bq. You will have to  migrate your indexes from 4 to 5, then 5 to 6,
and finally 6 to 7.

This actually has never been guaranteed. Lucene has guaranteed
back-compat for 1 major version. Using the IndexUpgrader tool will
rewrite the X-1 format in version X format, but cannot synthesize data
from thin air. So starting with X-2 -> X-1 -> X hasn't failed to run,
it just hasn't necessarily worked like an index that was created
entirely with version X.

This has been formalized starting with Lucene 6. A marker has been
written in each segment that is preserved across all merges and will
refuse to open indexes with a marker from X-2. If you merge two
segments, one created with X-1 and one created with X, the X-1 marker
is preserved. If you try to use Solr 8 on an index that has segments
that have _ever_ been touched by Lucene 6, Lucene will refuse to open
the index.

And anyway, the IndexUpgraderTool just optimizes down to 1 segment,
which has its own problems.

It's best to plan on re-indexing into a fresh collection...

Best,
Erick
On Tue, Dec 11, 2018 at 9:30 AM Emmanuel Keller
<[email protected]> wrote:
>
> About the migration strategy, you may have to choose between reindexing all 
> the data or migrating your current indexes.
>
> the IndexUpgrader tool will be able to handle that.
> You will have to  migrate your indexes from 4 to 5, then 5 to 6, and finally 
> 6 to 7.
> https://lucene.apache.org/solr/guide/7_6/indexupgrader-tool.html
>
> If you still have your original data, then the reindexing option is better. 
> And even more if you plan to use the new field options available, which means 
> changing the schema.
>
> If it can help :D
>
>
>
> On 11 Dec 2018, at 17:08, Cassandra Targett <[email protected]> wrote:
>
> The Solr Ref Guide also has extensive sections (particularly for upgrading to 
> Solr 6 and 7) describing summarizing major changes for various releases. 
> Start at https://lucene.apache.org/solr/guide/solr-upgrade-notes.html and 
> note the child pages from there.
>
> Cassandra
> On Dec 11, 2018, 10:30 AM -0600, Erick Erickson <[email protected]>, 
> wrote:
>
> The CHANGES.txt file in both the Solr and Lucene directories have a
> section listing the differences between every Solr version, major and
> minor that you could review. The reference guide has an upgrade
> section as well that will be informative.
>
> That said, there are many changes. Lot and lots and lots. What I'd do
> is plan on starting with a fresh Solr 7x setup, migrate your
> configurations (as in make similar changes in the 7x configs, do not
> just copy/paste). and start testing after indexing fresh. You haven't
> said whether you have any custom code and the like, so that is an
> unknown.
>
> Best,
> Erick
> On Tue, Dec 11, 2018 at 6:28 AM hello hello <[email protected]> wrote:
>
>
> Bonjour ,
>
> I am a project manager at La Poste française , I am managing a national 
> project using SORL 4.9 and I want to migrate to SOLR 7 .
> Can you give me a summary of all the spots that need to be done to make this 
> migration? Thank oyu your for help Fred
>
>
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