Tuesday, October 1, 2013

Enable Instant File Initialization to accelerate database restore

Today my colleague come to me and ask me why her database restore query was hang. She was going to restore a database with 200GB data file and 11GB log file.

She had run the restore command for about 15 minutes, but the restore process is still in 0%, it is not the normal situation she knows. I connected to the server and checked the running query status, the "restore database" command was waiting for the "ASYNC_IO_COMPLETION"

then I checked the disk performance, the database file is still in writing, 110MB/sec

before the database restore start writing data back to the data file, the data file needs to be initialized first. if instant file initialization is enabled, this step will be skipped for data file(only data file), looks like instant file initialization is not enabled on this server.

In order to prove it, I run the script from http://blogs.msdn.com/b/sql_pfe_blog/archive/2009/12/23/how-and-why-to-enable-instant-file-initialization.aspx

DBCC TRACEON(3004,3605,-1)
EXEC sp_readerrorlog
DBCC TRACEOFF(3004,3605,-1)

in the error log file, I do see the
2013-10-01 18:31:05.56 spid104     Zeroing E:\Program Files\Microsoft SQL Server\MSSQL10_50.MSSQLSERVER\MSSQL\DATA\TestFileZero.mdf from page 0 to 131232 (0x0 to 0x40140000)

so the server is not enabled  instant file initialization. the data file is 200GB, disk write speed is 100-110MB/sec, we get : the file initialization will take about 33 minutes.

I told her just waited for half hour, then you would see the progress. and finally, she told me the restore estimate time started changing after 35 minutes :)

This case proves that enabling Instant File Initialization is really important for sql performance, including database restore.

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