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HBase, mail # dev - Re: [Shadow Regions / Read Replicas ] Wal per region?


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Re: [Shadow Regions / Read Replicas ] Wal per region?
Devaraj Das 2013-12-03, 19:21
On Mon, Dec 2, 2013 at 10:20 PM, Jonathan Hsieh <[EMAIL PROTECTED]> wrote:

> > Deveraj:
> > Jonathan Hsieh, WAL per region (WALpr) would give you the locality (and
> hence HDFS short
> > circuit) of reads if you were to couple it with the favored nodes. The
> cost is of course more WAL
> > files... In the current situation (no WALpr) it would create quite some
> traffic cross machine, no?
>
> I think we all agree that wal per region isn't efficient on today's
> spinning hard drive world where we are limited to a relatively low budget
> or seeks (though may be better in the future with SSD's).
>
>
Yes, agree on this. My thought is also in the direction that with SSDs the
issue of many files per RS would be hopefully not an issue.
> With this in mind, I actually I making the case that we would group the all
> the regions from RS-A onto the same set of preferred regions servers.  This
> way we only need to have one or two other RS's tailing the RS.
>
> So for example, if region X, Y and Z were on RS-A and its hlog, the shadow
> region memstores for X, Y, and Z would be assigned to the same one or two
> other RSs.  Ideally this would be where the HLog files replicas have
> locality (helped by favored nodes/block affinity).  Doing this, we hold the
> number of readers on the active hlogs to a constant number, do not add any
> new cross machine traffic (though tailing currently has costs on the NN).
>
>
Yes, we did consider this but the issue is how much complex would the
failure handling be in order to maintain the grouping of the regions. So,
for example, if RS-A goes down, would the master be able to choose another
RS-A' quickly to maintain the grouping of the regions in RS-A. Or do we
then fallback to the regular single-region assignments and have the
balancer group the regions back... What's the grouping size. The same
issues apply to the assignments of the shadows.

But having said that, I agree that if the WALpr is expensive on non-SSD
hardware for example, we need to address the grouping of regions issues.
> One inefficiency we have is that if there is a single log per RS, we end up
> reading all the logs to tables that may not have the shadow feature
> enabled.  However, with HBase multi-wals coming, one strategy is to shard
> wals to a number on the order of the number of disks on a machine (12-24
> these days).  I think the a wal per namespaces (this could be used to have
> a wal per table) of the hlog would make sense.  This way of shardind the
> hlog would reduce the amount of reading of irrelevant log entries on a log
> tailing scheme. It would have the added benefit of reducing the log
> splitting work reducing MTTR and allowing for recovery priorities if the
> primaries and shadows also go down.  (this is an generalization of the
> separate out the META into a separate log idea).
>
> Jon.
>
> --
> // Jonathan Hsieh (shay)
> // Software Engineer, Cloudera
> // [EMAIL PROTECTED]
>

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