Re: RFC: How ZFS handles arc memory use

On Sun, Nov 02, 2025 at 11:48:06AM +0100, Alexander Leidinger wrote:
| Am 2025-10-29 22:06, schrieb Doug Ambrisko:
| > It seems around the switch to OpenZFS I would have arc clean task
| > running
| > 100% on a core.  I use nullfs on my laptop to map my shared ZFS /data
| > partiton into a few vnet instances.  Over night or so I would get into
| > this issue.  I found that I had a bunch of vnodes being held by other
| > layers.  My solution was to reduce kern.maxvnodes and vfs.zfs.arc.max so
| > the ARC cache stayed reasonable without killing other applications.
| > 
| > That is why a while back I added the vnode count to mount -v so that
| > I could see the usage of vnodes for each mount point.  I made a script
| > to report on things:
| 
| Do you see this also with the nullfs mount option "nocache"?

I seems to have run into this issue with nocache
  /data/jail/current/usr/local/etc/cups   /data/jail/current-other/usr/local/etc/cups     nullfs rw,nocache 0 0
  /data/jail/current/usr/local/etc/sane.d /data/jail/current-other/usr/local/etc/sane.d   nullfs rw,nocache 0 0
  /data/jail/current/usr/local/www        /data/jail/current-other/usr/local/www          nullfs rw,nocache 0 0
  /data/jail/current/usr/local/etc/nginx  /data/jail/current-other/usr/local/etc/nginx    nullfs rw,nocache 0 0
  /data/jail/current/tftpboot             /data/jail/current-other/tftpboot               nullfs rw,nocache 0 0
  /data/jail/current/usr/local/lib/grub   /data/jail/current-other/usr/local/lib/grub     nullfs rw,nocache 0 0
  /data/jail                              /data/jail/current-other/data/jail              nullfs rw,nocache 0 0
  /data/jail                              /data/jail/current/data/jail                    nullfs rw,nocache 0 0

After a while (a couple of months or more).  My laptop was running slow
with a high load.  The perodic find was running slow.  arc_prunee was
spinning.  When I reduced the number of vnodes then things got better.
My vfs.zfs.arc_max is 1073741824 so that I have memory for other things.

nocache does help taking longer to get into this situation.

Thanks,

Doug A.