Hacker News Reader

Comment by rfjakob | Hacker News Reader

I'm not sure I follow here. Linux does have LVM, what's the problem with it?

That's also what the article says: If you have LVM, you don't need any special tool.

LVM doesn't signal to databases and other user applications so it's definitely not a replacement for VSS. As for whether LVM is a replacement for nbdkit, I guess it depends on if you like writing kernel code, or prefer instead to write nbdkit plugins in userspace in a variety of programming languages (even shell script: https://www.youtube.com/watch?v=9E5A608xJG0)

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...

Reply on news.ycombinator.com