Re: zfs pool behavior - is it ever freed?

[Hauke Fath <hf%spg.tu-darmstadt.de@localhost>]

On Sat, 22 Jul 2023 07:55:41 -0400, Greg Troxel wrote:
> Using half the ram for pools feels like perhaps a bug, depending -- even
> if you are getting away with it.
> 
> I am curious:
> 
>   What VM approach?

An nvmm accelerated qemu

>   How much ram in the domU (generic term even if not xen)?

800 MB

>   Are you using NFS from the domU to dom0?  domU running zfs?  Something
>   else?

The machine is not running Xen - I found the Dom0 too limiting for the 
purpose.

It has a pair of SSDs (older intel SLC sata) for system partitions and 
L2ARC, and a pair of 4TB Seagate IronWolf disks for zfs storage, 
exported through nfs, afp, smb.

The VM nfs-mounts pkgsrc and distfiles from the base machine.	

>   Is the 16G for pools the sum of the dom0 and domU pools?  Or ?

That is for the base machine, so yes.

>> My guess would be that your 8 GB are simply not enough for sustaining 
>> both zfs and builds.
> 
> I think that's how it is, but it seems obviously buggy for that to be
> the case.  It is dysfunctional to run the system to lockup caching
> things that don't need to be cached.  The ffs vnode cache for example
> does not do this.

Agreed. But zfs has had a reputation of being memory-hungry for ~ever. 
And to my understanding NetBSD's zfs version is not exactly state of 
the art.

> The zfs howto currently talks about zfs taking 1G plus 1G per 1T of
> disk.  For me that would be 1.8G, which would be ok.  But that's not
> what happens.

I wonder: How much memory are you assigning to the Dom0? That alone 
would increase memory pressure compared to a native kernel.
 
> Thanks for the data point; I'll probably edit the zfs HOWTO.  As it is
> we should probably be recommending against zfs unless you have 64G of
> RAM :-( as even your system doesn't seem healthy memory usage wise.

That sounds overly pessimistic to me. I have run a department 
fileserver with dozens of clients (home on NFS) on 32 GB RAM for years 
- the same hardware that now serves as file and build server under 
NetBSD - without ever running into zfs related memory issues.

Your 8 GB machine would probably be just fine with zfs as a pure 
fileserver.

Cheerio,
Hauke

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