Application hangs and sql log shows "I/O is frozen" for minutes until "I/O was resumed"

Question

VSS is being invoked to backup our SQL Server 2012 databases throughout the day.  Occasionally, maybe once every few days, the application using the database hangs for a few minutes and coincidentally the sql server log shows that I/O was frozen for
a few minutes (rather than less then a second for all other backups):
11/25/2014 09:11:50,spid87,Unknown,I/O was resumed on database ourdatabase. No user action is required.
11/25/2014 09:09:10,spid87,Unknown,I/O is frozen on database ourdatabase. No user action is required. However<c/> if I/O is not resumed promptly<c/> you could cancel the backup.
AppAssure (backup software) support said this may be an issue with VSS.  What is the best way to go about troubleshooting this?  I've read many posts with people having similar issues dating back to 2010 and haven't really come across any
solutions.  Should I simply open a case with MS support?
SQL Server 2012
Windows Server 2012 2 node cluster
Database file on Dell Equallogic iCSCI SAN
Dell AppAssure for Backups
Thanks,
-Bob

Answer 1

Hi Bob,
AFAIK yes this issue of ''I/O was frozen'' might occur when you use backup tool which uses VSS to take backups. This is actually how VSS backups works . There are few steps when backup is being taken using TP tool which uses VS. One of the steps is to create
snapshot so that consistent backup could be taken
In the snapshot creation phase following happens
a) SQL Writer talks to SQL Server to prepare for a snapshot backup
b) Then all I/O for the database being backed up is frozen, and then we create the snapshot.
c) Once this is done, the I/O is resumed. This process is called "thaw".
The second step would freeze I/O momentarily and you might see little freezing. AFAIK this is default behavior how VSS backups work
You must read below excellent article It would help you in understanding how this backup works
http://blogs.msdn.com/b/sqlserverfaq/archive/2009/04/28/informational-shedding-light-on-vss-vdi-backups-in-sql-server.aspx
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