The IndexUpgraderTool does a forceMerge(1). If you have a large index,
that has its own problems, but will work. The threshold for the issues is
5G. See:
https://lucidworks.com/post/solr-and-optimizing-your-index-take-ii/ I should emphasize that if you have a very large single segment as a
result, it’ll eventually shrink if it accumulated deleted (or updated) documents,
it’ll just require a bunch of I/O amortized over time.
IndexUpgraderTool will _not_ allow you to take an index originally created with
7x to be used in 9x. (Uwe, I’ve been telling people this for a long time, if I’ve
been lying please let me know!). Starting with Lucene 6, a version is written into
each segment. Upon merge, the lowest version stamp is preserved. Lucene
will refuse to open an index where _any_ segment has a version stamp X-2 or
older.
Best,
Erick
> On Nov 20, 2020, at 7:57 AM, Michael Sokolov <msokolov@gmail.com> wrote:
>
> I think running the upgrade tool would also be necessary to set you up for
> the next upgrade, when 9.0 comes along.
>
> On Fri, Nov 20, 2020, 4:25 AM Uwe Schindler <uwe@thetaphi.de> wrote:
>
>> Hi,
>>
>>> Currently I am using Lucene 7.3, I want to upgrade to lucene 8.5.1.
>> Should
>>> I do reindexing in this case ?
>>
>> No, you don't need that.
>>
>>> Can I make use of backward codec jar without a reindex?
>>
>> Yes, just add the JAR file to your classpath and it can read the indexes.
>> Updates written to the index will use the new codecs. To force a full
>> upgrade (rewrite all segments), invoke the IndexUpgrader class either from
>> your code or using the command line. But this is not needed, it just makes
>> sure that you can get rid of the backwards-codecs jar.
>>
>> Uwe
>>
>>
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