SOLVED Metadata VDev in Fusion Pools -- RAIDZ2 vs. Three-Way Mirror

[bollar]

I'm planning to add a Metadata VDev to a pool in SCALE.

I note the following warning and see that the UI in SCALE requires the Metadata VDev to be the same structure as the Data VDev.

Using a Mirror layout is possible, but it is strongly recommended to keep the layout identical to the other vdevs. If the special vdev fails and there is no redundancy, the pool becomes corrupted and prevents access to stored data.

On CORE, I don't recall this decision being forced in the UI. On this SCALE pool, I had intended to add the Metadata VDev as a three-way mirror with the supposition that it's faster with similar data integrity to RAIDZ2. Am I wrong about this?

I'm fine to install a RAIDZ2, but if the three-way mirror is actually "better," I'd like to do that instead.

 

[c77dk]

If I remember correctly CORE 12.0 had to have a "force" checkbox ticked to allow mirrors to be used.
The important thing is to have the same amount of redundancy with the sVDEVs as in the data VDEVs. So for a raidz2 data you will go with 3-way mirrors.

 

[jgreco]

I would agree that this is probably unnecessary paranoia by the UI.

It used to be that loss of SLOG would kill a pool, but the recommendation was only ever for mirrored SLOG even so.

For a special vdev, I am not aware of any justifiable technical reason that you could not use three-way mirrors, and I would expect mirrors to be faster than RAIDZ2, but I haven't tested that particular thing for a special vdev.

So for a raidz2 data you will go with 3-way mirrors.

Or even two-way mirror on devices which are much more reliable than your pool devices, with the understanding that if you manage to kill both of them in rapid succession, your pool is toasty toast.

 

[morganL]

It's worth a suggestion in "report a bug" to improve this.
Mirrors on metadata VDEVs are recommended (should be default)

 

[bollar]

This winds up being a combination of user error and confusing interface. When you select to create a metadata VDev, the Metadata VDev drive selection panel is added below the Data VDev selection panel.

Now that I understand what's going on, I was able to add a three-way-mirror via the GUI.

I'll write an enhancement request for this in Jira.