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From Sean Busbey <bus...@cloudera.com>
Subject Re: better presplitting
Date Fri, 20 Jun 2014 19:41:08 GMT
When you add splits, they definitely start out on the server that is
hosting the tablet that has to split apart.  They have to, since the tablet
that hosted the previous key extent is the only one that can properly
handle requests for the new key extents.

We've run into this consistently when doing any testing that requires
pre-splitting for perf reasons.

In the case of YCSB tests, Mike scripted some nice manual pre-splitting in
waves:

* split table into X parts
* wait for balancing
* split each X part into Y parts
* wait for balancing

presuming the goal is to end up with X*Y presplits, this was way faster
than just asking for the total right off the bat.

We could generally look at improving the migration code to handle these
reassignments faster, but how often does this situation come up for people
who aren't making a new table? If the "do this offline" feature speeds up
the new table use case enough, I'm not sure optimizing the migration path
is worth the time investment right now.


On Fri, Jun 20, 2014 at 3:09 PM, Josh Elser <josh.elser@gmail.com> wrote:

> bq. They all started out on one server
>
> This seems.. weird. Would be good to start addressing this by identifying
> what the actual balancer code does so we can immediately start to test the
> assertions. We can then use the results to identify the deficiencies that
> exist.
>
> I think the 200splits per server was an Eric quote from some time ago
> (1.4-ish, maybe 1.5). I think this is relative to a bunch of things,
> workload and memory available most notably, and would be good to quantify
> too.
>
>
> On 6/20/14, 11:58 AM, Sean Busbey wrote:
>
>> One thing that jumped out from the most recent D4M paper was this quote:
>>
>>    One issue that was encountered is that after creating the pre-splits,
>> they all started out on one server. Accumulo load balanced the splits
>> across its servers at rate of ~50 splits/second, which is more than
>> adequate for normal operation, but can take ~20 minutes for 50,000 pre-
>> splits.[1]
>>
>> Do we already have an open ticket that would help this? I think maybe
>> there's one about being able to presplit a table that is offline?
>>
>> I believe our recommended sweet spot is like 100-200 tablets per server
>> (though I can't find the reference for *why* I believe this ATM), which
>> means for clusters in the ~100s of nodes this would be in the ballpark for
>> an expected number of pre-splits.
>>
>>
>> [1]:  arXiv:1406.4923v1 [cs.DB]
>>
>>


-- 
Sean

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