Can ZFS mirror a RaidZ1 set? (RAID 5+1 or 1+5)

I am building a small ZFS server using Truenas Scale. I am looking at 6 disk SATA HD configurations for the storage (with boot, L2ARC and SLOG on SSD).

I want the server to be able to survive simultaneous failure of 3 SATA HDs (because i am using 2nd hand drives and sticking the server in a cupboard and ignoring it for long periods of time).

With 6 disks my options seem to be:
(a) RAIDZ3 (4 disks failures will kill this) (50% capacity)
(b) A 3 disk RAIDZ1 array mirrored to the other 3 disks. (33% capacity) (This can survive in several scenarios if 4 disks fail.)

My questions are:

  1. does ZFS allow me to build option (b)?

  2. Is there any difference between:
    (i) create a (3 drive) RAIDZ1 and then mirror it
    (ii) create 3 pairs of mirrored drives and then form RAIDZ1 from them?

  3. Should I use 2 VDEVS each with 3 mirrored drives instead? (33% capacity)
    Simpler - even I can understand it!
    Less CPU intensive, better performance during failures etc.
    My server is not high performance (N100) so this is important.
    This will die if all three drives in one VDEV fail simultaneously, but otherwise can handle 4 failures and a drive experiencing a failure during a rebuild is much less likely due to the shorter duration and stress of this operation.

Thanks!

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I do not believe point b is possible- as data within a pool is striped, not mirrored, across vdevs. Meaning if a vdev fails, the pool will fail.

A raidz3 is perhaps a good choice for your needs. Do you have a backup solution? In my view backup is much more important than redundancy for home solutions. If you do not have a backup and assuming your budget is fixed, I’d suggest going with raidz1 to save money (less drives necessary) and using said savings to backup the data in some manner. How much data do you have? Running rsync to copy it to an external USB HDD is very simple. This solution has downsides but it is much better than no backup.

This thread has a lot of great information and links within it on setting up ZFS pool layouts. I recommend giving it a read:

There are probably a lot on the L1 forum too. ZFS is very popular here (for good reasons)

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zfs can do that. It’s called raidz3

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Hey @FelixO ,

It is not possible as data is being stripped across the drives and not mirrored.
RAIDZ 3 gives more usable space(50%) but the resources demands during normal operations and rebuilds would strain your N100 processor. The parity calculations and need to read from all remaining drives during rebuilds makes this unsuitable.

Mirrored RAIDZ1 setup works but adds complexity,
Simple mirrored vdevs are easier to manage while meeting reduncacy needs.

Mirrored vdevs also lets you replace drives one at a time without impacting array performance.The system handles multiple drive failure well as long as they are not all in the same vdev.

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I’ll echo what the previous comments have said in regards to using RAIDz3. I’d also just add the whole “RAID is not a backup” thing to the conversation.

One other thing that I wanted to pickup on though, is your intent to use an L2ARC and a SLOG.

L2ARC

When it comes to using and L2ARC, the advice of the TrueNAS devs is that you should always add more RAM as your first option. Not only is RAM a whole lot faster, an L2ARC actually uses a certain amount of RAM itself. So adding an L2ARC when you have insufficient RAM, can actually harm performance.

SLOG

A ZIL / SLOG will only benefit certain workloads. Without going into too much detail, (if memory serves) it comes down to Synchronous vs. Asynchronous Writes. If you don’t know the difference between the two or what your workload will use, then you might not need a SLOG… at the very least, you should research the question at hand.

The last thing I’ll say is that the questions you’ve asked, are answered by fairly basic / fundamental ZFS knowledge. I’m not saying this as a criticism or an insult but rather as a way of broaching the subject of learning the basics of ZFS. As @charles7 mentioned, with ZFS, data is striped across the Zpool. But I’ve lost count of the number of threads I’ve seen where new users didn’t know this, they then did something like add a mirrored Vdev to the same Zpool as their RAIDz2 vdev, then they found out the hard way why that’s such a bad idea… So with the best will in the world, I would urge you to get a solid understanding of Zpool topology and the basics and gotchas of ZFS, before you create your Zpool.

This community resource over on the TrueNAS forum may help you - Introduction to ZFS | TrueNAS Community

Good luck.

3 Likes

k

exactly what I do

Thanks! I spent too much time calculating possible variants of Raid and Mirroring layouts that I ended up creating a solution that doesn’t exist. :slight_smile:

(This device is more of a hobby project than an actual backup solution!)

Thanks! I think I’ll go with mirrored VDEVs.

Thanks for your reply.

This device is more of a hobby project than an actual backup solution - I am delving in to ZFS for thew first time as you can tell. :slight_smile:

L2ARC - the SSD that I have available for my boot drive in unnecessarily large for just that purpose so I was going to try to make use of the space.

SLOG - I picked up one of those 16GB Optane drives Wendell was talking about when they were on fire sale so I want to use it for something!