layering question.

["A. James Lewis" <james@fsck.co.uk>]

Hi all...

I've heard rumours that layering bcache with other block device drivers 
might not be recommended... I wonder what the truth really is... perhaps 
someone can advise.

I was planning to use 2 SSD's... combined with 4 large spinning drives 
to create a large filesystem with BTRFS...  my questions are as follows.

1. Is there a way to use 2 SSD's directly, or would it be OK to use MD 
to stripe them?... then used the MD array as the cache device?

2. I would be using BTRFS, so would it be better to create 4 separate 
bcache devices each attached to the single cache device, and then use 
BTRFS to raid 4 bcache devices... obviously this would be more flexible, 
or would I need to make an MD raid of the 4 devices, and then use that 
to create a single bcache device and build a BTRFS filesystem on top of 
that.

Hope that's clear, any clarification would be appreciated...

Also, there's talk about a pending on-disk cache format change some time 
around 3.19, but no details... is this over with, or still pending?

James