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Accumulo, mail # user - WAL - rate limiting factor x4.67


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Re: WAL - rate limiting factor x4.67
Adam Fuchs 2013-12-04, 14:55
One thing you can do is reduce the replication factor for the WAL. We have
found that makes a pretty significant different in write performance. That
can be modified with the tserver.wal.replication property. Setting it to 2
instead of the default (probably 3) should give you some performance
improvement, of course at some cost to durability.

Adam
On Wed, Dec 4, 2013 at 5:14 AM, Peter Tillotson <[EMAIL PROTECTED]>wrote:

> I've been trying to get the most out of streaming data into Accumulo 1.5
> (Hadoop Cloudera CDH4). Having tried a number of settings, re-writing
> client code etc I finally switched off the Write Ahead Log
> (table.walog.enabled=false) and saw a huge leap in ingest performance.
>
> Ingest with table.walog.enabled= true:   ~6 MB/s
> Ingest with table.walog.enabled= false:  ~28 MB/s
>
> That is a factor of about x4.67 speed improvement.
>
> Now my use case could probably live without or work around not having a
> wal, but I wondered if this was a known issue??
> (didn't see anything in jira), wal seem to be a significant rate limiter
> this is either endemic to Accumulo or an HDFS / setup issue. Though given
> everything is in HDFS these days and otherwise IO flies it looks like
> Accumulo WAL is the most likely culprit.
>
> I don't believe this to be an IO issue on the box, with wal off the is
> significantly more IO (up to 80M/s reported by dstat), with wal on (up to
> 12M/s reported by dstat). Testing the box with FIO sequential write is
> 160M/s.
>
> Further info:
> Hadoop 2.00 (Cloudera cdh4)
> Accumulo (1.5.0)
> Zookeeper ( with Netty, minor improvement of <1MB/s  )
> Filesystem ( HDFS is ZFS, compression=on, dedup=on, otherwise ext4 )
>
> With large imports from scratch now I start off CPU bound and as more
> shuffling is needed this becomes Disk bound later in the import as
> expected. So I know pre-splitting would probably sort it.
>
> Tnx
>
> P
>