FreeBSD 10.1 + ZFS

[Jon Radel]

On 5/27/15 5:12 PM, Baho Utot wrote:
>
>
> On 05/27/15 16:42, Jon Radel wrote:
>> On 5/27/15 3:27 PM, Baho Utot wrote:
>>>
>>> I do understand how ZFS works I am using it (ZFS RAID-Z1) on three 
>>> boxen but not as the root file system.
>>>
>>>
>> <snip>
>>>
>>> You cannot do RAID-Z2 with three drives only as RAID-Z2 has two 
>>> parity drives
>> You either contradict yourself or you have a shaky understanding of 
>> the word "cannot".  Not recommended, quite possibly not the best way 
>> to use 3 disks, sure....    Cannot, well, not so much.
>
> Ok tell me how you have two parity drives with using only three 
> drives.  It can  not be done.
>
Like this, more or less:

root at pumpkin:/etc # gpart add -t freebsd -s 10G ada0
ada0s2 added
root at pumpkin:/etc # gpart add -t freebsd -s 10G ada0
ada0s3 added
root at pumpkin:/etc # gpart add -t freebsd -s 10G ada0
ada0s4 added, but partition is not aligned on 4096 bytes
root at pumpkin:/etc # zpool create tada raidz2 /dev/ada0s2 /dev/ada0s3 
/dev/ada0s4
root at pumpkin:/etc # zpool status tada
   pool: tada
  state: ONLINE
   scan: none requested
config:

     NAME        STATE     READ WRITE CKSUM
     tada        ONLINE       0     0     0
       raidz2-0  ONLINE       0     0     0
         ada0s2  ONLINE       0     0     0
         ada0s3  ONLINE       0     0     0
         ada0s4  ONLINE       0     0     0

errors: No known data errors
root at pumpkin:/etc # df
Filesystem   1K-blocks   Used     Avail Capacity  Mounted on
/dev/ada0s1a 253911544 902528 232696096     0%    /
devfs                1      1         0   100%    /dev
tada          10033936     96  10033840     0%    /tada
root at pumpkin:/etc #

Of course, if one wanted a 3-device raidz2 vdev in production in the 
first place, you'd do
well to have the partitions on 3 different drives, much better 
performance and resilience
that way.  But the logical structure of the ZFS vdev is the same.

Why are you so certain that this is impossible?

--Jon Radel
jon at radel.com


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