Re: ZFS pool/filesystem space oddity

On Sun, 3 Mar 2024 05:33:02 -0700
"Edward Sanford Sutton, III" <mirror176@hotmail.com> wrote:

> On 3/3/24 04:58, Steve O'Hara-Smith wrote:
> > 	Hi,
> > 
> > 	My NAS runs a ZFS pool called archive striped over two mirrored
> > pairs of 4TB drives. It all works fine but I had a space eater recently
> > and so have been paying closer attention to space reports than normal -
> > which is why I noticed the oddity that the filesystems in the pool are
> > reported as having more free space than the pool.
> > 
> > 	Why is this ? Which is right ?
> > 
> >  From zpool iostat -v
> >                       capacity     operations     bandwidth
> > pool               alloc   free   read  write   read  write
> > -----------------  -----  -----  -----  -----  -----  -----
> > archive            4.38T  2.87T     59     52   880K  1.14M
> > 
> > One of the filesystems in the archive pool:
> > 
> > ✓ steve@holdall ~ $ df -H /data
> > Filesystem      Size    Used   Avail Capacity  Mounted on
> > archive/data    7.0T    4.0T    3.0T    57%    /data
> 
>    ZFS filesystems do not report space in a way that df expects and will 
> result in it giving oddities like a size that appears to adjust as 
> size=used+available and avail=realsize-. Space is further complicated 
> with topics like snapshots, refreservation, compression, etc. zpool also 
> shows space including reserved space that zfs and other tools would not 
> see/report as free while also understanding pool redundancy that is 
> outside the scope of the filesystem.

	All of these would lead me to expect zpool to show more space than
df rather than the other way round - hence my surprise.

-- 
Steve O'Hara-Smith <steve@sohara.org>