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Old 03-23-2016, 06:19 AM   #1
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ZFS RAID levels on Centos 7.2

Hey guys!

I have a monster server I'm provisioning at the minute.

There are 15 x 8TB HDD's connected to a SATA interface card that i'm using to create a ZFS volume. The drives are all detected and I've got the ZFS libraries installed and they're ready etc.

I created my ZFS volume using zpool. I opted for RAIDZ2, as I want double parity for the extra fault tolerence...

zpool create -f diskpool1 raidz2 sdc sdd sde sdf sdg sdh sdi sdj sdk sdl sdm sdn sdo sdp sdq
and if I check the status, thats all good...

[root@BACKUPNAS-I ~]# zpool status
  pool: diskpool1
 state: ONLINE
  scan: none requested

        NAME        STATE     READ WRITE CKSUM
        diskpool1   ONLINE       0     0     0
          raidz2-0  ONLINE       0     0     0
            sdc     ONLINE       0     0     0
            sdd     ONLINE       0     0     0
            sde     ONLINE       0     0     0
            sdf     ONLINE       0     0     0
            sdg     ONLINE       0     0     0
            sdh     ONLINE       0     0     0
            sdi     ONLINE       0     0     0
            sdj     ONLINE       0     0     0
            sdk     ONLINE       0     0     0
            sdl     ONLINE       0     0     0
            sdm     ONLINE       0     0     0
            sdn     ONLINE       0     0     0
            sdo     ONLINE       0     0     0
            sdp     ONLINE       0     0     0
            sdq     ONLINE       0     0     0

errors: No known data errors
But if I check the disk space, I'm only showing 87Tb

[root@BACKUPNAS-I ~]# df -h
Filesystem               Size  Used Avail Use% Mounted on
/dev/mapper/centos-root   50G  3.0G   47G   6% /
devtmpfs                 7.7G     0  7.7G   0% /dev
/dev/mapper/centos-home  154G   54M  154G   1% /home
/dev/md126p1             497M  188M  310M  38% /boot
diskpool1                 87T  256K   87T   1% /diskpool1

I know the raid levels vary from the RAID standards when using ZFS, due to its origins, however I anticipated that I'd have around 104TB usable with a RAID6-like configuration, giving me a fault tolerance of 2 disks in the pool.

Am I not doing something correctly, or is it simply the case that using what is essentially 'software RAID' with ZFS uses takes up alot of space (2 or so of my 8TB HDD's worth!)

Thanks in advance!
Old 03-24-2016, 12:59 AM   #2
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A fair proportion of what you see as "missing" is you falling for sales-speak. They sold you 8TB drives (8x1000*1000*1000), and Linux tells you you have something less than 8TiB. See wikipedia for terabyte vs tibibyte.
Essentially you take the number you were sold and divide by 1024^3 - you "lose" 7% into the ether. Then there is filesystem overhead on top of that. Wouldn't matter if it was ZFS or not. Basically you just have to suck it up.
Old 03-27-2016, 09:36 PM   #3
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Are you sure all the disks are 8TB?

15x8TB disks in a raidz2 configuration are leaving 13*8TB, roughly 96TiB. You only see 87TiB so there is something wrong. ZFS does have some overhead, but nothing that significant on an empty file system.

Please post the output of these commands:

dmesg | grep "logical blocks"
zpool list
zfs list


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