RAIDZ or MIRROR on SSD pool?

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RAIDZ or MIRROR on SSD pool?

Postby roemer » Sat Feb 11, 2023 10:57 pm

I am about to migrate an HDD-based ZFS pool to a SSD-based pool.
On the existing pool, I emphasized data protection and used RAIDZ2 over 4 hard drives.

For the new SSD-based pool, I am wondering whether RAIDZ1 or MIRROR is better.
A lot of documentation about ZFS pools seems to indicate that RAIDZ is better for throughput, especially for large files such as photos or videos.
But for better latency, especially on smaller I/O requests such as done by databases or VMs, MIRROR is said to have an advantage.
Then there is also the issue of a potential device failure where a RAIDZ puts extra burden on all remaining drives during re-silvering, while MIRROR only involves one drive top copy from.

But all the information I found was basically written for hard drives, where I understand the reasoning.
In fact I was originally planning for a MIRROR setup with my new pool...
... until I started benchmarking ;)

It seems that in my case (4 x Crucial P3 NVMe SSDs), a RAIDZ1 pool is faster than a 2xMIRROR for large file access, and at least comparable in speed for 4K access. My use case will involve mainly large file access. This leaves the argument about the re-silvering stress on the remaining drives in case of a vdev failure. But I can't see this a huge issue for SSDs anymore, as reading from an (old) SSD should not really have a burden on them. It is the writes which wears down SSDs, but during re-silvering the writing is mainly done on the new replacement drive.

Does anyone else have experience with an SSD-based zfs pool?
Do you use RAIDZ or MIRROR on your SSD pool?
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Joined: Sat Mar 15, 2014 2:32 pm

Re: RAIDZ or MIRROR on SSD pool?

Postby Bottacco » Thu Mar 16, 2023 11:31 pm

While researching ZFS on Mac I maid several trials with 4 SATA3 SSDs to compare against SATA3 HDDs. I talked to several BSD sysadmins that use ZFS in a regular basis and all of them recommended a stripped mirror zpool (2 mirrors stripped like in RAID 10), but they said that would make more sense with more mirrors (several mirrors striped). With that configuration you get good latency and fast access (the more mirrors you have, the better). The only downside is the storage efficiency, that is 50%.

For small VDEVs it all comes down to your main purpose, but being NVMe you won't have much of a problem in the throughput department, so between a RAIDZ2 and a pair of stripped mirrors I would go with the second option. But this is just my opinion based on all the videos, docs and post that I have researched.
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