Quick one today:
Rarely, but every once in a while, we see this entry in our production virtual database servers. Initially it seems benign, but upon further investigation we learnt that this is a repeating issue and there is an explanation for it.
spid125 I/O is frozen on database master. No user action is required. However, if I/O is not resumed promptly, you could cancel the backup
The same message could appear for all databases on this drive.
In short, this occurs when VSS backups are made through VDI.
Sql Server has a lot of APIs that enable independent software vendors to integrate Sql Server into their own products. Through these APIs, they carry out necessary functionality while maintaining the reliability, feature-functionality and performance of Sql Server. One such API interface is VDI (Virtual Device Interface); This works from within the VSS (Volume Shadow copy Service), through which 3rd party tools could perform backup & restore operations through BACKUPs using SNAPSHOT.
Using the VDI interface, when such command (BACKUP WITH SNAPSHOT) is executed, Sql Server needs to make sure that the backups are reliable and complete. Since data files are under constant lock by Sql Server with continuous updates going on, Sql Server has to temporarily cease all I/O into the data files to secure a volume snapshot.
Once that is complete, the same 3rd party tool tells Sql Server through VDI that I/O can now resume. This is usually for a short duration, but depending on the size of the file and the throughput of the drive, it could sometimes take longer.
In our environment, we see I/O freeze for a few seconds. This is not good. So we schedule these snapshot backups during daily scheduled down times.
2016-02-19 18:36:36.37 spid125 I/O is frozen on database master. No user action is required. However, if I/O is not resumed promptly, you could cancel the backup. 2016-02-19 18:36:47.91 spid125 I/O was resumed on database master. No user action is required.
Since this is internal Sql Server functionality that Microsoft controls, looks like they are working towards making it more efficient in future releases. Some Microsoft tools that rely on this are:
- Microsoft Backup (ntbackup.exe)
- Volume Shadow Copy (VSS)
- Data Protection Manager (DPM)
Some 3rd party tools that use VSS to perform such backup and restore operations are:
- Symantec Business Continuance Volume
- Veritas Net Backup
Hope this helps,