Comment by rfjakob | Hacker News Reader

Comment by rfjakob | Hacker News Reader

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For user application it does not signal anything I guess, but I'm not sure if that's important for databases. Databases are usually crash-consistent, which is equivalent to a snapshot.

However, it does signal to the filesystem, quoting https://manned.org/fsfreeze.8 :

       fsfreeze is unnecessary for device-mapper devices. The device-mapper (and
       LVM) automatically freezes a filesystem on the device when a snapshot
       creation is requested. For more details see the dmsetup(8) man page.

No one is doubting that databases are crash consistent. But if you read the other replies in this thread you can see there are still advantages to doing the backup in cooperation with the userspace programs, eg in time taken to restore instead of having to do full WAL replay. That's ignoring the non-database cases too.

I think I saw Oracle mentioned earlier, this has some info about the VSS integration. It's called 'Oracle VSS Writer': https://docs.oracle.com/database/121/NTQRF/vss.htm#BEIJJCAE

I guess MS-SQL does something similar, and that does seem useful beyond what LVM does.

Also VSS apparently can offload the snapshotting to a SAN ('VSS Hardware Provider'): https://docs.microsoft.com/en-us/windows-server/storage/file...

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