What is the signifigance of a fillfactor of 20?

Question

I have a database for our ticket system in SQL Server express. I was poking around and noticed that a lot of the indexes have a fillfactor set to 20. What is the signifigance of it being set to 20? Why would you set it so low?

Answer 1

@ 20 % you're consuming 5 times the disk actually needed for the index. I'm sure there's some business case where that might make sense, but I've never seen it. If you aren't rebuilding these regularly (rebuild returns to original fill factor level, in this case 20%), I'd be interested to see how full the indexes are after a month of operation. General ref: http://msdn.microsoft.com/en-us/library/ms177459.aspx

Answer 2

In SQL Server data is stored in to pages. And each page is of size 8K. And in simple terms fillfactor indicates % fullness of any page when index is created or rebuild operation is done.

Fillfactor plays important role if data in tables is modified very frequently. Having a fillfactor other than 0 or 100 actually leaves some free space in that page and it improves speed of insert operation, by avoiding an effect called "page splits".

But downside of having very small fillfactor is, it slows down read operations. And as @Eric mentioned it is very strange that someone has set it up to 20%. May be you want to change it between 70-90% with some trials.