Re: SSD caching an existing btrfs raid1

[Psalle <psalleetsile@gmail.com>]

On 19/09/17 17:47, Austin S. Hemmelgarn wrote:
(...)
>
> A better option if you can afford to remove a single device from that 
> array temporarily is to use bcache.  Bcache has one specific advantage 
> in this case, multiple backend devices can share the same cache 
> device. This means you don't have to carve out dedicated cache space 
> for each disk on the SSD and leave some unused space so that you can 
> add new devices if needed.  The downside is that you can't convert 
> each device in-place, but because you're using BTRFS, you can still 
> convert the volume as a whole in-place.  The procedure for doing so 
> looks like this:
>
> 1. Format the SSD as a bcache cache.
> 2. Use `btrfs device delete` to remove a single hard drive from the 
> array.
> 3. Set up the drive you just removed as a bcache backing device bound 
> to the cache you created in step 1.
> 4. Add the new bcache device to the array.
> 5. Repeat from step 2 until the whole array is converted.
>
> A similar procedure can actually be used to do almost any underlying 
> storage conversion (for example, switching to whole disk encryption, 
> or adding LVM underneath BTRFS) provided all your data can fit on one 
> less disk than you have.

Thanks Austin, that's just great. For some reason I had discarded bcache 
thinking that it would force me to rebuild from scratch, but this kind 
of incremental migration is exactly why I hoped was possible. I have 
plenty of space to replace the devices one by one.

I will report back my experience in a few days, I hope.