raidz2 a bit big

[Brooks Davis]

On Fri, May 22, 2009 at 02:05:39PM -0700, Randy Bush wrote:
> a dozen 2tb drives in a raidz2
> 
> dfw1.psg.com:/root# zpool status
>   pool: tank
>  state: ONLINE
>  scrub: none requested
> config:
> 
>         NAME        STATE     READ WRITE CKSUM
>         tank        ONLINE       0     0     0
>           raidz2    ONLINE       0     0     0
>             da0s3   ONLINE       0     0     0
>             da1s3   ONLINE       0     0     0
>             da2s1   ONLINE       0     0     0
>             da3s1   ONLINE       0     0     0
>             da4s1   ONLINE       0     0     0
>             da5s1   ONLINE       0     0     0
>             da6s1   ONLINE       0     0     0
>             da7s1   ONLINE       0     0     0
>             da8s1   ONLINE       0     0     0
>             da9s1   ONLINE       0     0     0
>             da10s1  ONLINE       0     0     0
>             da11s1  ONLINE       0     0     0
> 
> errors: No known data errors
> 
> i expected to get 10T effective because of the raidz2, but got 20T
>                                                     ^
> dfw1.psg.com:/root# df -H
> Filesystem        Size    Used   Avail Capacity  Mounted on
> /dev/da0s1a        17G    3.8G     12G    25%    /
> devfs             1.0k    1.0k      0B   100%    /dev
> /dev/md0          130M     90k    119M     0%    /tmp
> tank               19T      0B     19T     0%    /tank
> tank/usr           19T      0B     19T     0%    /tank/usr
> tank/usr/home      19T      0B     19T     0%    /tank/usr/home
> tank/var           19T      0B     19T     0%    /tank/var
> tank/var/spool     19T      0B     19T     0%    /tank/var/spool
> tank/data          19T      0B     19T     0%    /tank/data
> 
> clue bat?

raidz2 is RAID6ish in that two disks of a raid group go to parity to
allow you to survive double faults.  Thus your expected size is:

	(12-2)*2TB = 20TB.

-- Brooks
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 187 bytes
Desc: not available
Url : http://lists.freebsd.org/pipermail/freebsd-fs/attachments/20090522/99250a25/attachment.pgp