Database Free Space Monitoring – The right way
Lately I spent some time evaluating some monitoring tools for SQL Server and one thing that struck me very negatively is how none of them (to date) has been reporting database free space correctly.
I was actively evaluating one of those tools when one of my production databases ran out of space without any sort of warning.
I was so upset that I decided to code my own monitoring script.
Some things to take into account:
- Hard set limits for file growth have to be considered: a drive with lots of space is useless if the database file cannot grow and take it.
- If fixed growth is used, there must be enough space in the drive to accomodate the growth amount you set.
- If percent growth is used, you have to calculate recursively how much your database file will grow before taking all the space in the drive
- Some scripts found in blogs and books don’t account for mount points. Use
sys.dm_os_volume_statsto include mount points in your calculation (unless you’re running SQL Server versions prior to 2012).
- Database free space alone is not enough. NTFS performance start degrading when the drive free space drops below 20%. Make sure you’re monitoring that as well.
- 20% of a huge database can be lots of space. You can change that threshold to whatever you find appropriate (for instance, less than 20% AND less than 20 GB)
That said, here is my script, I hope you find it useful.
-- create a temporary table to hold data from sys.master_files IF OBJECT_ID('tempdb..#masterfiles') IS NOT NULL DROP TABLE #masterfiles; CREATE TABLE #masterfiles ( database_id int, type_desc varchar(10), name sysname, physical_name varchar(255), size_mb int, max_size_mb int, growth int, is_percent_growth bit, data_space_id int, data_space_name nvarchar(128) NULL, drive nvarchar(512), mbfree int ); -- extract file information from sys.master_files -- and correlate each file to its logical volume INSERT INTO #masterfiles SELECT mf.database_id ,type_desc ,name ,physical_name ,size_mb = size / 128 ,max_size_mb = CASE WHEN max_size = 268435456 AND type_desc = 'LOG' THEN -1 ELSE CASE WHEN max_size = -1 THEN -1 ELSE max_size / 128 END END ,mf.growth ,mf.is_percent_growth ,mf.data_space_id ,NULL ,d.volume_mount_point ,d.available_bytes / 1024 / 1024 FROM sys.master_files AS mf CROSS APPLY sys.dm_os_volume_stats(database_id, file_id) AS d; -- add an "emptyspace" column to hold empty space for each file ALTER TABLE #masterfiles ADD emptyspace_mb int NULL; -- iterate through all databases to calculate empty space for its files DECLARE @name sysname; DECLARE c CURSOR FORWARD_ONLY READ_ONLY STATIC LOCAL FOR SELECT name FROM sys.databases WHERE state_desc = 'ONLINE' OPEN c FETCH NEXT FROM c INTO @name WHILE @@FETCH_STATUS = 0 BEGIN DECLARE @sql nvarchar(max) DECLARE @statement nvarchar(max) SET @sql = ' UPDATE mf SET emptyspace_mb = size_mb - FILEPROPERTY(name,''SpaceUsed'') / 128, data_space_name = ISNULL( (SELECT name FROM sys.data_spaces WHERE data_space_id = mf.data_space_id), ''LOG'' ) FROM #masterfiles AS mf WHERE database_id = DB_ID(); ' SET @statement = 'EXEC ' + QUOTENAME(@name) + '.sys.sp_executesql @sql' EXEC sp_executesql @statement, N'@sql nvarchar(max)', @sql FETCH NEXT FROM c INTO @name END CLOSE c DEALLOCATE c -- create a scalar function to simulate the growth of the database in the drive's available space IF OBJECT_ID('tempdb..calculateAvailableSpace') IS NOT NULL EXEC tempdb.sys.sp_executesql N'DROP FUNCTION calculateAvailableSpace' EXEC tempdb.sys.sp_executesql N' CREATE FUNCTION calculateAvailableSpace( @diskFreeSpaceMB float, @currentSizeMB float, @growth float, @is_percent_growth bit ) RETURNS int AS BEGIN IF @currentSizeMB = 0 SET @currentSizeMB = 1 DECLARE @returnValue int = 0 IF @is_percent_growth = 0 BEGIN SET @returnValue = (@growth /128) * CAST((@diskFreeSpaceMB / (@growth / 128)) AS int) END ELSE BEGIN DECLARE @prevsize AS float = 0 DECLARE @calcsize AS float = @currentSizeMB WHILE @calcsize < @diskFreeSpaceMB BEGIN SET @prevsize = @calcsize SET @calcsize = @calcsize + @calcsize * @growth / 100.0 END SET @returnValue = @prevsize - @currentSizeMB IF @returnValue < 0 SET @returnValue = 0 END RETURN @returnValue END ' -- report database filegroups with less than 20% available space ;WITH masterfiles AS ( SELECT * ,available_space = CASE mf.max_size_mb WHEN -1 THEN tempdb.dbo.calculateAvailableSpace(mbfree, size_mb, growth, is_percent_growth) ELSE max_size_mb - size_mb END + emptyspace_mb FROM #masterfiles AS mf ), spaces AS ( SELECT DB_NAME(database_id) AS database_name ,data_space_name ,type_desc ,SUM(size_mb) AS size_mb ,SUM(available_space) AS available_space_mb ,SUM(available_space) * 100 / CASE SUM(size_mb) WHEN 0 THEN 1 ELSE SUM(size_mb) END AS available_space_percent FROM masterfiles GROUP BY DB_NAME(database_id) ,data_space_name ,type_desc ) SELECT * FROM spaces WHERE available_space_percent < 20 ORDER BY available_space_percent ASC IF OBJECT_ID('tempdb..#masterfiles') IS NOT NULL DROP TABLE #masterfiles; IF OBJECT_ID('tempdb..calculateAvailableSpace') IS NOT NULL EXEC tempdb.sys.sp_executesql N'DROP FUNCTION calculateAvailableSpace'
I am sure that there are smarter scripts around that calculate it correctly and I am also sure that there are other ways to obtain the same results (PowerShell, to name one). The important thing is that your script takes every important aspect into account and warns you immediately when the database space drops below your threshold, not when the available space is over.
Last time it happened to me it was a late saturday night and, while I really love my job, I can come up with many better ways to spend my saturday night.
I’m pretty sure you do as well.