RAID-Z2 with only 3 drives?

JDLH

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Aug 10, 2019
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Hi, folks:

I'm new to FreeNAS. I want to build a server for my archive of home documents and photos. My top priority is to be robust against data loss. I don't demand much performance, and data efficiency is nice but secondary to robustness. My old server is a RAID1 with three 3TB drives rotating through two bays. My data has survived, but I know I've been lucky. I want to do better than that with FreeNAS. I currently store 2+ TB, but I will outgrow 3TB in a few months.

I have read a lot of the introduction resources. Thank you to everyone who took the time to write them. It's clear that I want RAID-Z2 instead of RAID-Z1, for robustness. I'm buying FreeNAS Mini hardware with 4 bays. I hear the importance of NAS-grade drives and ECC RAM loud and clear.

I have bought 3 x 10TB WD Red drives. It isn't clear to me if I can set them up as a 3-drive RAID-Z2 vdev. According to the concept of data + 2 parity, it should be possible, and result in a 10TB vdev with two parity. But guides seem to talk about 3-drive vdevs as RAID-Z1 only, and implies that one can only start considering RAID-Z2 with a minimum of 4 drives. What's the real situation? Where in the documentation are the actual constraints described?

The concrete options in front of me seem to be:
  1. Create a RAID-Z2 vdev with the 3 x 10TB drives. Expect 10 TB capacity. Don't expand it until I buy three larger drives, or buy a bigger case and create a second vdev, or until FreeNAS someday starts to allow adding drives to vdevs.
  2. Create a RAID-Z1 vdev with the 3 x 10TB drives. Expect 14 TB capacity. Manage the increased risk of data loss by a more disciplined backup strategy. Accept the same limitations against expanding as for #1.
  3. Buy a 4th drive. Create a RAID-Z2 vdev with the 4 x 10TB drives. Expect 14TB capacity. (But I'd be happy with 10TB, and option 1.)
  4. Buy a 4th drive. Create a RAID-Z3 vdev with the 4 x 10TB drives. Expect 10TB capacity. Celebrate the increased robustness against data loss.
  5. Create a RAID-Z1 vdev with the 2 x 10TB drives. Expect 10 TB capacity. Use the 3rd drive standalone for backups. (It seems like ZFS could put the drive to better use.)
What would you recommend? Thanks in advance for your insight.
—Jim DeLaHunt, Vancouver, Canada
 

sretalla

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RAIDZ2 on 3 drives is (technically not RAIDZ2) rather a 3-way mirror.

You can go with your option 1 if you're focused on fault tolerance over cost and capacity.

Option 2 would be frowned upon in the forum due to the disk capacity being over 1TB and using RAIDZ1.

Option 3 is reasonable (20TB by the way).

Option 4 not technically RAIDZ3... you would need a 4-way mirror (similar to option 1).

Option 5 not RAIDZ1, rather a mirror. A workable option that if you use it right also gives you granular and flexible recovery in addition to fault tolerance.

In all cases, don't forget you lose about 15% of raw capacity to overhead and shouldn't fill a ZFS volume over 80%, so 10TB really means a little under 7TB in real-world capacity.
 

Ericloewe

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RAIDZ2 on 3 drives is (technically not RAIDZ2) rather a 3-way mirror.
You can do RAIDZn with n+1 disks. I have no idea why you'd want to and FreeNAS won't let you, but it exists. n+1 mirrors would only have advantages in that scenario, unless you're banking on RAIDZ expansion.
 

otpi

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Feb 23, 2017
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I use a z2 with 4 drives. It's not optimal with respect efficiency, but works just fine. 50% of total capacity is used for redundancy, vs 33% in a 6 wide raidz2, or even 25% in an 8 wide z2.

For my 8 TB drives that's 16 TB available and 16 used for redundancy (32TB total). I could get the same available capacity using 6x4TB z2 giving 16 TB available and 8 for redundancy (24TB total). i.e. I only need to buy 24 TB total vs 32TB.

(Yes, I know, I used the sticker TB's in the example)

My reason was to keep number of hdds low, and the cost/TB was better for 8 TB hdds at the time of purchase. I also prefer 8 TB hdds for future upgrades, since I do not see a problem with doing a backup of existing data and restoring to a new expanded pool (i.e.: destroying the pool and adding more hdds). YMMW.
 
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