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From "stack (JIRA)" <j...@apache.org>
Subject [jira] Created: (HBASE-2053) Upper bound of outstanding WALs can be overrun
Date Thu, 17 Dec 2009 18:54:18 GMT
Upper bound of outstanding WALs can be overrun

                 Key: HBASE-2053
                 URL: https://issues.apache.org/jira/browse/HBASE-2053
             Project: Hadoop HBase
          Issue Type: Bug
            Reporter: stack

Kevin Peterson up on hbase-user posted the following.  Of interest is the link on the end
which is logs of WAL rolls and removals.  In once place we remove 70plus logs because the
outstanding edits have moved passed the outstanding sequence numbers -- so our basic WAL removal
mechanism is working -- but if you study the log, the tendency is steady climb in the number
of logs.   HLog#cleanOldLogs needs to notice such an upward tendency and work more aggressively
cleaning the old in this case.  Here is Kevin's note:

n Tue, Dec 15, 2009 at 3:17 PM, Kevin Peterson <x@y.com> wrote:
This makes some sense now. I currently have 2200 regions across 3 tables. My
largest table accounts for about 1600 of those regions and is mostly active
at one end of the keyspace -- our key is based on date, but data only
roughly arrives in order. I also write to two secondary indexes, which have
no pattern to the key at all. One of these secondary tables has 488 regions
and the other has 96 regions.

We write about 10M items per day to the main table (articles). All of these
get written to one of the secondary indexes (article-ids). About a third get
written to the other secondary index. Total volume of data is about 10GB /
day written.

I think the key is as you say that the regions aren't filled enough to
flush. The articles table gets mostly written to near one end and I see
splits happening regularly. The index tables have no pattern so the 10
millions writes get scattered across the different regions. I've looked more
closely at a log file (linked below), and if I forget about my main table
(which would tend to get flushed), and look only at the indexes, this seems
to be what's happening:

1. Up to maxLogs HLogs, it doesn't do any flushes.
2. Once it gets above maxLogs, it will start flushing one region each time
it creates a new HLog.
3. If the first HLog had edits for say 50 regions, it will need to flush the
region with oldest edits 50 times before the HLog can be removed.

If N is the number of regions getting written to, but not getting enough
writes to flush on their own, then I think this converges to maxLogs + N
logs on average. If I think of maxLogs as "number of logs to start flushing
regions at" this makes sense.



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