[dm-devel] howto sent scsi failover commands from dm

[Mike Christie]

Hi Jens,

I just had a quick question, becuase there are concerns that we are 
abusing your block layer sg io code.

For DM (which as you know just remaps bios) multipath, we sometimes need 
to send a special vendor specific scsi command to the device to initiate 
failover. To accomplish this we could just have DM alloc and init a 
request and then send it down like a block sg io by calling 
elv_add_request or blk_insert_request.

One of the problems with that is if a another remapper driver like MD or 
DM is under dm-multipath then the underlying device has no request_fn 
and this model will break. Is it acceptable to just say that this is not 
a valid config, should we begin to make it possible to pass requests 
around through remapper drivers like we do with bios, or should we avoid 
the mess and goto userspace (with userspace there would have to be a way 
to properly traverse mappers too though)?

Thanks,

Mike