Nearly all SharePoint farms I look at show the error:

Drives used for SQL databases are running out of free space

However, this seems to happen even on SharePoint farms where there is tonnes of space. It’s always puzzled me, and it results in many customers simply ignoring the warning, ‘cos they can’t figure out why they’re getting it, and having checked the drives they think they’re fine (and they probably are).

Well Zoltan Kovacs has a very good analysis. It’s well worth a read.

I’m going to plagiarize a little – in case his blog vanishes in the future – but the essential elements are:

It compares the current size and “some” autogrowth value with the free space on that drive. It does not care about your free space in your database; if you have a 1 GB db and have 990 MB free space in it, it is still counted as 1 GB as current size.

So you are safe if:

all your databases files can grow with two times its growth percentage [if specified as percentage] OR can grow 40% [if autogrowth is specified as a fixed value] compared to current free space.

Nice. That’s useful, and might make these rules useful again.