[zfs-discuss] How can I determine used and free space?

Tim K tk-zol at idpax.com
Sun Sep 13 18:01:09 EDT 2015


I've been using ZoL for the past 18 months and I like it but one of the 
most confusing things to me is figuring out used and available space, in 
particular when using multiple datasets under the same pool, with 
snapshots too.

So here's my actual situation:

4 x 3TB WD Red HDDs in a RAID-Z2 pool

# zpool list nas
NAME   SIZE  ALLOC   FREE  EXPANDSZ   FRAG    CAP  DEDUP HEALTH  ALTROOT
nas   10.9T  4.86T  6.01T         -     3%    44%  1.00x ONLINE  -

The 10.9T number above makes some sense given how HDD manufacturers rate 
the space and some ZFS overhead/reserved space I assume. I understand 
this is the raw space in the pool, before RAID-Z2.

This is where I start getting lost:

# zfs list -r nas
NAME                   USED  AVAIL  REFER  MOUNTPOINT
nas                   2.36T  2.75T   256K  /nas
nas/archive            416G  2.75T   251G  /nas/archive
nas/backup             591G  2.75T   255G  /nas/backup
nas/data              11.5G  2.75T  9.66G  /nas/data
nas/fast_backup        516M  2.75T   198K  none
nas/fast_backup/home   516M  2.75T   507M  none
nas/imp                147G  2.75T   147G  /nas/imp
nas/media             1.22T  2.75T  1.06T  /nas/media

I'm not sure how much space I actually used and how much I still have 
available. Is it really that I only have USED + AVAIL = 5.11T usable 
space out of the raw 10.9T? Any way I look at it doesn't seem to make a 
lot of sense. Maybe it's the snapshots that I don't know how to take 
into account.

Any good resources that can help me make sense of the used and available 
space? Or maybe a kind soul could walk me through the numbers, please.

Thanks for your help.

Tim


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