Confused on zvol size usage (too)

[EtienneB]

I am lost with the ZVOL sizes, like many I read

The ZVOL for the 2 VMs were set at 20 and 48GB respectively upon creation (see table below).

I assume with the compression of 1.86 the 20GB turns into 36GB and the 48GB with 1.51 into 79GB?

But where does the Available space of these Volumes come from? That would turn my VMs in to 36+42=78GB and 79+64 = 143GB sizes, way different from the set 20GB and 48GB.
Inside the VM's, Ubuntu sees disk spaces of 20GB and 48GB only, so it can't expand there anyway automatically.
Snapshot usage of the VMs is around 4Gb, so not really of influence here.

I think the Available space of the VOLUME reduces the overall Available space on the pool?
I would like to reduce the free space of the VMs in order to have more overall free space on the pool.

I am lost here how free space works/manage that. Should I have set Force under the zvol option? Or set -S refreservation?

I ran into out-of-disk space on this pool last night, so I wiped 3 snapshots of around 5-9Gb each to get going again but I think the VMs are a big culprit here. Any explanations and suggestions are welcome.

I did read these links:

ZVOL why is it so confusing?

Hi Folks I am confused about a ZVOL i created in TRUE NAS. When i look at the properties of the ZVOL some the properties that are puzzling me are shown below. This is from the output of zfs get all . PROPERTY VALUE SOURCE used 115G - available 16.0T -...

www.truenas.com

avail size mismatch : zpool list vs zfs list

I've created a 1Tb RAID-1 pool named NASBackup, and enabled dedup & compression. I'm getting confusing numbers for the available size on the pool vs the dataset. Pool has 209G available, dataset has only 77G available. There is only one dataset in the pool. [root@freenas ~]# zpool list...

www.truenas.com

Reservation & Ref Reservation - An Explanation (Attempt)

So in this article I'm going to try to explain and answer a lot of the questions I get and misconceptions I see in terms of ZFS and space ut...

nex7.blogspot.com

ssdpool(System Dataset Pool)ONLINE | 199.69 GiB (92%) Used | 17.31 GiB Free

Name	Type	Used	Available	Compression	Compression Ratio
ssdpool	FILESYSTEM	199.69 GiB	17.31 GiB	zstd	1.70
iocage	FILESYSTEM	61.06 GiB	17.31 GiB	Inherits (zstd)	2.00
share	FILESYSTEM	22.48 GiB	17.31 GiB	Inherits (zstd)	1.03
virtual-machines	FILESYSTEM	115.44 GiB	17.31 GiB	Inherits (zstd)	1.60
freenasubuntu-w5xza	VOLUME	36.46 GiB	42.53 GiB	Inherits (zstd)	1.86
truenaspython-ev6kqg	VOLUME	78.98 GiB	64.21 GiB	Inherits (zstd)	1.51

 

[Etorix]

"Free space" is not an easy notion in ZFS.

199.69 GiB used means just that: You have nearly 200 GiB of data in this pool (current data, older data in snapshots and metadata). "Compression ratio 1.70" means this would use 340 GiB if it were not compressed.
Inside your zvols, the guest OS sees exactly the space that was allocated to it—irrespective of how ZFS manages to compress it when storing on drive, or of how much ZFS has to store because it still retains data from older snapshots of the dataset.
I suspect you confuse the space occupied by snapshot metadata with the size of the data they refer to. 4 GB of snapshot metadata potentially refers to A LOT of (badly fragmented) data.

Incidentally, 92% is way too high for block storage—less than 50% is recommended for such applications.

 

[EtienneB]

Thanks, after deleting the snapshots I went from 500Kbytes to this 17Gb.

The impression I have that the 42 and 62 GB Available is somehow limiting the iocage e.g. is untrue?

BTW, the cli gives this info

Code:

NAME      SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
ssdpool   224G   128G  95.9G        -         -    56%    57%  1.00x    ONLINE  /mnt

 

[EtienneB]

As I have an external backup, I decided to remove all snapshots of this pool.
And indeed it freed up a lot more than I expected. The 4GB of snapshot space, has a lot of more underlying space, like 40Gb. Thanks for that!

ssdpool(System Dataset Pool)ONLINE check_circle | 129.98 GiB (60%) Used | 87.02 GiB Free

Name	Type	Used	Available	Compression	Compression Ratio	Readonly	Dedup
ssdpool	FILESYSTEM	129.98 GiB	87.02 GiB	zstd	1.54	false	OFF
iocage	FILESYSTEM	32.76 GiB	87.02 GiB	Inherits (zstd)	1.84	false	OFF
share	FILESYSTEM	22.48 GiB	87.02 GiB	Inherits (zstd)	1.03	false	OFF
virtual-machines	FILESYSTEM	74.15 GiB	87.02 GiB	Inherits (zstd)	1.53	false	OFF
freenasubuntu-w5xza	VOLUME	25.39 GiB	105.65 GiB	Inherits (zstd)	1.79	false	OFF
truenaspython-ev6kqg	VOLUME	48.75 GiB	107.24 GiB	Inherits (zstd)	1.47	false	OFF
website	FILESYSTEM	1.04 MiB	87.02 GiB	Inherits (zstd)	1.56	false	OFF

and the CLI:

Code:

NAME      SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
ssdpool   224G  91.1G   133G        -         -    46%    40%  1.00x    ONLINE  /mnt

(the CLI says only 40% cap utilization, the GUI 60%)

The only thing I can't get my head around to is the 105Gb and 107Gb of Available space, given the 87GB of the parent Virtual-Machines.