I know that Lucene indexes cannot be shared:
Sitecore keeps a local copy of the Lucene index in the file system of each instance and does not support sharing the indexes between instances.
Is it possible to xcopy Lucene indexes between CM and CD?
Is there some other approach or recommendation to maintain a indexes in 30+ content delivery servers ?
Update: I’m fully aware that CDs must kick off their own index update. With over 30 CD servers I’m thinking that maybe there will be a period of time that not all CD server have the same set of indexes. I’m afraid that for some reason the indexes will fail in some of the CD servers and tracking why/where will be hell. That’s why trying to discover if there some alternative approach where indexes are maintain in one place (some sort shared) and basically instantly replicated to all CD
You need to enable History Engine for the CM and CD servers web database.
See this extract from the Sitecore Scaling Guide.
To enable History Engine for a Sitecore database: In the web.config file, add the following section to the /configuration/sitecore/databases/database element, where id equals the name of the database:
<Engines.HistoryEngine.Storage>
<obj type="Sitecore.Data.$(database).$(database)HistoryStorage, Sitecore.Kernel">
<param connectionStringName="$(id)" />
<EntryLifeTime>30.00:00:00</EntryLifeTime>
</obj>
</Engines.HistoryEngine.Storage>
<Engines.HistoryEngine.SaveDotNetCallStack>false</Engines.HistoryEngine.SaveDotNetCallStack>
When a Sitecore item is changed, the Lucene indexes are updated immediately on the Sitecore instance where the change was made. On remote servers in a multi-server environment, the Lucene indexes are not updated immediately after an item is changed. The Lucene indexes are automatically updated after the interval that is defined in the web.config file, in the Indexing.UpdateInterval setting and with the minimum wait time between the two consequent updates defined in the Indexing.UpdateJobThrottle setting.
See here