Assume that you use Availability Groups in Microsoft SQL Server. When you use a Volume Shadow Copy Services (VSS)-based application to back up an availability database, the backup operation may Freeze the system I/O indefinitely. This causes the rendering of any other successive operations to wait until VSS explicitly issues a Thaw. If you try to run a transaction log backup of the same database during this time, you may receive the following error message:

BACKUP failed to complete the command BACKUP LOG database_name. Check the backup application log for detailed messages.

Note Before you receive this error message, you may receive an error message that resembles the following:

I/O is frozen on database database_name. No user action is required. However, if I/O is not resumed promptly, you could cancel the backup.


This fix is included in the following cumulative updates for SQL Server:

Cumulative Update 1 for SQL Server 2017

Cumulative Update 9 for SQL Server 2016 RTM

Cumulative Update 5 for SQL Server 2016 Service Pack 1

Cumulative Update 8 for SQL Server 2014 SP2

About SQL Server builds:

Each new build for SQL Server contains all the hotfixes and security fixes that were in the previous build. We recommend that you install the latest cumulative updates for SQL Server:

Latest cumulative update for SQL Server 2017

Latest cumulative update for SQL Server 2016

Latest cumulative update for SQL Server 2014


Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.


Learn about the terminology Microsoft uses to describe software updates.

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