# Saturday, 07 April 2007

Module 3 of the course 2780 is about backup and restore. When you combine this module with the database best-practice of separating the data files and transaction log files on different disk, you can even demonstrate how to recover from disk failure using the right backup and restore strategy without losing data... that is, recover if the failing disks contain the data files!!!

But what if the failing disk contains the transaction log files? Backup and restore are not going to help you to recover up to the moment of disaster. Sure, you can restore the database from last backup, but if last backup was on Sunday and the incident occurs Friday afternoon, about a weeks work is lost!

Yet we know all data is still sitting in the data file(s), the only sad part is, we can't bring the database up without the log file. So the question is, how to give the database its log file(s) back. Not knowing the answer from the top of my head, in which case I usually say "Let's see what happens if we stop the service and throw away a logfile", results were inconclusive and since (lack of) time is a major issue on the 2780, the subject of lost transaction log files landed on the todo-list.

/* BEGIN UPDATE 1 */

HALT, don't do anything yet if you have this problem in a production environment. Just read this post and the post Recovery continued, as there is no need to detach the failed database if your instance is still up and running.

/* END UPDATE 1 */

Now for the answer, which get's pretty complicated, depending on the state of your database. IF your database shutdown was clean:

Detach the damaged database. Then attach the data file(s) again, but remove the missing logfile from the list (see picture). This way, SQL Server recreates the logfile.

Depending on the state of the database SQL Server may even perform this operation at database startup. So you won't even notice it when you don't visit the log-files (Windows Application EventLog or SQL Server Activity log). Explains why results were inconclusive during class. This is easily tested (not on production databases, naturally) by stopping the database service, throwing away the logfile of a user database and starting the database service again. Either, you'll see the recreation of the transaction log in the Event log and Activity log, or you have to perform the detach and attach yourself.

Both for detaching and attaching, you may get error messages. However, these are warnings, the operations do complete successfully.

That was the easy part, but surely you've seen the big, fat IF! Databases don't shutdown clean if the log file crashes. To get a picture of what happens when a log-file is lost due to a disk crash, I created a database with the data file on disk (D:\SQL_DATA\LogCrash_Demo.mdf) and the transaction log on a memory stick (G:\SQL_DATA\LogCrash_Demo_Log.ldf). Next I added a table and started inserting new records. And... pull the stick out!

Msg 9001, Level 21, State 4, Line 1
The log for database 'LogCrash_Demo' is not available. Check the event log for related error messages. Resolve any errors and restart the database.
Msg 0, Level 20, State 0, Line 0
A severe error occurred on the current command. The results, if any, should be discarded.

After detaching the database and trying to attach (CREATE DATABASE ... FOR ATTACH_REBUILD_LOG) it again, the following error comes up;

File activation failure. The physical file name "G:\SQL_DATA\LogCrash_Demo_log.ldf" may be incorrect.
The log cannot be rebuilt because the database was not cleanly shut down.
Msg 1813, Level 16, State 2, Line 1
Could not open new database 'LogCrash_Demo'. CREATE DATABASE is aborted.

/* BEGIN UPDATE 2 */

I was going to look into what DBCC REBUILD_LOG could have done in this scenario and how that could be applied to SQL Server 2005... as of SQL Server 2005 DBCC REBUILD_LOG is no more and since DBCC REBUILD_LOG is an undocumented command, so the succession isn't documented either (if you care for an overview of DBCC changes, including "undocumented" stuff, look at Norb Technologies).

But just forget about DBCC REBUILD_LOG, Paul Randal is pretty clear about it (and all of a sudden it's gone in SQL Server 2005). Besides, the promised later post, how to get to the data in the data file is available too.

/* END UPDATE 2 */

UPDATED 2007-04-13

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