Disk space full but logical space available in database We have a fairly big MS SQL 2008R2 database that resides on a SSD drive. The drive itself only has ~110Gb of space, and the database files are the only files on the drive.

The database is in "Simple" recovery mode, and only has two files, .MDF and .LDF. 

The disk is now nearly full: the MDF is currently 109Gb in size. However, SSMS tells me there's nearly 18Gb of "Space Available" (in the 'General' properties page), and if I go through the motions of Shrinking the file it also tells me there's 18Gb of space free. SSMS also tells me the database size is around 132Gb, which surprises me - that wouldn't fit on the drive!

From what I've read, shrink is a really bad idea. However, I'm starting to see replication errors (could not allocate space for object). We've previously tried shrinking the database, but within a few hours the file was back to it's original size.

How should we proceed - given there's apparently 18Gb of free space, should SQL automatically use that free space? Or is it as simple as: we really need more disk space?
Space is available inside the database because data has been moved around. Perhaps you have very high levels of page splits, or have recently deleted a large portion of data that had previously caused the data file to grow.

SQL Server does not shrink database files automatically when you've freed up space within them, because the logical assumption is that if you've used that space once, you'll use it again. Autogrow can be an expensive event and unnecessary if you've only freed up space temporarily (what were you able to do with all that free space in the meantime?). For the same reasons, you shouldn't try to temporarily reclaim space, either. Just let SQL Server use the 18 GB of available space as you add more data. If you think you will need more than 18 GB of additional space going forward (in which case, you will need to add file(s) on other disks, or move to a bigger disk).

sp_spaceused (and in turn the UI dialog you're looking at) may return more space than is possible because of synchronization issues in the metadata about your tables/indexes/files. In order to make sure it reflects accurate space, run this:

DBCC UPDATEUSAGE(0);


I have also seen scenarios where rebuilding indexes was required in order to rectify the counts/space, but I haven't seen that specific case since SQL Server 2000.

(I suspect this isn't a simple case where your database already spans multiple disks, or you surely would have mentioned that in the question.)

That all said, the fact that when you shrink the data file expands almost immediately leads me to believe that you are actually using the space, but must also be performing big deletes or updates that are freeing it up (which is when you see 18 GB free). Unfortunately it's impossible for us to see exactly why the data file is expanding and then clearing itself out - perhaps you have transactions where you are truncating / re-populating large tables, performing massive archive operations, etc.