Question

We’ve been using the sys.database_files view and the FILEPROPERTY function to get the size of the database. Unfortunately, as I have just found in the documentation, the values returned by sys.database_files are not always reliable:

When you drop or rebuild large
indexes, or drop or truncate large
tables, the Database Engine defers the
actual page deallocations, and their
associated locks, until after the
transaction commits. Deferred drop
operations do not release allocated
space immediately. Therefore, the
values returned by sys.database_files
immediately after dropping or
truncating a large object may not
reflect the actual disk space
available. For more information
about deferred allocations, see
Dropping and Rebuilding Large Objects.

So, if a large table has just been dropped then when can I query the sys.database_files view reliably? How long shall I wait?

Answer 1

You can use the sys.allocation_units DMV.

Whilst a deferred drop is pending the allocation unit will still be showing with a 0 value in the type column.