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From Greg Pendlebury <greg.pendleb...@gmail.com>
Subject Re: SolrCloud leaders using more disk space
Date Tue, 01 Jul 2014 02:01:15 GMT
Thanks for the reply Tim.

>> "Can you diff the listings of the index data directories on a leader vs.
replica?"

It was a good tip, and mirrors some stuff we have been exploring in house
as well. The leaders all have additional 'index.XXXX' directories on disk,
but we have come to the conclusion that this is a coincidence and not
related to the fact that they are leaders.

Current theory is that they are the result of an upgrade rehearsal that was
performed before launch where the cluster was split into two on different
versions of Solr and different ZK paths. I suspect that whilst the ops team
where doing the deployment there were a number of server restarts that
triggered leader elections and recovery events that weren't allowed to
complete gracefully, leaving the old data on disk.

The coincidence is simply that the ops team did all their initial practice
stuff on the same 3 hosts, which later became our leaders. I've found a few
small similar issue on hosts 4-6, and none at all on hosts 7-9.

I hoping we get a chance to test all this soon, but we need to re-jig our
test systems first, since they don't have any redundancy depth to them
right now.

Ta,
Greg


On 28 June 2014 02:59, Timothy Potter <thelabdude@gmail.com> wrote:

> Hi Greg,
>
> Sorry for the slow response. The general thinking is that you
> shouldn't worry about which nodes host leaders vs. replicas because A)
> that can change, and B) as you say, the additional responsibilities
> for leader nodes is quite minimal (mainly per-doc version management
> and then distributing updates to replicas). The segment merging all
> happens at the Lucene level, which has no knowledge of SolrCloud
> leaders / replicas. Since this is SolrCloud, all nodes pull the config
> from ZooKeeper so should be running the same settings. Can you diff
> the listings of the index data directories on a leader vs. replica?
> Might give us some insights to what files the leader has that the
> replicas don't have.
>
> Cheers,
> Tim
>
> On Tue, Jun 3, 2014 at 8:32 PM, Greg Pendlebury
> <greg.pendlebury@gmail.com> wrote:
> > Hi all,
> >
> > We launched our new production instance of SolrCloud last week and since
> > then have noticed a trend with regards to disk usage. The non-leader
> > replicas all seem to be self-optimizing their index segments as expected,
> > but the leaders have (on average) around 33% more data on disk. My
> > assumption is that leader's are not self-optimising (or not to the same
> > extent)... but it is still early days of course.
> >
> > If it helps, there are 45 JVMs in the cloud, with 15 shards and 3
> replicas
> > per shard. Each non-leader shard is sitting at between 59GB and 87GB on
> > their SSD, but the leaders are between 84GB and 116GB.
> >
> > We have pretty much constant read and write traffic 24x7, with just
> 'slow'
> > periods overnight when write traffic is < 1 document per second and
> > searches are between 1 and 2 per second. Is this light level of traffic
> > still too much for the leaders to self-optimise?
> >
> > I'd also be curious to hear about what others are doing in terms of
> > operating procedures. We load test before launch what would happen if we
> > turned off JVMs and forced recovery events. I know that these things all
> > work, just that customers will experience slower search responses whilst
> > they occur. For example, a restore from a leader to a replica under load
> > testing for us takes around 30 minutes and response times drop from
> around
> > 200-300ms average to 1.5s average.
> >
> > Bottleneck appears to be network I/O on the servers. We haven't explored
> > whether this is specific to the servers replicating, or saturation of the
> > of the infrastructure that all the servers share, because...
> >
> > This performance is acceptable for us, but I'm not sure if I'd like to
> > force that event to occur unless required... this is following the line
> of
> > reasoning proposed internally that we should periodically rotate leaders
> by
> > turning them off briefly. We aren't going to do that unless we have a
> > strong reason though. Does anyone try to manipulate production instances
> > that way?
> >
> > Vaguely related to this is leader distribution. We have 9 physical
> servers
> > and 5 JVMs running on each server. By virtue of the deployment procedures
> > the first 3 servers to come online are all running 5 leaders each. Is
> there
> > any merit in 'moving' these around (by reboots)?
> >
> > Our planning up to launch was based on lots of mailing list response we'd
> > seen that indicated leaders had no significant performance difference to
> > normal replicas, and all of our testing has agreed with that. The disk
> size
> > 'issue' (which we aren't worried about... yet. It hasn't been in prod
> long
> > enough to know for certain) may be the only thing we've seen so far.
> >
> > Ta,
> > Greg
>

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