[zfs-discuss] RAIDZ-2 disk space

Andreas Dilger adilger at dilger.ca
Thu Apr 5 18:04:20 EDT 2018

On Apr 5, 2018, at 3:08 PM, Gandalf Corvotempesta <gandalf.corvotempesta at gmail.com> wrote:
> I have a RAIDZ-2 volume made with 4x480GB SSDs
> zpool list show me 1.73GB available space.
> AFAIL, zpool doesn't consider the RAIDZ, thus is showing me more or
> less 4x480GB as available space.
> zfs list is showing 647GB as avail, and 187GB as used, a total of
> 834GB. this is more or less the real usable space including the parity
> Some questions: why linux is seeing disks as 447GB (based on fdisk
> output) and not as 480GB ?

480GB = 480000000000 bytes / 1024 / 1024 / 1024 = 447 GiB

Disks are sold with decimal "GB" to make them look larger, but Linux
prints out the binary "GiB" since this is what it uses internally.

> The 647GB available, can be used fully or should I reserve about 20%
> for ZFS? (i've read somewhere that is not suggested to fill ZFS
> totally)

With a 4-disk RAID-Z2 VDEV you will only get at most 2x447 = 894 GiB
of usable space, and 647+187 = 834 matches this approximately (less
internal overhead).  I would recommend not going above 90% full on any
filesystem, due to fragmentation causing persistent performance issues.

Cheers, Andreas

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