Re: ZFS reservations for type=volume

A dataset's reservation is local.  It doesn't contribute to its
parent's reservation.  Otherwise, you wouldn't be able to separately
set a reservation on the parent.  But it _does_ contribute to the
parent's "used" value.  In that way, you're prevented from reserving
too much data on the parent's children.
-Alan

On Thu, Apr 21, 2022 at 2:28 PM Michael Jung
<mikej@paymentallianceintl.com> wrote:
>
> I have a zfs block dataset raid-5400-1/esxi-store1 that I share as an iscsi target and that works great.  I have set a reservation
>
> on that block device equal to its size so that is not sparse and thus while I could over provision guests on the provided LUN,
>
> the storage presented as the LUN capacity will always be available.  At least this is what I want to achieve.
>
>
>
> What I find strange is that the reservation does not seem to be applied to the ZFS pool ‘raid-5400-1’.  Do you really need
>
> to set your maximum reservation at the pool level, and then apply reservations to all datasets on that volume?  And if so
>
> I would assume you could never set reservations for datasets totaling more than what was reserved for the pool ‘raid-5400-1’.
>
>
>
> I could build out a test environment and figure out constraints but I’d really like to know the “how it is supposed to work”
>
> not the “how I find it to work”.
>
>
>
> Thanks in advance.
>
>
>
> FreeBSD 14.0-CURRENT #4 main-n253875-8e72f458c6d:
>
>
>
> (this is a raidz2 pool – not my draid pool)
>
>
>
>
>
> root@draid:/usr/src/contrib/bearssl # zfs list -o name,type,used,avail,ratio,compression,reservation,refreservation,volsize raid-5400-1
>
> NAME         TYPE   USED  AVAIL  RATIO  COMPRESS        RESERV  REFRESERV  VOLSIZE
>
> raid-5400-1  filesystem  18.0T  2.08T  1.36x  on                none       none        -               <- no reservation @pool
>
> root@draid:/usr/src/contrib/bearssl #
>
>
>
> root@draid:/usr/src/contrib/bearssl # zfs list -o name,type,used,avail,ratio,compression,reservation,refreservation,volsize raid-5400-1/esxi-store1
>
> NAME                     TYPE   USED  AVAIL  RATIO  COMPRESS        RESERV  REFRESERV  VOLSIZE
>
> raid-5400-1/esxi-store1  volume  16.9T  18.5T  1.78x  zstd             15.6T      16.9T    15.6T        <- reservation @dataset
>
>
>
> root@draid:/usr/src/contrib/bearssl # zpool list
>
> NAME          SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
>
> ccache       9.50G  9.10G   406M        -         -    88%    95%  1.00x    ONLINE  -
>
> raid-5400-1  24.6T  1.52T  23.1T        -         -     2%     6%  1.00x    ONLINE  -                     <- Free does not reflect reservation @pool
>
> tank         18.5T   605G  17.9T        -         -     0%     3%  1.00x    ONLINE  -
>
> zfsroot       103G  33.3G  69.7G        -         -    31%    32%  1.00x    ONLINE  -
>
> root@draid:/usr/src/contrib/bearssl #
>
>
>
>
>
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