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Date: Fri, 25 Dec 2009 00:51:29 +0000 (UTC)
From: "stack (JIRA)" <jira@apache.org>
To: hbase-dev@hadoop.apache.org
Subject: [jira] Commented: (HBASE-2053) Upper bound of outstanding WALs can
 be overrun
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    [ https://issues.apache.org/jira/browse/HBASE-2053?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12794527#action_12794527 ] 

stack commented on HBASE-2053:
------------------------------

Hey Billy.

.bq I thank a simple fix for this would be to flushed the oldest outstanding seqnum and loop or queue flushes until it flushes all outstanding memstore edits for the oldest log file.

The way it currently works is that if too many log files,  we'll find the region that has the oldest sequence number and schedule a flush on that.  This doesn't seem to be enough.  We need to do something like you suggest.... keep flushing the oldest till we pass out at least one WAL file. 

Can you post a regionserver log that has too many running WALs (if you have DEBUG enabled) so can study the issue?  Thanks.

> 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:
> {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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