Dennis Lee

ZFS on 2 differently sized drives

for dummies

2020-07-08

This is how I set up a mirrored ZFS file system on a system with one 500 GB HDD and one 250 GB HDD.

At this point, I have done the following:

lsblk -o NAME,FSTYPE,SIZE,TYPE,MOUNTPOINT

NAME   FSTYPE   SIZE MOUNTPOINT
sda           465.8G
├─sda1 vfat     512M /boot/efi
├─sda2 btrfs    229G /
├─sda3 swap     3.5G [SWAP]
└─sda4 ext4   232.9G
sdb           232.9G
└─sdb1 ext4   232.9G

Optional: you can wipefs(8) to remove the file system information (but not data) from the 250 GB partition and disk. The setup will work whether or not you do this.

sudo wipefs /dev/sda4
sudo wipefs -a /dev/sdb
lsblk -o NAME,FSTYPE,SIZE,TYPE,MOUNTPOINT

NAME   FSTYPE   SIZE MOUNTPOINT
sda           465.8G
├─sda1 vfat     512M /boot/efi
├─sda2 btrfs    229G /
├─sda3 swap     3.5G [SWAP]
└─sda4        232.9G
sdb           232.9G

Before creating the ZFS pool, find the unique IDs of the relevant drives. Don’t use those short identifiers with the letters (/dev/sdX) because the system might assign them differently on a future reboot.

ls -ls /dev/disk/by-id/ | awk '{print $10,$11,$12}'

ata-HGST_HTS725050A7E630_TF755AWHHA2J9M -> ../../sda
ata-HGST_HTS725050A7E630_TF755AWHHA2J9M-part1 -> ../../sda1
ata-HGST_HTS725050A7E630_TF755AWHHA2J9M-part2 -> ../../sda2
ata-HGST_HTS725050A7E630_TF755AWHHA2J9M-part3 -> ../../sda3
ata-HGST_HTS725050A7E630_TF755AWHHA2J9M-part4 -> ../../sda4
ata-ST250DM000-1BD141_5VY93J76 -> ../../sdb
wwn-0x5000c5005ce4e07d -> ../../sdb
wwn-0x5000cca77fd2acb2 -> ../../sda
wwn-0x5000cca77fd2acb2-part1 -> ../../sda1
wwn-0x5000cca77fd2acb2-part2 -> ../../sda2
wwn-0x5000cca77fd2acb2-part3 -> ../../sda3
wwn-0x5000cca77fd2acb2-part4 -> ../../sda4

Apparently there are 2 sets of IDs linked to the same disks. I’ll just show the wwn- ones because they’re shorter.

The following creates a mirrored zpool (software RAID, basically) named tank with the 250 GB partition and disk.

sudo zpool create tank mirror wwn-0x5000cca77fd2acb2-part4 wwn-0x5000c5005ce4e07d
lsblk -bo NAME,FSTYPE,SIZE,TYPE,MOUNTPOINT

NAME   FSTYPE             SIZE TYPE MOUNTPOINT
sda               500107862016 disk
├─sda1 vfat          536870912 part /boot/efi
├─sda2 btrfs      245809283072 part /
├─sda3 swap         3700424704 part [SWAP]
└─sda4 zfs_member 250059350016 part
sdb               250059350016 disk
├─sdb1 zfs_member 250049724416 part
└─sdb9                 8388608 part

ZFS will automatically mount tank to /tank (persisting across reboots), but there are options to change the mount point or handle mounting manually with fstab, mount(8), umount(8) and such. When I create the data file system inside tank, I will change its mount point. data doesn’t have to be mounted inside of tank's mount point, as you will see.

sudo zfs create tank/data
sudo zfs set mountpoint=/home/dennis/data tank/data
sudo zfs get mountpoint tank/data

NAME       PROPERTY    VALUE              SOURCE
tank/data  mountpoint  /home/dennis/data  local

At this point, /home/dennis/data will be owned by root:root, which is probably not desired (I mounted it inside my ~ for a reason!). Let’s fix that.

cd /home/dennis
sudo chown -R dennis:dennis data
ls -l

drwxr-xr-x 2 dennis dennis 2 Jul  8 20:36 data/