We are looking to upgrade our entire environment from 6.6.0 to 6.6.3, a software bug keeps causing the Indexers to crash so we have lost an entire site for the past few days. When it comes to upgrading the docs state to wait for the sites replication and search factor to be met, as the indexers have been down for a few days this may take some time. So is it advised to wait for this to happen before upgrading the next site i.e waiting 24 hours before starting to upgrade the second site? Will having each site on different versions cause any issues, or having one site on a different version to the Cluster Master or Search Heads?
The docs for clustered environment upgrades state that you can do one site at a time in a multisite cluster and that as long as you stay within a "point release" (i.e. 6.3.x to 6.4.x), you can have a mixture of versions during the upgrade.
So I'd upgrade the sites where SF and RF are met, give the bad site time to fixup, and then upgrade it.
Here's a link to the exact document I speak of:
In fact, if you read a bit further, upgrading to a maintenance release (i.e. 6.3.0 to 6.3.x) can be done in a rolling restart method:
However it does caution you to do so as quickly as possible. See the warnings under the Upgrade to a maintenance release.
In the end, I believe you'll be fine because nothing major has changed in the way the buckets are handled between 6.3.0 and 6.3.3 (according to the release notes I've read).