Is there still an "optimal" number of drives for each raidz configuration?

anaxagorasbc

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Dec 3, 2021
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I'm upgrading my nas and converting to truenas/zfs. I would like to do an 8 drive raidz2 of 18tb disks. To me 8 drives feels like the sweet spot for redundancy vs space efficiency due to parity for the type of data i'm storing. At 10 drives i'm not sure i'm comfortable with a raidz2 with the resilver time on 18tb drives.


I found some rather dated posts that sugested "2^n + parity" as the number of drives to have in an array

thus for a raidz2, the "optimal" number of drives would be: 4, 6, 10
and raidz2 that would be: 5, 7, 11

Does this still hold true for any reason? Or is it outdated as things have changed?

I've read some stuff that using compression negates this, but since my storage will all be "media", video, pictures, audio - data that generally doesn't compress that well i don't plan on using compression on my main dataset.

So i'm currently debating a single vdev of 8x18tb, or starting with a single 6x18tb vdev and then as i need more storage in the future adding a second 6x18tb vdev to the pool.
 

Heracles

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Feb 2, 2018
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I would like to do an 8 drive raidz2 of 18tb disks

That would be a good design for file server / media server. Not as good if you are planing to run VMs from that pool.

At 10 drives i'm not sure i'm comfortable with a raidz2 with the resilver time on 18tb drives.

Usually, it is considered that up to 12 disks in a single Raid-Z vDev is good. Above that, the risk increases. Also, for 10 to 12 drives, Raid-Z3 may be of interest. The more drives in the vDev, the more often many of them will have problems at the same time.

Does this still hold true for any reason? Or is it outdated as things have changed?

It is not an absolute rule and do not mind too much about it.

To have a large Raid-Z vDev makes it harder to do auto-expand on it (replace all its drives with bigger ones). It also makes it harder to add another vDev that follows the same design (same number of drives and same redundancy).

So i'm currently debating a single vdev of 8x18tb, or starting with a single 6x18tb vdev and then as i need more storage in the future adding a second 6x18tb vdev to the pool.

You are a million time better to put all your storage online day 1.
 

anaxagorasbc

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Usually, it is considered that up to 12 disks in a single Raid-Z vDev is good.
I did consider a raidz2 or raidz3 of 11 drives plus 1 hotspare, but i read that over 10 drives in a vdev is generally not recommended. Any thoughts?

My current config for my largest storage 8x8tb drives in a raid6 with 1 hotspare and it's been bulletproof for about 4 years now, but it's at 95% capacity.

Use case is a home nas. no vmstorage pools, mainly large video files, images for digital photography, nextcloud , photoprism, various backups (veam, acronis, time machine), that sort of stuff. While i do run some VMs, i use a pair of 4tb SSD's in a raid 0 for vm storage that meets my needs.
 

Heracles

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About spares, you may have a look at this thread.... It talks about RaidZ1 + Spare vs RaidZ2 but the reality is the same when comparing RaidZ2 + Spare vs RaidZ-3.

The more drives you have in a vDev, the less flexibility you have.
The more vDev you have, the more performance you get.

As such, to put too many drives in a single vDev will not be as beneficial as having more vDevs with fewer disks each.

Now, there is no silver bullet or magical answer here. There is not a single number of drives that is maximum. For your case, a single large RaidZ vdev makes sense, so you can fo it. 10 RaidZ2 or 12 RaidZ3 are your two best options in my opinion.
 
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