Re: Can I add a raidz vdev as a mirror of existing vdevs

On 1/9/22 1:28 AM, David Christensen wrote:
> On 1/8/22 6:11 PM, Mel Pilgrim wrote:
>> I have a zpool with a "stripe of mirrors" structure:
>>
>> mech
>>    mirror-0
>>      disk0
>>      disk1
>>    mirror-1
>>      disk2
>>      disk3
>>    ...
>>
>> I want to convert it from mirrors to raidz as part of an upgrade, with 
>> the final form looking something like this:
>>
>> mech
>>    raidz-0
>>      disk20
>>      disk21
>>      ...
>>
>> I'd like to be able to attach the new raidz vdev as a mirror to the 
>> entire pool, have it silver, then detach the old vdevs.  Is this at 
>> all possible?  I'd rather not create a whole new pool and copy 
>> everything over because doing so would require downtime.
> 
> 
> On 1/8/22 8:32 PM, Mel Pilgrim wrote:
>  > On 2022-01-08 18:52, Paul Procacci wrote:
>  >> Found an answer to this on facebook of all places .......
>  >>
>  >> 
> https://www.facebook.com/notes/unix-in-egypt/converting-zfs-2-way-mirror-to-a-raidz-array-wihtout-losing-data/10151919826687996/ 
> 
>  >> 
> <https://www.facebook.com/notes/unix-in-egypt/converting-zfs-2-way-mirror-to-a-raidz-array-wihtout-losing-data/10151919826687996/> 
> 
>  >>
>  >>
>  >> My initial reaction was going to be 'no'....but after reading this, it
>  >> does seem it is possible.
>  >> I've never done it personally, but with proper backups .... why not?
>  >
>  > The degraded pool shenanigans described are to avoid needing separate
>  > disks for the old and new structure, but that's exactly what I have, so
>  > I can skip all of that.  With those bits skipped, that becomes a regular
>  > exercise of creating a new pool and streaming snapshots, which requires
>  > downtime.
> 
> 
> It would help if you posted the actual disk sizes (disk0, disk1, disk2, 
> disk3, etc., and disk20, disk21, disk22, etc.), drive slot quanities 
> (total, used, and available), the current pool structure ('zpool status 
> mech'), the raidz level you want (1, 2, or 3), and the desired pool 
> structure.
> 
> 
> Assuming suitable drive quantities and sizes, spare drive slots, etc., I 
> believe the following can be done without downtime (but performance will 
> be impacted during evacuation/ copy):
> 
> 1.  Add drives disk20, disk21, etc., to pool "mech" as raidzN  vdev's. 
> For example:
> 
>      zpool add mech raidz1 disk20 disk21 disk22
> 
>      zpool add mech raidz1 disk23 disk24 disk25
> 
>      ...
> 
> 2.  Remove mirrors from the pool:
> 
>      zpool remove mech mirror-0
> 
>      zpool remove mech mirror-1

That is better written:

	zpool remove mech mirror-0 mirror-1 ...


Or, apply the mirror-X comment; below.


> Alternatively, interleave add's and remove's:
> 
>      zpool add mech raidz1 disk20 disk21 disk22
> 
>      zpool remove mech mirror-0
> 
>      zpool add mech raidz1 disk23 disk24 disk25
> 
>      zpool remove mech mirror-X
> 
>      ...
> 
> Verify "mirror-X" for the second and subsequent removals.
> 
> 
> Beware that FreeBSD 12.2-R man zpool(8) states that 'zpool remove' can 
> remove mirrored top-level vdevs, but not raidz.  So, if you botch adding 
> a raidz vdev, you may need to wipe the pool, try again, and restore from 
> backup.
> 
> 
> David
>