Re: postgres large database backup

On 11/30/22 20:41, Michael Loftis wrote:
>
> 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.
>
Hi Michael,

I am not sure that such statement holds water. When a snapshot is taken,
the amount of necessary I/O requests goes up dramatically. For every
block that snapshot points to, it is necessary to read the block, write
it to the spare location and then overwrite it, if you want to write to
a block pointed by snapshot. That gives 3 I/O requests for every block
written. NetApp is trying to optimize it by using 64MB blocks, but ZFS
on Linux cannot do that, they have to use standard CoW because they
don't have the benefit of their own hardware and OS. And the standard
CoW is tripling the number of I/O requests for every write to the blocks
pointed to by the snapshot, for every snapshot. CoW is a very expensive
animal, with horns.

Regards

--
Mladen Gogala
Database Consultant
Tel: (347) 321-1217
https://dbwhisperer.wordpress.com