Using your requirement for 2 disks of redundancy and general speed, (not highest speed), RAID-Z2 is likely what you want. This is similar to RAID-6 in that you can loose any 2 disks and still have full access to your data.
In general, you don't want more than 10 to 12 disks in a vDev, (virtual Device, a ZFS term). Too many disks in a RAID-Zx vDev can cause problems with speed and disk replacement speed. Of course, ZFS supports multiple vDevs in a single ZFS pool... so if you have 20 disks, 2 x 10 disk RAID-Z2 vDevs are possible. That's 4 disks of redundancy, 2 per vDev. Redundancy is at the vDev level, so in the example of 2 x 10 disk RAID-Z2 vDevs, you can loose 4 disks total, as long as it is only 2 disks per vDev.
Next, if you have a spare disk slot, you can perform a replace in place for any disk not yet completely failed. This allows the failing disk to supply some data, and when it can't, then other redundancy from RAID-Z2 is used. In essence, this "replace in place" feature mirrors the failing disk, unless the failing disk can't supply valid data. Then the other disks are used for that data. And when this special "mirror" is complete, the failing disk is freed up and no longer part of the vDev. Please note that ZFS checks the checksum of all data, so ZFS knows "valid data" verses corrupt data.
I recommend reading this;
[---- 2024/01/16: Still relevant. Virtualization really doesn't change much. Updates made as appropriate. ----] [---- 2018/02/27: This is still as relevant as ever. As PCIe-Passthru has matured, fewer problems are reported. I've updated some...
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It is also highly recommended to read up on both ZFS and TrueNAS. Both have limitations and some people find one or both not suitable for their needs.
This is a short introduction to ZFS. It is really only intended to convey the bare minimum knowledge needed to start diving into ZFS and is in no way meant to cut Michael W. Lucas' and Allan Jude's book income. It is a bit of a spiritual...
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We realize that new users have a lot to learn when they come to FreeNAS. There's a certain amount of confusion added to discussions when users pick random/approximate terms to describe things. I've spent a lot of time quietly trying to...
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This resource was originally created by user: @Davvo on the TrueNAS Community Forums Archive. https://www.truenas.com/community/resources/zfs-storage-pool-layout.201/download [1] This amazing document, created by iXsystems in February 2022 as a “White Paper”, cleanly explains how to qualify...
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All those links are to TrueNAS Resources, a special place in these forums for community supplied helpful guides and documentation. See the top any forum page for a direct link to the Resources section, (or the Documentation section, or the Report a Bug page).
