[lustre-discuss] ZFS zpool/filesystem operations while mounted with '-t lustre'

[Peter Grandi]

>>> On Thu, 18 May 2023 15:13:24 +0100, Peter Grandi via lustre-discuss <lustre-discuss at lists.lustre.org> said:

>> You might want to take a look at this: https://www.opensfs.org/wp-content/uploads/2017/06/Wed06-CroweTom-lug17-ost_data_migration_using_ZFS.pdf
> I was indeed reading that but I was a bit hesitant because the 
> "zpool"/"zfs" operations are bracketed by 'service lustre stop 
> ...'/'service lustre start ...' commands which I hope to avoid.

So doing a snapshot of the MDT "filesystem" and then doing 'zfs
send' of it just works well. I think that:

* It may be important that the shapshot be read-only

* The snapshotting should be preceded by 'barrier_freeze' and
  followed by 'barrier_thaw'.

* It is also useful to have the option '-s' on 'zfs receive' to
  make the 'zfs send' restartable.

* Probably the snapshot can be done *without* a 'barrier_freeze'
  first, then once it is done one can do 'barrier_freeze', then
  take another snapshot, then 'barrier_thaw' and then send that
  very short incremental, to minimize the time where the Lustre
  instance is frozen.

I will find the time to request an update of the backup sexction
of the Lustre operations manual to document this.

With the ZFS backend improvements in recent years I guess that
ZFS rather than 'ldiskfs' should currently be the default
storage backend for MDTs too.