Reducing the ZVOL refreservation size?

[Marek Salwerowicz]

Hi list,

My box is running FreeBSD 10.3-RELEASE-p11:
FreeBSD storage2 10.3-RELEASE-p11 FreeBSD 10.3-RELEASE-p11 #0: Mon Oct 
24 18:49:24 UTC 2016 
root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC amd64

More than 3 years ago I created a 20 TB ZVOL, to be used as an iSCSI 
drive for Win2k8 Server. Currently the Server uses around 7 TB of the 
storage space. However, the total space consumed by the ZVOL is around 
27TB (instead of desired 20TB).

What I have noticed is that the 'refreservation' size value does not 
decrease while the 'referenced' size value increases.

As I can predict, after filling the ZVOL with data, it will consume 
around 40 TB in total.

Is it safe to reduce the 'refreservation' from 20 TB to eg. 15 TB?   So 
that there will be around 5TB of storage space freed?


Please find below all details for the ZVOL:

#zfs get all tank1/PROD/WIN-Drive
NAME                   PROPERTY VALUE                  SOURCE
tank1/PROD/WIN-Drive  type                  volume -
tank1/PROD/WIN-Drive  creation              Wed May 21 10:53 2014 -
tank1/PROD/WIN-Drive  used                  27.6T -
tank1/PROD/WIN-Drive  available             26.9T -
tank1/PROD/WIN-Drive  referenced            6.96T -
tank1/PROD/WIN-Drive  compressratio         1.00x -
tank1/PROD/WIN-Drive  reservation           none default
tank1/PROD/WIN-Drive  volsize               20T local
tank1/PROD/WIN-Drive  volblocksize          8K -
tank1/PROD/WIN-Drive  checksum              on default
tank1/PROD/WIN-Drive  compression           off default
tank1/PROD/WIN-Drive  readonly              off default
tank1/PROD/WIN-Drive  copies                1 default
tank1/PROD/WIN-Drive  refreservation        20.6T local
tank1/PROD/WIN-Drive  primarycache          all default
tank1/PROD/WIN-Drive  secondarycache        all default
tank1/PROD/WIN-Drive  usedbysnapshots       48.8G -
tank1/PROD/WIN-Drive  usedbydataset         6.96T -
tank1/PROD/WIN-Drive  usedbychildren        0 -
tank1/PROD/WIN-Drive  usedbyrefreservation  20.6T -
tank1/PROD/WIN-Drive  logbias               latency default
tank1/PROD/WIN-Drive  dedup                 off default
tank1/PROD/WIN-Drive  mlslabel -
tank1/PROD/WIN-Drive  sync                  standard default
tank1/PROD/WIN-Drive  refcompressratio      1.00x -
tank1/PROD/WIN-Drive  written               18.0G -
tank1/PROD/WIN-Drive  logicalused           6.96T -
tank1/PROD/WIN-Drive  logicalreferenced     6.91T -
tank1/PROD/WIN-Drive  volmode               default default
tank1/PROD/WIN-Drive  snapshot_limit        none default
tank1/PROD/WIN-Drive  snapshot_count        none default
tank1/PROD/WIN-Drive  redundant_metadata    all default


Thanks in advance for your help


Cheers


Marek


-- 
Marek Salwerowicz