Sounds like making addsplits a FATE op would make sense.
On 6/20/14, 4:26 PM, dlmarion wrote:
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At least in 1.4 anyway, my information may be outdated.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 <[email protected]>
</div><div>Date:06/20/2014 6:33 PM (GMT-05:00) </div><div>To: [email protected] </div><div>Subject: Re:
better presplitting </div><div>
</div>On Jun 20, 2014 12:41 PM, "Sean Busbey" <[email protected]> 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 <[email protected]> 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
