Is I/O Frozen in VSS backup increase base on the size ?

Question

Hello,

I find out while performing VSS method backup process, 2 error message appear in ERROR log:-

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

These 2 messages gap was less than a second.
However I would like to know if increasing of database size, will this I/O frozen time increase ?
And then if I configure Mirroring, SQL Always-On, Multi A-Z, any impact to I/O frozen time ?

Appreciate your input.

Answer 1

Hello @KC2022JAN-0951 ,

Thank you for reaching out to the Microsoft Q&A platform. Happy to answer your question.

This sequence of messages would be present for all databases which got backed-up. These messages indicate that SQL Server has frozen the IO so that snapshots can be taken for the volume. This is coordination between VSS and SQL Server so that consistent database backup is taken across the volumes.

Message 1: I/O is frozen on database XXXX. No user action is required. However, if I/O is not resumed promptly, you could cancel the backup.
I/O was resumed on database XXXX. No user action is required.

Message 2: I/O was resumed on database XXXX. No user action is required.

Answering the particular questions:

Q) I would like to know if increasing of database size, will this I/O frozen time increase?
The total time taken for I/O freeze doesn't depend on the size of the DB, it depends on the rate at which the activity of SQL transactions that happens on the DB. If the SQL DB is too busy processing the transactional data(reads/writes) then the I/O freeze might experience failures.

Q) If I configure Mirroring, SQL Always-On, Multi A-Z, any impact to I/O frozen time?
No, it won't have any direct impact on I/O frozen time. As mentioned in the above answer, it depends on how busy the server is. Please see the below explanation on what happens during I/O frozen time during a VSS snapshot.

What happens during a VSS snapshot?
The Volume Shadow Copy Service tells the writers to temporarily freeze application write I/O requests (read I/O requests are still possible) for few seconds that are required to create the shadow copy of the Databases. The application freeze is not allowed to take longer than 60 seconds.

Hope this helps!

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