Re: postgres large database backup

On 11/30/22 19:41, Michael Loftis wrote:
>
>
> On Wed, Nov 30, 2022 at 18:03 Mladen Gogala <gogala(dot)mladen(at)gmail(dot)com> wrote:
>
> On 11/30/22 18:19, Hannes Erven wrote:
>> You could also use a filesystem that can do atomic snapshots - like ZFS.
>
> Uh, oh. Not so sure about that. Here is a page from the world of the
> big O: https://blog.docbert.org/oracle-on-zfs/
>
> However, similar can be said about ZFS. ZFS snapshots will slow down
> the I/O considerably. I would definitely prefer snapshots done in
> hardware and not in software. My favorite file systems, depending on
> the type of disk, are F2FS and XFS.
>
>
> ZFS snapshots don’t typically have much if  any performance impact versus
> not having a snapshot (and already being on ZFS) because it’s already
> doing COW style semantics.
>
> Postgres write performance using ZFS is difficult because it’s super
> important to match up the underlying I/O sizes to the device/ZFS ashift,
> the ZFS recordsize, and the DB’s page/wal page sizes though, but not
> getting this right also cause performance issues without any snapshots,
> because again COW. If you’re constantly breaking a record block or sector
> there’s going to be a big impact. It won’t be any worse (in my own
> testing) regardless of if you have snapshots or not. Snapshots on ZFS
> don’t  cause any crazy write amplification by themselves (I’m not sure
> they cause any extra writes at all, I’d have to do some sleuthing)
>
> ZFS will yes be slower than a raw disk (but that’s not an option for Pg
> anyway), and may or may not be faster than a different  filesystem on a HW
> RAID volume or storage array volume. It absolutely takes more
> care/clue/tuning to get Pg write performance on ZFS, and ZFS does
> duplicate some of Pg’s resiliency so there is duplicate work going on.

I wonder what percentage of /Big Databases/ (like Op's and Vijaykumar's) are
still on physical servers, as opposed to VMs connected to SANs.  Even many
physical servers are connected to SANs.  (That is, of course, in the dreaded
Enterprise environment.)

--
Angular momentum makes the world go 'round.