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Pavel
Asked: 2012-01-21 09:51:43 +0800 CST

ZFS, raidz1: Why is the volume size (42T) so much smaller than the pool size (50T)?

  • 772

I have a pool of 28 2TB-disks (56T) in 4 arrays of 7 disks. Since it's raidz1 (~RAID5), I'd expect 1 disk to be used for parity in each array, so the resulting volume should be 2TB*4*(7-1)=48TB, right?

Now, what I see on my system:

$ zpool list volume
NAME     SIZE  ALLOC   FREE  CAP  DEDUP  HEALTH  ALTROOT
volume  50.5T   308K  50.5T   0%  1.00x  ONLINE  -

$ zfs list volume
NAME     USED  AVAIL  REFER  MOUNTPOINT
volume  2.00T  40.3T  75.8K  /volume

$ df -h /volume
Filesystem             Size   Used  Available Capacity  Mounted on
volume                  42T    75K        40T     1%    /volume

So, there are only 42T instead of 48T. Where are the missing 6TB? And where does the number 50.5T come from?

zfs raid5 zpool raidz

3 Answers

  1. Best Answer

    A 2TB disk is not 2 TiBi in size - it's only 2*10^12 / 2^30 ~ 1862 GiBi.

    4 arrays of 6 effective disks each would be 24 * 1862 = 44703 GiBI, or 43.6 TiBi of real, usable storage.

    I reckon it has some additional overhead you're not taking into account - IIRC RAIDZ also does snapshots and scrubbing, which take up additional space.

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  2. To clarify the discrepancy in output between the commands:

    The zpool command counts the disks that are being used for redundancy as space, while the zfs command does not; thus, the 50.5 TB number is your raw disk size, while the 42T is after taking out the 4 disks for redundancy.

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  3. Hard drive manufactures measure disk size in Base 10. Computers measure bytes in Base 2.
    It should say 42 TiB to clarify the SI use of the tera- prefix.

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