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From Jean-Daniel Cryans <jdcry...@apache.org>
Subject Re: Region number and allocation advice
Date Fri, 06 Jul 2012 21:22:12 GMT
Hey Nick,

I'd say it has nothing to do with spindles/CPUs. The reason is that
locking happens only at the row level thus having 1 or 100 regions
doesn't change anything, then you only have 1 HLog to write to
(currently at least) so spindles don't really count either. It might
influence the number of threads that can compact tho.

I gave some explanations at the HBase BoF we did after the Hadoop
Summit[1] and, from the writes POV, you want to have as much MemStore
potential as you have dedicated heap for it and the HLog need to match
that too. One region can make sense for that except for the issues I
demonstrated and also it's harder to balance. Instead, having a few
regions will give you more agility. For the sake of giving a number
I'd say 10-20 but then you need to tune your memstore size
accordingly. This really is over-optimization tho and it's not even
something we put in practice here since we have so many different use
cases.

The cluster size in my opinion doesn't influence the number of regions
you should have per machine, as long as you keep it low it will be
fine.

Hope this helps,

J-D

1. http://www.slideshare.net/jdcryans/performance-bof12

On Fri, Jul 6, 2012 at 1:53 PM, Nick Dimiduk <ndimiduk@gmail.com> wrote:
> Heya,
>
> I'm looking for more detailed advice about how many regions a table should
> run. Disabling automatic splits (often hand-in-hand with disabling
> automatic compactions) is often described as advanced practice, at least
> when guaranteeing latency SLAs. Which begs the question: how many regions
> should I have? Surely this depends on both the shape of your data and
> expected workload. I've seen "10-20 Regions per RS" thrown around as a
> stock answer. My question is: why? Presumably that's 10-20 regions per RS
> for all tables rather than per-table. That advice is centered around a
> regular region size, but surely distribution of ops/load matters more. But
> still, where does 10-20 come from? Is it a calculation vs the number of
> cores on the RS, like advice given around parallelizing builds? If so, how
> many cores are we assuming the RS has? Is it a calculation vs the amount of
> RAM available? Is 20 regions based on a trade-off between static
> allocations and per-region memory overhead? Does 10-20 become 5-15 in a
> memory-restricted environment and bump to 20-40 when more RAM is available?
> Does it have to do with the number of spindles available on the machine?
> Threads like this one [0] give some hint about how the big players work.
> However, that advice looks heavily influenced by concerns when there are
> 1000's of regions to manage. How does advice for larger clusters (>50
> nodes) differ from smaller clusters (<20 nodes)?
>
> Thanks,
> -n
>
> [0]: http://thread.gmane.org/gmane.comp.java.hadoop.hbase.user/22451

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