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https://issues.apache.org/jira/browse/SOLR-12259?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16662554#comment-16662554
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Erick Erickson commented on SOLR-12259:
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bq. can deal with "safe" X+2 upgrades if there ever are any.
Certainly X+2 is not supported. If (and only if) there are situations where it
_would_ be safe, then it could conceivably do some low-level tricks to spoof
the rewritten segments with version stamp X. That is _not_ something I advocate
be part of Solr, but with this notion it's possible to do if an existing Solr
installation wants to.
I'm not sure index locking is necessary, conceptually it's simpler and I wanted
to throw it out. Another idea is to rewrite just a few (or just one) segment(s)
at a time that needed whatever modification the merge policy is implementing
and optionally do a commit periodically to flush the fieldCache. Or flush the
field cache as a byproduct. Or.... The point would be to keep the fieldCache
from getting too big.
One thing I'm not sure of (this is brainstorming after all) is _if_ we do use a
new endoint that rewrites segments, and assuming we don't lock out updates,
insure that any segments rewritten by this new endpoint don't _also_ get merged
by the background merging process.
> Robustly upgrade indexes
> ------------------------
>
> Key: SOLR-12259
> URL: https://issues.apache.org/jira/browse/SOLR-12259
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Reporter: Erick Erickson
> Assignee: Erick Erickson
> Priority: Major
>
> 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.
> Issues:
> 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.
> Proposal:
> 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.
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