Adding 4 disks to existing 4 disk raidz2

[buzuddha]

Hello all,

This is my first post here, so I hope I'm not breaking too many forum rules just yet! I am aware that this question has been asked and answered in some form or another many times, however I can't find it asked according to my case (though this appears to come close):

I have a 4 x 12TB raidz2 to which I would like to add 4 more drives. Based on my poor understanding of the raid calculators, I could have 2x4 raidz2 or 1x8 raidz2 in a pool. It appears that 2x4 has only 50% usable storage while the 1x8 would be 75% usable (if I'm misunderstanding this please let me know!). I'm thinking that 1x8 makes more sense for my needs (all personal data, mostly media, can lose all of it and will have backups of critical docs).

Now, before I copy over 17TB of data from my old raid5 array, the question is: to achieve the 1x8, do I need to have this in place upon creation of the pool or can I add these 4 disks in once they arrive next week?

Thanks for any help here!

 

[Arwen]

You need to have all 8 disks in a single RAID-Z2 vDev, (Virtual Device).

ZFS does not at present allow growing a RAID-Zx vDev's width. So a 4 disk RAID-Z2 remains a 4 disk RAID-Z2.

You can, as you found out, add a second 4 disk RAID-Z2 vDev to increase storage. Or replace, (one at time time), the existing 4 disks with larger ones, to grow the size of the 4 disk RAID-Z2.

The only way to gain that extra efficiency, (from 50% to 75%), is to destroy the existing 4 disk RAID-Z2 and re-create it with 8 disks. This of course requires either a full backup and restore. Or willingness to loose all existing data stored on the 4 disk RAID-Z2.


Unfortunately ZFS is not as flexible as some other RAID and volume manager schemes.

 

[buzuddha]

Awesome. Thanks for the prompt reply. I'll await the arrival of the remaining drives, destroy and rebuild and migrate the data at that point.