Grokbase Groups HBase dev June 2010
FAQ
Compaction requests should be prioritized to prevent blocking
-------------------------------------------------------------

Key: HBASE-2646
URL: https://issues.apache.org/jira/browse/HBASE-2646
Project: HBase
Issue Type: Improvement
Components: regionserver
Affects Versions: 0.20.4
Environment: ubuntu server 10; hbase 0.20.4; 4 machine cluster (each machine is an 8 core xeon with 16 GB of ram and 6TB of storage); ~250 Million rows;
Reporter: Jeff Whiting


While testing the write capacity of a 4 machine hbase cluster we were getting long and frequent client pauses as we attempted to load the data. Looking into the problem we'd get a relatively large compaction queue and when a region hit the "hbase.hstore.blockingStoreFiles" limit it would get block the client and the compaction request would get put on the back of the queue waiting for many other less important compactions. The client is basically stuck at that point until a compaction is done. Prioritizing the compaction requests and allowing the request that is blocking other actions go first would help solve the problem.

You can see the problem by looking at our log files:

You'll first see an event such as a too many HLog which will put a lot of requests on the compaction queue.
{noformat}
2010-05-25 10:53:26,570 INFO org.apache.hadoop.hbase.regionserver.HLog: Too many hlogs: logs=33, maxlogs=32; forcing flush of 22 regions(s): responseCounts,RS_6eZzLtdwhGiTwHy,1274232223324, responses,RS_0qhkL5rUmPCbx3K-1274213057242,1274513189592, responses,RS_1ANYnTegjzVIsHW-12742177419
21,1274511001873, responses,RS_1HQ4UG5BdOlAyuE-1274216757425,1274726323747, responses,RS_1Y7SbqSTsZrYe7a-1274328697838,1274478031930, responses,RS_1ZH5TB5OdW4BVLm-1274216239894,1274538267659, responses,RS_3BHc4KyoM3q72Yc-1274290546987,1274502062319, responses,RS_3ra9BaBMAXFAvbK-127421457
9958,1274381552543, responses,RS_6SDrGNuyyLd3oR6-1274219941155,1274385453586, responses,RS_8AGCEMWbI6mZuoQ-1274306857429,1274319602718, responses,RS_8C8T9DN47uwTG1S-1274215381765,1274289112817, responses,RS_8J5wmdmKmJXzK6g-1274299593861,1274494738952, responses,RS_8e5Sz0HeFPAdb6c-1274288
641459,1274495868557, responses,RS_8rjcnmBXPKzI896-1274306981684,1274403047940, responses,RS_9FS3VedcyrF0KX2-1274245971331,1274754745013, responses,RS_9oZgPtxO31npv3C-1274214027769,1274396489756, responses,RS_a3FdO2jhqWuy37C-1274209228660,1274399508186, responses,RS_a3LJVxwTj29MHVa-12742
{/noformat}

Then you see the too many log files:
{noformat}
2010-05-25 10:53:31,364 DEBUG org.apache.hadoop.hbase.regionserver.CompactSplitThread: Compaction requested for region responses-index,--1274799047787--R_cBKrGxx0FdWjPso,1274804575862/783020138 because: regionserver/192.168.0.81:60020.cacheFlusher
2010-05-25 10:53:32,364 WARN org.apache.hadoop.hbase.regionserver.MemStoreFlusher: Region responses-index,--1274799047787--R_cBKrGxx0FdWjPso,1274804575862 has too many store files, putting it back at the end of the flush queue.
{/noformat}

Which leads to this:
{noformat}
2010-05-25 10:53:27,061 INFO org.apache.hadoop.hbase.regionserver.HRegion: Blocking updates for 'IPC Server handler 60 on 60020' on region responses-index,--1274799047787--R_cBKrGxx0FdWjPso,1274804575862: memstore size 128.0m is >= than blocking 128.0m size
2010-05-25 10:53:27,061 INFO org.apache.hadoop.hbase.regionserver.HRegion: Blocking updates for 'IPC Server handler 84 on 60020' on region responses-index,--1274799047787--R_cBKrGxx0FdWjPso,1274804575862: memstore size 128.0m is >= than blocking 128.0m size
2010-05-25 10:53:27,065 INFO org.apache.hadoop.hbase.regionserver.HRegion: Blocking updates for 'IPC Server handler 1 on 60020' on region responses-index,--1274799047787--R_cBKrGxx0FdWjPso,1274804575862: memstore size 128.0m is >= than blocking 128.0m size
{/noformat}



Once the compaction / split is done a flush is able to happen which unblocks the IPC allowing writes to continue. Unfortunately this process can take upwards of 15+ minutes (the specific case shown here from our logs took about 4 minutes).


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postedJun 1, '10 at 6:44p
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