Reading database transaction log with fn_dump_dblog()

Reading database transaction log with fn_dump_dblog()

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While the format of SQL Server transaction log is not publicly documented, there is a number of ways to view the contents of the log. This is sometimes necessary for troubleshooting and forensic purposes. One way is the DBCC LOG command. Another is fn_dblog() table-valued function. Both are undocumented, however you can easily find unofficial documentation on various web sites and blogs.

If you use Intellisense in SSMS 2008, you may notice another function, called fn_dump_dblog():

This function provides the same rowset as fn_dblog(), but has some interesting functionality that makes it useful is some troubleshooting and recovery scenarios. Specifically, it can read not only transaction log of the current database, but also transaction log backups on either disk or tape.

The first two parameters, @start and @end, can be used to filter the output by a range of LSN values (you need to make sure that the LSN values start with '0x'). This is similar to fn_dblog(). If you use default values for all other parameters, the fn_dump_dblog() function behaves just like fn_dblog(), returning a rowset over the log of the current database.

The third parameter, @devtype, is where the differences between fn_dump_dblog() and fn_dblog() start. This parameter determines the type of backup device. You can specify either 'DISK' or 'TAPE'. 'DISK' is the default.

The fourth parameter, @seqnum, is an integer that can be used to specify a particular backup within a backup device, when the same device has been used for multiple backups. Most backup devices contain only one backup, so usually this will be 1.

The fifth parameter is the fully-qualified path to the backup file on disk, when backup device type is 'DISK'. Note that Intellisense displays this parameter simply as @fname1. Note that the SQL Server service account will need read permission on the backup file.

The sixth parameter, displayed as @fname2, can be used to provide the name of a backup file in the default backup directory for the SQL Server instance.

All other parameters appear to be unused (please post a comment if you find otherwise). Update 2012-05-18: Paul Randal just blogged about the fn_dump_dblog() function - the rest of the parameters are used to specify multiple media families in a media set used for a log backup, i.e. a log backup striped over multiple backup files.

The fn_dump_dblog() function exists in SQL Server 2005/2008. Do note that the function is undocumented – use it at your own risk.

Update 2012-02-08: I just found out that the function can also work against database backups, not just log backups. A database backup contains a portion of the log that will be rolled forward on restore to make the restored database transactionally consistent, and that portion of the log can be viewed with the fn_dump_dblog() function. This is potentially useful to discover the LSN and the timestamp of the checkpoint that occurs during the backup - look for LOP_END_CKPT in the Operation column.

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