment with changing the HDFS sharding, you should be able to just change
{{DatanodeUtil#idToBlockDir}}.  I am curious how well a simple 1-level sharding scheme would
work on ext4.  Of course, you'd also have to come up with an upgrade process to the new layout

> 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
>            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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