Ric Wheeler wrote:

(Adding Tejun & Greg KH to this thread)

Adding linux-ide to this thread.

Leon Woestenberg wrote:

[--snip--]

quoted

In short, I use ext3 over /dev/md0 over 4 SATA drives /dev/sd[a-d]
each driven by libata ahci. I unplug then replug the drive that is
rebuilding in RAID-5.

When I unplug a drive, /dev/sda is removed, hotplug seems to work to
the point where proc/mdstat shows the drive failed, but not removed.

Yeap, that sounds about right.

quoted

Every other notion of the drive (in kernel and udev /dev namespace)
seems to be gone after unplugging. I cannot manually removed the drive
using mdadm, because it tells me the drive does not exist.

I see.  That's a problem.  Can you use /dev/.static/dev/sda instead?  If 
you can't find those static nodes, just create one w/ 'mknod 
my-static-sda b 8 0' and use it.

quoted

Replugging the drive brings it back as /dev/sde, md0 will not pick it up.

No, it won't.

I have a similar setup, AHCI + 4 drives but using a RAID-1 group.  The 
thing that you are looking for is "persistent device naming" and should 
work properly if you can tweak udev/hotplug correctly.

I have verified that a drive pull/drive reinsert on a mainline kernel 
with a SLES10 base does provide this (first insertion gives me sdb, pull 
followed by reinsert still is sdb), but have not tested interaction with 
RAID since I am focused on the bad block handling at the moment.  I will 
add this to my list ;-)

quoted

The expected behaviour (from me) is that the drive re-appears as 
/dev/sda.

Apart from persistent naming Ric mentioned above, the reason why you 
don't get sda back is md is holding the internal device.  It's removed 
from all visible name spaces but md still holds a reference, so the 
device cannot be destroyed.  So, when a new device comes along, sda is 
occupied by the dead device, and the new one gets the next available 
slot, which happens to be sde in your case.

quoted

What is the intended behaviour of md in this case?

Should some user-space application fail-remove a drive as a pre-action
of the unplug event from udev, or should md fully remove the drive
within kernel space??

I'm curious too.  Would it be better for md to listen to hotplug events 
and auto-remove dead devices or is it something which belongs to userland?

Thanks.

-- 
tejun