2 ms·

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 too

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.

rwmj 5y ago
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 https://www.youtube.com/watch?v=9E5A608xJG0)
- rfjakob 5y ago
  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 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.
  - rwmj 5y ago
    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.
    - rfjakob 5y ago
      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 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-server/volume-shadow-copy-service https://docs.microsoft.com/en-us/windows-server/storage/file...

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.

rwmj 5y ago

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 https://www.youtube.com/watch?v=9E5A608xJG0)

rfjakob 5y ago
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 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.
- rwmj 5y ago
  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.
  - rfjakob 5y ago
    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 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-server/volume-shadow-copy-service https://docs.microsoft.com/en-us/windows-server/storage/file...