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Posts Tagged ‘Write on failed: 112(There is not enough space on the disk’

Interesting one today:

On a production box, the backup jobs have been failing with an interesting and perplexing error. Its says “Not enough disk space“; As you can guess, this is one of those confusing or misleading error messages that’s not what it seems on the surface — Making it worthwhile for a post of its own.

Detailed error message is below:

BACKUP DATABASE DummyDB
TO        DISK = N''
	, DISK = N''
	, DISK = N''
	, DISK = ''
WITH STATS = 1
GO
...
...
...
68 percent processed. 
69 percent processed. 
70 percent processed. 
Msg 3202, Level 16, State 1, Line 1 

Write on "F:\MSSQL\Backup\DummyDB.BAK" failed: 
112(There is not enough space on the disk.) 

Msg 3013, Level 16, State 1, Line 1 
BACKUP DATABASE is terminating abnormally.

This error occurs in both backups with & without compression; And in FULL & Differential backups.

This is a fairly large database, ranging up to 18 TB. So, backups are an ordeal to perform. So, when DIFF backups started failing, it was a bit concerning too.

After attempting several backups on local  & remote storage with plenty of space, a pattern still did not emerge. The only constant is that it fails around 70% completion progress.

At that point, one of  my colleagues (Thanks Michael) pointed out that, as part of backup operation, Sql Server will first run some algorithm that calculates the amount of space needed for the backup file. If the backup drive has enough free space well  and good, if not, it throws this error.

But, as you can guess, we had plenty of free space i.e. peta bytes of free space.

Occasionally, manual backups are successful. So, I’m not sure what is going on, but here is my theory:

At different points, Sql  Server  runs the algorithm (“pre-allocation algorithm”) to determine if there is enough space. Initially it comes back saying “yes” — and the backup proceeds with writing to the backup file; Again a little later, it checks, and it comes back with “Yes”; But at someone on subsequent checks (in our case between 70% to 72% complete), the algorithm decides there is  not enough disk space.

So, turns out there is a TRACE FLAG called 3042 that could disable this algorithm from making any assessments — that way backups could progress to completion.

From  MSDN:

Bypasses the default backup compression pre-allocation algorithm to allow the backup file to grow only as needed to reach its final size. This trace flag is useful if you need to save on space by allocating only the actual size required for the compressed backup. Using this trace flag might cause a slight performance penalty (a possible increase in the duration of the backup operation).

Using this trace flag might cause a slight performance penalty (a possible increase in the duration of the backup operation).

Caution: Manually make sure there is plenty of space for backup to complete — since we are disabling the algorithm.

--
-- Disable pre-allocation algorithm
--
DBCC TRACEON (3042)
GO

BACKUP DATABASE DummyDB
TO        DISK = N''
    , DISK = N''
    , DISK = N''
    , DISK = ''
WITH STATS = 1
GO
DBCC TRACEOFF (3042)
GO

Make sure you test this in a non-production environment, before enabling it in production.

Hope this helps,
_Sqltimes

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