HDD spin down and ZFS

[Mjasnik]

Hi,

I have an ZFS installation on Ubuntu 22.04, which is not being used as before and before it was 24x7, always on. When I added enterprise disks some years ago, oldest SATA was 11 years running non-stop. All disks running all the time doing some smaller and bigger jobs.
Now, due to some changes I have no need for full blown "thingy" as before, I have moved the system and have added "spare" SSD into the mix with ext4 and it does all the things I need now and which before ended up on on HDDs. So it is a relief to HDDs, but I need to keep all that ZFS mirror as safe storage deposit box, the rest light tasks as I said - moved to SSD.

In total there are 3 pools (all mirrors) on top of LUKS with bunch of datasets on partitions. One pool spans 2 partitions on 2 disks, other pool 3 partitions on 3 disks and last pool 3 partitions on 3 disks.

So to the point. I need the system to be quiet, it just cannot be noisy at that place for most of the time and you can imagine what 4 disks do when they spin - yes, they make noise.
So I am doing backup of my stuff once in a while and maybe someone would want to access some stuff on those disks once in a while, but I plan that they will be powered on not many times a week and when they will, it will not be for long, but I need it to be ready.

So, I though that putting them to sleep is a good idea. However it just does not really work with ZFS!
I have tested that disks just being there - spin down works, if I just decrypt the LUKS - spin down works, if I import ZFS pools or just one pool - spin down does not work. Actually while writing this, I have verified another thing - importing pool with readonly flag, allows disks to spin down :)

So I figured it is just plain ZFS thing and I just don't know why an imported rw pool that no-one is even accessing does not allow disks to spin down.
I have verified that every 10 minutes there are some funny writes to disks using iostat, but what it is or why it is doing so, I have no idea. So rw pool does not really sleep well, why?

Maybe someone has ideas whether is it possible at all and what can I do to put that on auto and forget about manual mount, unmount (+ NFS or SMB or whatever) every time someone may need smth from the pool for the backup purposes or smth.
Or I just have to remount rw before writing to it manually.

br,
Eduardo

[Mjasnik]

Ha,
I have found an issue about this #18082 . So it is essentially the same, as I describe above.
I am fairly certain that I looked everywhere for this behavior, but apparently not enough.
So closing the discussion.