Re: 2.6.0, SiI3112, md raid1 problem: bio too big device (128 > 15)

[Jeff Garzik <jgarzik@pobox.com>]

Jim Lawson wrote:
> Hi all,
> 
> I am having trouble creating a raid1 array under 2.6.0.  I am able to
> create raid0 and raid5 mds, but raid1s fail with "bio too big device hde3
> (128 > 15)", which doesn't tell me a lot.  I can see it's in
> drivers/block/ll_rw_blk.c, right at the boundary with the device driver,
> but I'm not enough of a kernel wonk to find out a lot more.
> 
> I'm not sure if this has to do with a kernel bug, a bug in the driver for
> the controller I have (SiI3112, Silicon Image 3112), or the disks I am
> using (Seagate Barracuda 7200.7, 160 GB SATA disks) ... or the combination
> thereof :-)


Hum... that's kinda interesting.

AFAICS, the basic problem is that Silicon Image's sector size limitation 
means that md cannot submit stripe-sized bio's as it wishes.

md does indeed split bio's in raid0, so it makes sense that it works (to 
my naive eye).  I'm amazed raid5 works, since raid5 appears to hardcode 
STRIPE_SIZE.  And I don't see splitting code in raid1, so it would make 
sense that raid1 would fail.

In general though, I'm surprised that each block driver has to 
reimplement the pain of splitting its own bio's, to conform to the 
underlying device.  Since bio's can be merged after 
generic_make_request(), surely it makes sense to implement bio splitting 
-once- in the block layer, rather than in each block driver that has to 
care about stackable block devices?

	Jeff