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https://issues.apache.org/jira/browse/HDFS-8791?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14637440#comment-14637440
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Colin Patrick McCabe commented on HDFS-8791:
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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 version...
> 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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