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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2782) QOS for META table access
Date Thu, 24 Jun 2010 13:55:51 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2782?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12882158#action_12882158
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Todd Lipcon commented on HBASE-2782:
------------------------------------

Ehh, I'm still unconvinced. Moving META to ZK means extra work for things like snapshots,
backups, etc, where currently we can use the same mechanisms for user tables as for meta tables.
Plus these issues that we see on META are issues that we'll also see on user tables. For example,
right now it's very easy for a MR job to completely monopolize the capacity of a cluster to
the point that interactive queries start having 30sec+ latencies. Really good QoS is hard,
but I think a simple solution like above can get us a lot of benefit for not much work. Especially
if we can make the QoS policy pluggable, maybe someone will just write a really good one and
contribute it back.

Multi-tenancy is a huge problem we haven't even begun to tackle, and QoS is just a tiny bit
of it, but my point is that we need to solve this problem regardless of what happens to META.

> QOS for META table access
> -------------------------
>
>                 Key: HBASE-2782
>                 URL: https://issues.apache.org/jira/browse/HBASE-2782
>             Project: HBase
>          Issue Type: Improvement
>          Components: regionserver
>            Reporter: Todd Lipcon
>
> I'd like to brainstorm some ideas on how we can prioritize reads and writes to META above
reads and writes to other tables. I've noticed that if the regionserver hosting META is under
heavy load, then lots of other operations take much longer than they should. For example,
I'm currently running 120 threads of YCSB across 3 client nodes hitting a 5-node cluster.
Doing a full scan of META (only 600 rows) takes upwards of 30 seconds in the shell, since
all of the handler threads are tied up and there's a long RPC queue. 

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