57,5G missing from pool

[dd_rambo]

Hi, I currently have a single disk (2Tb) stripe that has 127G free acording to zpool list but the Volume in it self reports only 69,5Gb of free space. Where has that 57,5Gb of space gone to?

I understand that if you for an example have a 3 disk raidz1 of 1Tb disk that zpool list will show you 3Tb while zfs list will show you 2Tb of AVAIL space.

But as I mentioned this is a single disk stripe so where are my 57,5Gb going?

Code:

zpool list VolGreen
NAME	   SIZE  ALLOC   FREE  EXPANDSZ   FRAG	CAP  DEDUP  HEALTH  ALTROOT
VolGreen  1.81T  1.69T   127G		 -	44%	93%  1.00x  ONLINE  /mnt

Code:

zfs list VolGreen
NAME	   USED  AVAIL  REFER  MOUNTPOINT
VolGreen  1.69T  69.5G	96K  /mnt/VolGreen

Code:

zpool status VolGreen
  pool: VolGreen
 state: ONLINE
  scan: scrub repaired 0 in 2h50m with 0 errors on Sun Jul 16 08:50:57 2017
config:

		NAME										  STATE	 READ WRITE CKSUM
		VolGreen									  ONLINE	   0	 0	 0
		  gptid/3d5481f0-f155-11e6-a627-000c296f98a8  ONLINE	   0	 0	 0

errors: No known data errors

 

[m0nkey_]

Please paste the output of zfs get used,logicalused,copies VolGreen using the [ code ] tags.

 

[dd_rambo]

@m0nkey_

Code:

zfs get used,logicalused,copies VolGreen
NAME	  PROPERTY	 VALUE   SOURCE
VolGreen  used		 1.69T   -
VolGreen  logicalused  1.69T   -
VolGreen  copies	   1	   default

 

[m0nkey_]

OK, I think I know what's going on. On *NIX based systems, Roughly ~5% of the available space is reserved for root. Doing back of the envelope math suggests this may be the case. Also, the space may be used by metadata.

Here's a Stack Exchange answer for the reserved space for root: https://unix.stackexchange.com/questions/7950/reserved-space-for-root-on-a-filesystem-why

 

[dd_rambo]

Hmm ok.
I read somewhere that ZFS also has a reservation for CoW (Copy-on-Write) wich should be fixed at 1/64th of the pool size, but this is more like 1/32th in my case.
I gues I have to accept that a portion of disk space will always be unusable to the user and will be reserved for the system in one way or the other.

Seems dumb though to have a fixed ratio for example 5% or 1/64th of the pool. What if you have a large pool say 72Tb. That reserved space is then going to "waste" alot of space.

 

[danb35]

Since your pool should really never be more than 80% full anyway, this shouldn't cause problems in the real world.

 

[dd_rambo]

Since your pool should really never be more than 80% full anyway, this shouldn't cause problems in the real world.

Yhe good thinking, BUT...
With a large portion of the space unavalible to the pool for user data you will hit that 80% much sooner ;)