Hi James,

Zitat von "A. James Lewis" [off-list ref]:

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.

to me it's more than rumors. We're facing severe difficulties (server  
reboots, disks marked faulty by MDRAID, hangs) in our layered setup:

- physical disks MD-RAID6 (data) plus two SSDs MD-RAID1 (cache)
- bcache
- LVM
- DRBD for many of the logical volumes (always primary, no fail-overs)
- ext4 fs
- NFS / Samba / SCST (fileio)

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?

MD-RAID1 is what our current configuration looks like. We've also  
combined the spinning disks into a RAID6.

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.

I have no btrfs experience, so I cannot answer that one. I went for a  
single data and cache device (via RAID) so I won't have to partition  
my SSDs - that would not have been scalable (we're planning to add  
plenty of physical disks over time, and to use many LVs/file systems).

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?

Someone else might want to help with that one as well?

Regards,
Jens