Re: Setting up ZFS L2ARC on a zvol

On Wed, 26 Oct 2022, at 13:32, julio@meroh.net wrote:
> Hello,
> And here comes the question: can the L2ARC of the hard disk pool be 
> backed by a zvol on the NVMe pool (again, so that I don't have to use 
> fixed-size partitions)?

There's a sysctl that may enable this, but it's not advised. I
experimented with nested zpools a few years ago also:

https://forums.freebsd.org/threads/geli-encrypted-nested-zpool.59826/

https://reviews.freebsd.org/D4998 said:

> Change 294329 removed the ability to build ZFS pools that are backed
> by zvols, because having that ability (even if it's not used) leads
> to deadlocks. By popular demand, I'm adding an off-by-default sysctl
> to reenable that ability.

Your idea looks awfully similar to me. Of course in intervening years
this could well have changed.

Reserving some of your NVMe for L2ARC etc in a subsequent partition
seems like a good idea, less hassle than the above, albeit with a loss
of flexibility.

https://youtube.com/embed/yHgSU6iqrlE is old but may be helpful.

A+
Dave