We're running Sitecore 6.4 with OMS on SQL Server 2008. I've seen blogs with Sitecore index rebuilding scripts where it explicitly uses a fill factor of 80 in the script. What is the official Sitecore recommendation?
As Mark Cassidy says above, the OMS tuning guide uses figures like 80 and 90% throughout the script examples. This isn't included in the CMS Performance Tuning guide though.
Sitecore support suggest that the precise fill factor to use can vary with index, database, and the precise implementation and usage of the application. Values between 50-75% were suggested as feasible, depending on performance.
In particular, the Sitecore analytics database - which performs limited read operations and a huge amount of write operations - can have indexes with a far lower fill factor than the CMS databases. Indexes with a lower fill factor are slower to read due to the amount of empty pages.
While a ballpark figure of 80% is a reasonable starting point, this should be tested index by index by watching fragmentation rate and measuring the performance of read operations.
The frequency and cost of index rebuilds must also be factored in.
With a low fill factor the indexes will be minimally fragmented for longer periods, requiring fewer scheduled rebuilds, but may be read more slowly. With a high fill factor the indexes will become fragmented far faster, requiring more frequent scheduled rebuilds, but will offer better read performance while not fragmented.