Solr 4, optimizing while doing other updates?

[Shawn Heisey]

For Solr 4.0 and higher, is it possible to optimize the index while 
other updates are happening?  Based on some behavior I just saw, I think 
it might be.

I ran a full-import using DIH -- six index shards with 13 million 
records each and a seventh shard (hot shard) with 317000. On a few of 
those large indexes, after DIH reported idle and successful completion, 
I noticed that the index size was still increasing -- Solr was doing one 
last background merge.

In the meantime, my indexing program had noticed that the DIH was done, 
and began indexing backed-up content to the new indexes.  That indexing 
worked flawlessly, even though the indexes were still merging.  I don't 
think there's any way for me to detect the "DIH done, but still merging" 
state ... but I am guessing that I don't have to worry about it.

Can anyone confirm?  I know that on older Solr versions, if I tried to 
index while optimizing, my program would not work right.

Thanks,
Shawn