Tuesday, April 23, 2013

TSQL to identify databases with high number of VLFs

If you aren't aware there can be significant impact to performance when a database has a high number of VLFs within it's Transaction Log. SQL Server divides each physical transaction log file internally into a number of virtual log files (VLFs) and it is these that contain the log records.  There is no fixed size for a VLF, and there is no fixed number which would be created within a physical log file. These values are all determined dynamically by SQL Server when creating or extending physical log files.  While SQL Server tries to maintain a small number of VLFs for the best performance, file growth can result in a database having a large number of VLFs because the following equation will be used to determine the number of files to create within the new space.

- Less than 64MB = 4 VLFs
- Greater than 64MB and less than 1GB = 8 VLFs
- Greater than or equal to 1GB = 16 VLFs

So for example if I initially create a 1GB transaction log file it will be divided into 16 64MB VLFs, then if it grew a few times by 10 Mb before being switched to 10% the following trend would be experienced and result in a quickly increasing number of VLFs when then compared to manually growing the file:

ActionSpace AddedVLFs CreatedTotal SizeTotal VLFs
Initial Size1024 Mb161024 Mb16
Auto Grow10 Mb41034 Mb20
Auto Grow10 Mb41044 Mb24 
Auto Grow10 Mb41054 Mb28
Auto Grow10 Mb41064 Mb32
Auto Grow106 Mb81170 Mb40
Auto Grow117 Mb81287 Mb48
 Auto Grow128 Mb81415 Mb56
 Auto Grow141 Mb81556 Mb64
 Auto Grow155 Mb81711 Mb72
 Auto Grow171 Mb81882 Mb80
 Auto Grow188 Mb82070 Mb88

The only way to reduce the number of VLFs is to then Shrink the file and manually grow the file by a set size such as:

ActionSpace AddedVLFs CreatedTotal SizeTotal VLFs
Initial Size1024 Mb161024 Mb16
Manual Grow1024 Mb162048 Mb32
Auto Grow204 Mb82252 Mb 40

Ideally you should keep the number of VLFs to as small as possible. In SQL 2012 there is now a warning raised when a database has >10,000 VLFs, although there is no warning available in earlier versions of SQL Server. You can use the following TSQL though to report on the number of VLFs per database within your SQL Instance. Then if you compare this with your auto-grow settings for the database you can determine the reason for why the count is the way it is.

SET NOCOUNT ON;
/* declare variables required */DECLARE @DatabaseId INT;
DECLARE @TSQL varchar(MAX);
DECLARE cur_DBs CURSOR FOR
SELECT database_id FROM sys.databases;
OPEN cur_DBs;
FETCH NEXT FROM cur_DBs INTO @DatabaseId

--These table variables will be used to store the data
DECLARE @tblAllDBs Table (DBName sysname
   , FileId INT
   , FileSize BIGINT
   , StartOffset BIGINT
   , FSeqNo INT
   , Status TinyInt
   , Parity INT
   , CreateLSN NUMERIC(25,0)
)IF '11' = substring(convert(char(12),serverproperty('productversion')), 1, 2)
BEGIN   DECLARE @tblVLFs2012 Table (RecoveryUnitId BIGINT
      , FileId INT
      , FileSize BIGINT
      , StartOffset BIGINT
      , FSeqNo INT
      , Status TinyInt
      , Parity INT
      , CreateLSN NUMERIC(25,0)
   );
END
ELSE
BEGIN
   DECLARE @tblVLFs Table (
      FileId INT
      , FileSize BIGINT
      , StartOffset BIGINT
      , FSeqNo INT
      , Status TinyInt
      , Parity INT
      , CreateLSN NUMERIC(25,0)
   );
END
--loop through each database and get the info
WHILE @@FETCH_STATUS = 0
BEGIN   PRINT 'DB: ' + CONVERT(varchar(200), DB_NAME(@DatabaseId));
   SET @TSQL = 'dbcc loginfo('+CONVERT(varchar(12), @DatabaseId)+');';
   IF '11' = substring(convert(char(12),serverproperty('productversion')), 1, 2)
   BEGIN
      DELETE FROM @tblVLFs2012;
      INSERT INTO @tblVLFs2012
      EXEC(@TSQL);
      INSERT INTO @tblAllDBs
      SELECT DB_NAME(@DatabaseId)
         , FileId
         , FileSize
         , StartOffset
         , FSeqNo
         , Status
         , Parity
         , CreateLSN
      FROM @tblVLFs2012;
   END
   ELSE
   BEGIN
      DELETE FROM @tblVLFs;
      INSERT INTO @tblVLFs
      EXEC(@TSQL);
      INSERT INTO @tblAllDBs
      SELECT DB_NAME(@DatabaseId)
         , FileId
         , FileSize
         , StartOffset
         , FSeqNo
         , Status
         , Parity
         , CreateLSN
      FROM @tblVLFs;
   END
   FETCH NEXT FROM cur_DBs INTO @DatabaseId
ENDCLOSE cur_DBs;
DEALLOCATE cur_DBs;

--just for formating if output to Text
PRINT '';
PRINT '';
PRINT '';

--Return the data based on what we have found
SELECT a.DBName
    , COUNT(a.FileId) AS [TotalVLFs]
    , MAX(b.[ActiveVLFs]) AS [ActiveVLFs]
    , (SUM(a.FileSize) / COUNT(a.FileId) / 1024) AS [AvgFileSizeKb]
FROM @tblAllDBs a
INNER JOIN (
    SELECT DBName
        , COUNT(FileId) [ActiveVLFs]
    FROM @tblAllDBs
    WHERE Status = 2
    GROUP BY DBName
) b
ON b.DBName = a.DBName
GROUP BY a.DBName
ORDER BY TotalVLFs DESC;

SET NOCOUNT OFF;


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1 comment:

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