Convert single disk cache pool to RAID1 cache pool?

[Brandon_K]

Subject pretty much says it all.  What is the correct process to convert an existing / in operation single disk cache pool to a mirrored cache pool?  I can find guides on the inverse, but not single to two disk.

Backstory; I have a NVME that is being used for container data.  I have a matching NVME that was formerly used as a separate cache pool and is now empty / unused.  Both drives are formatted as btrfs.

I know the basics of needing to stop the array, add another drive slot to the existing cache pool, but what I don't know how to do convert the pool to RAID1.  Specifically, I'm concerned about it mirroring the empty drive, thus wiping out the container data. 

Any help would be appreciated!

[JonathanM]

4 hours ago, Brandon_K said:

I'm concerned about it mirroring the empty drive, thus wiping out the container data. 

You could use the blkdiscard command to wipe the empty drive. Very carefully, of course.

[Brandon_K]

3 hours ago, JonathanM said:

You could use the blkdiscard command to wipe the empty drive. Very carefully, of course.

The drive is empty, at least there is nothing in the root of the drive.  I'm assuming the few megs on it is for Unraid share info (even though it's not assigned as a share/cache to any other share) or filesystem purposes?  I know my way around Unraid pretty well, but understanding the deep dive portions of the file system and how Unraid works with the shares in the background is out of my wheelhouse. Beyond that, without knowing how Unraid will mirror the disks, I'm still worried that it would still mirror a completely wiped disk.  

[JorgeB]

12 hours ago, Brandon_K said:

I know the basics of needing to stop the array, add another drive slot to the existing cache pool, but what I don't know how to do convert the pool to RAID1. 

That's it, and it will be raid1 by default. 

[steve9992000]

But will it destroy existing data in the already-working first drive? Does it have to be flushed first to the array before proceeding?

[JorgeB]

1 minute ago, steve9992000 said:

But will it destroy existing data in the already-working first drive?

Not if existing cache is btrfs, it cannot be done for xfs.