What happens when SQL query is executing prior to snapshot backup and query executed during snapshot backup?

Question

During vss based snapshot backup, databases IO is frozen one by one. And subsequently resumed.

Questions:

1. When there is a query running, and vss based snapshot backup is executed, will the freeze get delayed (that is - wait for running query to complete)?

2. Assuming the freeze and resume interval is 60 seconds. What happens to user queries fired during this 60 seconds interval. Are they queued? Or rejected?

Answer 1

summer snow

The post I wrote linked in the comments has a follow up:

• Wait Stats When VSS Snaps Are Slow

In the post are a couple DBCC commands you can use to test what happens to queries during a snapshot:

DBCC FREEZE_IO('Crap');
DBCC THAW_IO('Crap');

Of course, modification queries are blocked by frozen I/O, but I couldn't get a read only query to be blocked by it. I think one thing I omitted in testing was reading pages from disk rather than pages that were already in the buffer pool, which is left as an exercise to the reader at this point.