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https://issues.apache.org/jira/browse/HBASE-8778?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13692454#comment-13692454
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Dave Latham commented on HBASE-8778:
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{quote}Putting tabledesc in a subdir only solves the problem for region
opening. But other tools, which does an ls on directory will still run into
this. {quote}
It would solve it for any tools loading the table descriptor information. It
would not solve it for any tools which want to scan the table directory, but
not the full directory. However, I'm not aware of any other cases where you
want only some data from the table directory, but don't know it's name. Either
you want a single region, or all the regions in which case even if they are
organized hierarchically you are still reading all of them.
> Region assigments scan table directory making them slow for huge tables
> -----------------------------------------------------------------------
>
> Key: HBASE-8778
> URL: https://issues.apache.org/jira/browse/HBASE-8778
> Project: HBase
> Issue Type: Improvement
> Reporter: Dave Latham
> Attachments: HBASE-8778-0.94.5.patch, HBASE-8778-0.94.5-v2.patch
>
>
> On a table with 130k regions it takes about 3 seconds for a region server to
> open a region once it has been assigned.
> Watching the threads for a region server running 0.94.5 that is opening many
> such regions shows the thread opening the reigon in code like this:
> {noformat}
> "PRI IPC Server handler 4 on 60020" daemon prio=10 tid=0x00002aaac07e9000
> nid=0x6566 runnable [0x000000004c46d000]
> java.lang.Thread.State: RUNNABLE
> at java.lang.String.indexOf(String.java:1521)
> at java.net.URI$Parser.scan(URI.java:2912)
> at java.net.URI$Parser.parse(URI.java:3004)
> at java.net.URI.<init>(URI.java:736)
> at org.apache.hadoop.fs.Path.initialize(Path.java:145)
> at org.apache.hadoop.fs.Path.<init>(Path.java:126)
> at org.apache.hadoop.fs.Path.<init>(Path.java:50)
> at
> org.apache.hadoop.hdfs.protocol.HdfsFileStatus.getFullPath(HdfsFileStatus.java:215)
> at
> org.apache.hadoop.hdfs.DistributedFileSystem.makeQualified(DistributedFileSystem.java:252)
> at
> org.apache.hadoop.hdfs.DistributedFileSystem.listStatus(DistributedFileSystem.java:311)
> at
> org.apache.hadoop.fs.FilterFileSystem.listStatus(FilterFileSystem.java:159)
> at org.apache.hadoop.fs.FileSystem.listStatus(FileSystem.java:842)
> at org.apache.hadoop.fs.FileSystem.listStatus(FileSystem.java:867)
> at org.apache.hadoop.hbase.util.FSUtils.listStatus(FSUtils.java:1168)
> at
> org.apache.hadoop.hbase.util.FSTableDescriptors.getTableInfoPath(FSTableDescriptors.java:269)
> at
> org.apache.hadoop.hbase.util.FSTableDescriptors.getTableInfoPath(FSTableDescriptors.java:255)
> at
> org.apache.hadoop.hbase.util.FSTableDescriptors.getTableInfoModtime(FSTableDescriptors.java:368)
> at
> org.apache.hadoop.hbase.util.FSTableDescriptors.get(FSTableDescriptors.java:155)
> at
> org.apache.hadoop.hbase.util.FSTableDescriptors.get(FSTableDescriptors.java:126)
> at
> org.apache.hadoop.hbase.regionserver.HRegionServer.openRegion(HRegionServer.java:2834)
> at
> org.apache.hadoop.hbase.regionserver.HRegionServer.openRegion(HRegionServer.java:2807)
> at sun.reflect.GeneratedMethodAccessor64.invoke(Unknown Source)
> at
> sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
> at java.lang.reflect.Method.invoke(Method.java:597)
> at
> org.apache.hadoop.hbase.ipc.WritableRpcEngine$Server.call(WritableRpcEngine.java:320)
> at
> org.apache.hadoop.hbase.ipc.HBaseServer$Handler.run(HBaseServer.java:1426)
> {noformat}
> To open the region, the region server first loads the latest
> HTableDescriptor. Since HBASE-4553 HTableDescriptor's are stored in the file
> system at "/hbase/<tableDir>/.tableinfo.<sequenceNum>". The file with the
> largest sequenceNum is the current descriptor. This is done so that the
> current descirptor is updated atomically. However, since the filename is not
> known in advance FSTableDescriptors it has to do a FileSystem.listStatus
> operation which has to list all files in the directory to find it. The
> directory also contains all the region directories, so in our case it has to
> load 130k FileStatus objects. Even using a globStatus matching function
> still transfers all the objects to the client before performing the pattern
> matching. Furthermore HDFS uses a default of transferring 1000 directory
> entries in each RPC call, so it requires 130 roundtrips to the namenode to
> fetch all the directory entries.
> Consequently, to reassign all the regions of a table (or a constant fraction
> thereof) requires time proportional to the square of the number of regions.
> In our case, if a region server fails with 200 such regions, it takes 10+
> minutes for them all to be reassigned, after the zk expiration and log
> splitting.
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