Multiple Time Machine and quotas

[Dotty]

I know if you have one Time Machine share and put a quota on the Dataset, the clients will start dropping the older backups as the HDD becomes full (or quotas reach the limits).
What happens if there are multiple Time Machine shares on the same box, with quotas adding up to more than the actual available HDD space, when the freeNAS box actual pool gets full?

Lets say the pool is 10 TB and I have 20 users.
I give each user 1 TB quota on their own AFP share for Time Machine.
Eventually I will run out of the 10TB volume, for maybe one of the user is using only 200GB space.

What will happen here?

(I know, someone will say "never assign quotas that total to more the space on the pool", but Im just curious what would happen in the case above)

 

[Ericloewe]

Easy solution is to make those datasets all child datasets of a dataset on which you set the absolute max quota for everyone.

Otherwise, nastiness happens and ponies will be sad.

 

[Dotty]

Easy solution is to make those datasets all child datasets of a dataset on which you set the absolute max quota for everyone.

Otherwise, nastiness happens and ponies will be sad.

I though about the nested datasets, but the question remains.
Lets say the pool is 20TB and the parent dataset is 10TB.
What happens on the Mac using 200GB when all the others used already the 10TB of the parent dataset?
(hint: the little Mac doesnt know the parent Dataset is full, since its quota is 1TB and it cant see it from within its dataset)

 

[Ericloewe]

As is typical for time machine, I fully expect it to crash, burn and set a flag saying that all previous backups are corrupted even though they are not.

 

[Ericloewe]

More realistically, I expect that ZFS will quote the smallest of the quotas or available disk space.

 

[Dotty]

More realistically, I expect that ZFS will quote the smallest of the quotas or available disk space.

It doesnt,
Parent Quota: 250GB
Child1 Quota:250GB
Child2 Quota:250GB
Mac1 pointing to TimeMachine Child1 : see quota as 250GB
Mac2 pointing to TimeMachine Child2 : see quota as 250GB

Test 1 - Dummy 50GB file on Child1 share.
--- Mac1 sees the available space on Time Machine reduced by aprox 50GB
--- Mac2 still sees 250GB available on Time Machine
So, chances are that Mac1 will eat all space from the Parent, so Mac2 will keep trying to fill up 250GB that doesnt exist.
:-(

I know, the parent should be reserving about 600GB (potential size plus 20%).