Occurs when serializing output produced by RESTORE HEADERONLY, RESTORE FILELISTONLY, or RESTORE LABELONLY.
I asked him, my friend, what exact command he is running, and he informed that he is running below.
RESTORE FILELISTONLY FROM DISK='F:\Backups\ProductionDB.bak'
This is matching with documentation but what is “serializing output”? He informed that he was checking sys.dm_exec_requests DMV and filtering for the SPID which was running this command.
I asked him to check sys.sysprocesses and we found that there were two threads for that SPID. One of them was in a suspended state with wait type parallel_backup_queue and another was running and showing wait of BACKUPIO. I captured process monitor to check IO and I was surprised to see that the above command was reading the whole file in chunks. The database size was close it 1.5 TB.
So, I asked him to leave it running as it was reading the backup file. Finally, we found that it failed with an error:
The media family on device is incorrectly formed.
This made me believe that if there is a problem with the backup media, the commands would take a long time and we might see wait of parallel_backup_queue.
I have not been able to reproduce this in my lab even with 50 GB backup. If you find a way, please share via comments. Or if you have some other findings of Parallel Backup Queue, please share that as well.
Here is the link to the blog post where I discussed SQL Server Wait Stats in detail.
Reference: Pinal Dave (https://blog.sqlauthority.com)