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From "ivan.bella" <ivan.be...@comcast.net>
Subject Re: better presplitting
Date Sat, 21 Jun 2014 03:52:14 GMT
Right...pre splitting more gradually might be worthwhile...

<div>-------- Original message --------</div><div>From: dlmarion <dlmarion@comcast.net>
</div><div>Date:06/20/2014  7:26 PM  (GMT-05:00) </div><div>To: dev@accumulo.apache.org
</div><div>Subject: Re: better presplitting </div><div>
</div>We have always had issues with splitting taking a long time. Its a serial process
that has to compete with the balancer for a lock on the metadata table. At least in 1.4 anyway,
my information may be outdated. Trying to add threads to create splits in parallel was never
faster. It would be nice if you could manually acquire a lock on the metadata table in the
shell, add all your split points, then release the lock and let the tservers figure it out.
In this case you could parallelize the splitting by avoiding splitting the last tablet, but
split at the midpoint of the last tablet and last split.



<div>-------- Original message --------</div><div>From: Josh Elser <josh.elser@gmail.com>
</div><div>Date:06/20/2014  6:33 PM  (GMT-05:00) </div><div>To: dev@accumulo.apache.org
</div><div>Subject: Re: better presplitting </div><div>
</div>On Jun 20, 2014 12:41 PM, "Sean Busbey" <busbey@cloudera.com> wrote:
>
> 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.

I'd have to pull up the split code, but it seems like a simple fix could be
to let all but one result of the split of a tablet remain local. That way
the current server doesn't get bogged down, and the master would just use
the regular assignment path instead of waiting for the balancer to kick in.

Maybe there's a reason this doesn't work though :)

> 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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