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From "stack (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-1644) [hbase] Compactions should take no longer than period between memcache flushes
Date Fri, 20 Jul 2007 17:59:06 GMT
[hbase] Compactions should take no longer than period between memcache flushes

                 Key: HADOOP-1644
                 URL: https://issues.apache.org/jira/browse/HADOOP-1644
             Project: Hadoop
          Issue Type: Wish
          Components: contrib/hbase
            Reporter: stack
            Assignee: stack
            Priority: Minor

Currently, compactions take a long time.  During compaction, updates are carried by the HRegions'
memcache (+ backing HLog).  memcache is unable to flush to disk until compaction completes.

Under sustained, substantial --  rows that contain multiple columns one of which is a web
page -- updates by multiple concurrent clients (10 in this case), a common hbase usage scenario,
the memcache grows fast and often to orders of magnitude in excess of the configured 'flush-to-disk'

This throws the whole system out of kilter.  When memcache does get to run after compaction
completes -- assuming you have sufficent RAM and the region server doesn't OOME -- then the
resulting on-disk file will be way larger than any other on-disk HStoreFile bringing on a
region split ..... but the resulting split will produce regions that themselves need to be
immediately split because each half is beyond the configured limit, and so on...

In another issue yet to be posted, tuning and some pointed memcache flushes makes the above
condition less extreme but until compaction durations come close to the memcache flush threshold
compactions will remain disruptive. 

Its allowed that compactions may never be fast enough as per bigtable paper (This is a 'wish'

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