Advice for Choosing an 8-Bay NAS: TrueNAS Mini XL+ and Synology DS1821+

With a mirror, regardless of whether it’s a two way, three way, or more, your data capacity is the size of one disk since the other disks are duplicates of the first.

As for your pool description, you mean you’re starting out with your pool having two vdevs, with each having a mirrored pair of 3TB drives? So you’d have 6TB of storage.

My pool considerations are 4 x 3TB disks in a RAIDZ2 configuration or 2 x 3TB mirrored pairs giving as you say circa 6TB.

Okay, so no three way mirrors as mentioned in your original post. Then you’re in the exact same boat I am, same number of drives and same drive size. So I can just copy and paste the notes I’ve made for myself:

Capacity

Recently discovered online ZFS capacity calculator:

Using that, a pool with a two 3TB mirror vdevs will give ~5.8TB, while the four drives in a pool with a single raidz2 vdev would give ~ 5.6TB. So available space would be different but not significantly different as compared to the 12TB or raw storage space starting with. Would see a big difference if another drive added, taking it to five drives. Then raidz2 capacity jumps to ~ 8.6TB.

Reliability

With raidz2, could lose any two disks and the pool would still be intact. With a pool made of two mirrored vdevs, once a single drive is lost, the failure of the other disk in that vdev means the loss of the entire pool. This is because redundancy is at the level of the vdev. Loose a vdev and loose the entire pool and have to go to backup.

This is balanced against the fact that rebuilding a disk in a mirror is much faster than resilvering a disk in a raid Z1 or raidz2 vdev. How much faster I have yet to determine. The resilvering also stresses the remaining disks in a raidz vdev, making another disk failure more likely, presuming they are all the same age.

Hope that helps.