FW: Safe to change numVersionBuckets?

-----Original Message-----
From: Randy Fradin [mailto:randy.fradin@gmail.com]
Sent: 31 March 2018 09:54
To: solr-user@lucene.apache.org
Subject: Safe to change numVersionBuckets?

I have a SolrCloud cluster (version 6.5.1) with around 3300 cores per
instance. I've been investigating what is driving heap utilization since it
is higher than I expected. I took a heap dump and found the largest driver
of heap utilization is the array of VersionBucket objects in the
org.apache.solr.update.VersionInfo class. The array is size 65536 and there
is one per SolrCore instance. Each instance of the array is 1.8MB so the
aggregate size is 6GB in heap.

I understand from reading the discussion in SOLR-6820 that 65536 is the
recommended default for this setting now because it results in higher
document write rates than the old default of 256. I would like to reduce my
heap utilization and I'm OK with somewhat slower document writing
throughput. My question is, it is safe to reduce the value of
numVersionBuckets on all of my existing cores without reindexing my data?

My solrconfig.xml contains this for all of my collections:

<updateHandler class="solr.DirectUpdateHandler2">
<updateLog>
<str name="dir">${solr.ulog.dir:}</str>
<int name="numVersionBuckets">${solr.ulog.numVersionBuckets:65536}</int>
</updateLog>
</updateHandler>

Assuming it is safe to change, can I just add a vm arg to the Solr process
like "-Dsolr.ulog.numVersionBuckets=256" to override the value for all cores
at once? Or do I have to change and re-upload the solrconfig.xml files and
reload the cores?

Thanks