ZVOL fundamentals Query

Hi Folks

I am confused about a ZVOL i created in TRUE NAS. When i look at the properties of the ZVOL i see the following.


used 115G -
available 16.0T -
referenced 21.3G -
compressratio 1.33x -
reservation none default
volsize 100G local
volblocksize 32K -
checksum on default
compression lz4 inherited from VDEV1
refreservation 115G local
usedbysnapshots 0B -
usedbydataset 21.3G -
usedbychildren 0B -
usedbyrefreservation 93.7G -
My queries

  • How is it that used value at 115G is greater that vol size of 100G
  • How is Available 16T when ZVOL size is 100G
  • What is usedbyrefreservation and how do i reduce the size of it.
  • How do i find out what is the actual usage so that i know when to step in and increase the ZVOL Size
  • I am assuming compression
  • I think that is the total ZFS pool availability
  • not sure
  • In TrueNAS it shows under Pools

Thanks Tom.

  1. Compressions is enabled so ideally used space should be less. This ZVOL contains only a windows VM and actual used space is about 40GB (without compression). Total drive allocated inside the VM is 100GB (the size of the ZVOL).
  2. Yes you are right that is Pool availability and in TRUENAS under Pools it shows the same thing against the ZVOL. If this is the availability what is the point of setting the size of ZVOL at the time of creation.

Wow this is confusing.

usedbydataset and usedbyrefreservation added totals 115GB, which is your “used”.

usedbydataset is around where your Windows VM would be after compression based on your ratio.

There’s also a “referenced” 21.3G, which matches your usedbydataset.

Based on that, I’d guess usedbyrefreservation is “used by reference reservation” and is used in conjunction with compression.

The 115GB could be overprovisioning for slop by TrueNAS.