Affects Version/s: None
Fix Version/s: None
This needs more careful planning, see the SIP here: https://cwiki.apache.org/confluence/display/SOLR/SIP-2+Support+safe+index+transformations+without+reindexing
The general problem statement is that the current upgrade path is trappy and cumbersome. It would be a great help "in the field" to make the upgrade process less painful.
Additionally one of the most common things users want to do is enable docValues, but currently they often have to re-index.
1> if I upgrade from 5x to 6x and then 7x, theres no guarantee that when I go to 7x all the segments have been rewritten in 6x format. Say I have a segment at max size that has no deletions. It'll never be rewritten until it has deleted docs. And perhaps 50% deleted docs currently.
2> IndexUpgraderTool explicitly does a forcemerge to 1 segment, which is bad.
3> in a large distributed system, running IndexUpgraderTool on all the nodes is cumbersome even if <2> is acceptable.
4> Users who realize specifying docValues on a field would be A Good Thing have to re-index. We have UninvertDocValuesMergePolicyFactory. Wouldn't it be nice to be able to have this done all at once without forceMerging to one segment.
Somehow avoid the above. Currently
LUCENE-7976 is a start in that direction. It will make TMP respect max segments size so can avoid forceMerges that result in one segment. What it does not do is rewrite segments with zero (or a small percentage) deleted documents.
So it doesn't seem like a huge stretch to be able to specify to TMP the option to rewrite segments that have no deleted documents. Perhaps a new parameter to optimize?
This would likely require another change to TMP or whatever.
So upgrading to a new solr would look like
1> install the new Solr
2> execute "http://node:port/solr/collection_or_core/update?optimize=true&upgradeAllSegments=true"
What's not clear to me is whether we'd require UninvertDocValuesMergePolicyFactory to be specified and wrap TMP or not.
Anyway, let's discuss. I'll create yet another LUCENE JIRA for TMP do rewrite all segments that I'll link.
I'll also link several other JIRAs in here, they're coalescing.