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https://issues.apache.org/jira/browse/HBASE-8778?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13714646#comment-13714646
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Lars Hofhansl commented on HBASE-8778:
--------------------------------------
Looking at the code. If the modtime of the tabledescriptor has changed after
the cached version, we do what Dave described twice! First getTableInfoModtime
is called, if that determines that the cache was changed,
getTableDescriptorModtime is called, which does the same work of stat'ing the
dir all over again.
I can think of a few ways to make this better:
# When the table descriptor is cached, check the mod time of the table
directory first; if that mod time is <= the cached descriptor's mod time we're
good and do not need to stat the table directory. In a high churn table that
might not help much, though, as new region dirs are constantly added.
# Record the sequence number of a table descriptor when cached. Instead of
checking mod time, we can check whether next highest sequence number exists. If
so, we need to reload (but no need to check the mod time by stat'ing the dir).
Can there be gaps in the sequence numbers?
> 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
> Assignee: Dave Latham
> Fix For: 0.98.0, 0.95.2, 0.94.11
>
> 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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