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From "Billy Pearson (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2053) Upper bound of outstanding WALs can be overrun
Date Tue, 29 Dec 2009 01:30:29 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2053?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12794990#action_12794990

Billy Pearson commented on HBASE-2053:

I set to 5 mins to help lower the number of log files in down times when there is no imports
The hlogs roll even with 0 edits in it and starts the flushing of memcache to get back under
seams to help over time to reduce the log file count in slow import times.

The log files that you see 70+ are all full thought so we still have a problem of building
up logs 
But I figured that when replication comes along the hbase.regionserver.logroll.period 
would get reduced to 5 mins or less to help roll the logs so the logs could be process and
sent to the other cluster.
So I figured I would try the lower roll time and see if I could find any problems now.

> 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
>         Attachments: hbase-root-regionserver-server-2.log.2009-12-22.gz
> 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:
> {code}
> 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.
> http://kdpeterson.net/paste/hbase-hadoop-regionserver-mi-prod-app35.ec2.biz360.com.log.2009-12-14
> {code}

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