🛠️SQL Server Administration: Monitoring the Database Logs

One of the aspects to monitor on a SQL Server instance is the size of the logs available for each database, respectively the degree to which the logs are used. Starting with SQL Server 2005 this could be achieved by using the 'Log File(s) Used Size (KB)' and 'Log File(s) Size (KB)'  counters via the sys.dm_os_performance_counters DMV as follows:

-- log files - size (kb)
SELECT lfu.instance_name database_name
, lfu.cntr_value size_kb
, Cast(lfu.cntr_value/1024.00 as decimal (18,2)) size_MB
FROM sys.dm_os_performance_counters lfu 
WHERE lfu.counter_name LIKE  'Log File(s) Size (KB)%' 
  AND lfu.object_name LIKE 'SQLServer:Databases%'
  AND lfu.instance_name IN ('tempdb', 'master', 'model', 'msdb')
ORDER BY lfu.instance_name

-- log files - used size (kb)
SELECT lfs.instance_name database_name
, lfs.cntr_value used_size_kb
, Cast(lfs.cntr_value/1024.00 as decimal (18,2)) used_size_MB
FROM sys.dm_os_performance_counters lfs
WHERE lfs.counter_name LIKE  'Log File(s) Used Size (KB)%' 
  AND lfs.object_name LIKE 'SQLServer:Databases%'
  AND lfs.instance_name IN ('tempdb', 'master', 'model', 'msdb')
ORDER BY lfs.instance_name

The two queries can be combined into one as follows:

-- database log space allocation (SQL Server 2005+)
SELECT db.name database_name
, db.log_reuse_wait_desc 
, Cast(lfs.cntr_value/1024.00 as decimal(28,2)) size_MB
, Cast(lfu.cntr_value/1024.00 as decimal(28,2)) AS used_MB
, Cast(100.00*lfu.cntr_value/lfs.cntr_value as decimal(10,2)) used_percent 
, CASE WHEN CAST(lfu.cntr_value AS float) / CAST(lfs.cntr_value AS float) > .5 THEN 
   CASE 
    WHEN db.name = 'tempdb' AND log_reuse_wait_desc NOT IN ('CHECKPOINT', 'NOTHING') THEN 'WARNING'  
    WHEN db.name <> 'tempdb' THEN 'WARNING' 
    ELSE 'OK' 
    END 
  ELSE 'OK' END log_status 
FROM sys.databases db 
     JOIN sys.dm_os_performance_counters lfs 
       ON db.name = lfs.instance_name 
      AND lfs.counter_name LIKE 'Log File(s) Size (KB)%' 
     JOIN sys.dm_os_performance_counters lfu 
       ON db.name = lfu.instance_name 
      AND lfu.counter_name LIKE  'Log File(s) Used Size (KB)%' 
WHERE db.name IN ('tempdb', 'master', 'model', 'msdb')
ORDER BY db.name 

Output:

database_name	log_reuse_wait_desc	size_MB	used_MB	used_percent	log_status
master	NOTHING	1.99	1.11	55.78	WARNING
model	NOTHING	7.99	1.74	21.80	OK
msdb	NOTHING	28.80	1.96	6.81	OK
tempdb	NOTHING	999.99	0.69	0.07	OK

Starting with SQL Server 2012 the same information can be obtained via the sys.dm_db_log_space_usage DMV, however the view returns information only for the current database:

-- getting the log space only for a database (SQL Server 2012+)
SELECT db_name(database_id) database_name 
, Cast(total_log_size_in_bytes/1024.00/1024.00 as decimal(28,2)) size_MB
, Cast(used_log_space_in_bytes/1024.00/1024.00 as decimal(28,2)) used_MB
, Cast(used_log_space_in_percent as decimal(28,2)) used_percent
FROM sys.dm_db_log_space_usage

Output:

database_name	size_MB	used_MB	used_percent
master	1.99	1.19	59.61

With less flexibility one can obtain the size in MB and the used percentage by using the DBCC utility as follows:

-- retrieving the log usage for all databases
DBCC SQLPERF(LOGSPACE); 

Output:

Database Name	Log Size (MB)	Log Space Used (%)	Status
master	1,992188	33,13726	0
tempdb	999,9922	0,06352589	0
model	7,992188	21,11437	0
msdb	28,80469	6,617846	0
AdventureWorks2014	33,99219	14,76672	0
AdventureWorksDW2014	17,99219	22,6878	0

Notes:
1. All the mentioned objects require VIEW SERVER STATE permissions.
2. The solution based on the performance counters returns slightly different values than the other solutions, though the differences are neglectable. 

Resources: 
[1] SQL Docs (2017) sys.dm_os_performance_counters [source]
[2] SQL Docs (2017) DBCC SQLPERF [source]
[3] SQL Docs (2017) sys.dm_db_log_space_usage [source]