Even though all the data drives are xfs, and /disk settings / default file system = xfs.

Is this a recent fs preference for unraid?  I'm using 6.1.4.  Just curious... 

Because BTRFS is the only filesystem supported on unraid that can handle multiple drives in one volume, you can not format cache as anything else if you have more than one cache slot showing available when the array is stopped.

 

If you wish to reformat it as XFS, you can only have one cache drive. You will need to stop the array, set the number of cache slots to 1, then you can set the cache drive format to XFS.

 

After I had an "incident" with my BTRFS cache drive, I switched to XFS and have had no more issues. My BTRFS issue did not result in any data loss, but it sure made me nervous for several hours until I got it resolved.

Sorry obviously I was not clear.  I just replaced my existing, aging xfs cache drive with a new drive - stopped the array, shut down, replaced the cache drive.

 

There was only one (unformatted) cache drive, not two, when I started the array.  The new cache drive showed as unformatted, as it should.  I simply went ahead and clicked 'format', assuming that it'd be formatted with xfs; but instead I realized btrfs was somehow selected.

 

I have read about xfs > btrfs in terms of reliability in several places, which is why I'm sticking with xfs for the time being.

Btrfs the default for the cache drive if you left it set to auto.    If you want to change it then stop the array; change the format to the one you want; start the array and then format the drive.  Note that doing this will lose any existing contents, so make sure anything that matters is backed up elsewhere.

Oh, I see, good to know, and thanks for clarifying this.  If I may be really anal here, it seems as though there should be two separate default file system settings then: one for data drives, one for cache drives.    Or at least a reminder that the default file system setting only applies to data drives.