Attaching a device as mirror to an existing ZFS storage pool

Got a MC-32 from ovh last year on Black Friday. Recently I have reinstalled it to Proxmox VE 5.0, with the preinstallation template that comes with ZFS provided by ovh.

UNCONFIGURED RAID

MC-32 has 2x240GB SSD with soft raid, I wanted to build a RAID 1 using the 2 SSDs. Since Proxmox VE 5.0 doesn’t support soft raid, I thought that the installation template would build a ZFS RAID 1 using 2 SSDs but unfortunately it didn’t.

After the installation had finished, I found that only one SSD was used.

Here is the partition table after the installation.

$ fdisk -l

Disk /dev/sda: 223.6 GiB, 240057409536 bytes, 468862128 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: FF368BD5-2B0B-404B-AE3C-28B3F2D64E9E

Device         Start       End   Sectors   Size Type
/dev/sda1         34      2047      2014  1007K BIOS boot
/dev/sda2       2048 468845710 468843663 223.6G Solaris /usr & Apple ZFS
/dev/sda9  468845710 468862094     16385     8M Solaris reserved 1

Disk /dev/sdb: 223.6 GiB, 240057409536 bytes, 468862128 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes

/dev/sdb was not used. More importantly, rootfs had been installed to a ZFS pool named rpool, and rpool was using the whole partition /dev/sda2.

REBUILDING RAID IN ZFS

I have a running disk which had data in it and an empty disk. What to do in order to have RAID 1 on these 2 disks? Fortunately I was using ZFS, it’s rather simple. Let’s rebuild the RAID 1 using mirror in ZFS.

I wanted to create the same partition table on /dev/sdb, mirror all data from /dev/sda2 to /dev/sdb2, and have rpool backed by 2 partitions in 2 disks which are mirrored.

(Optional) Wipe the disk to be used as the mirrored disk.

This is an irreversible operation, it will permanently all delete the existing data.
```
shred -vz /dev/sdb
```
Copy the partition table from /dev/sda to /dev/sdb
```
sgdisk --replicate=/dev/sdb /dev/sda
```
Randomize the disk’s GUID and all partitions’ unique GUIDs of /dev/sdb to prevent conflict
```
sgdisk --randomize-guids /dev/sdb
```
Install boot partition to /dev/sdb
```
grub-install /dev/sdb
```
Mirror the current data from /dev/sda2 to /dev/sdb2

By attaching /dev/sdb2 to rpool as a mirror device, ZFS will automatically sync the data.
```
zpool attach -f rpool /dev/sda2 /dev/sdb2
```
Check the mirroring progress by the following command.
```
zpool status -v
```

End up /dev/sda2 and /dev/sdb2 formed a mirror for rpool.

$ zpool status -v
  pool: rpool
 state: ONLINE
  scan: scrub repaired 0 in 0h35m with 0 errors on Sun Oct  8 00:59:39 2017
config:

	NAME        STATE     READ WRITE CKSUM
	rpool       ONLINE       0     0     0
	  mirror-0  ONLINE       0     0     0
	    sda2    ONLINE       0     0     0
	    sdb2    ONLINE       0     0     0

ZFS AND PROXMOX VE

ZFS plays well with Proxmox VE and it’s mirrored zpool covers the drawback of Proxmox VE not supporting soft raid. Sometimes you just want to pick up a budget server with limited options, like without hardware raid, and play with Proxmox VE while keeping your valuable data safe by utilizing multiple disks, ZFS seems to be a good option. But if you have multiple hosts, maybe Proxmox VE on Ceph is your friend.