Re: libata hotplug and md raid?

[Mark Lord <mlord@pobox.com>]

Leon Woestenberg wrote:
> Hello all,
> 
> On 9/13/06, Tejun Heo <htejun@gmail.com> wrote:
>> Ric Wheeler wrote:
>> > Leon Woestenberg wrote:
>> >> 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.
>>
>> >> 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.
>>
> 
> I did further testing of the ideas set out in this thread.
> 
> Although I can use (1) static device nodes, or (2) persistent naming
> with the proper udev rules, each has its own kind of problems with md.
> 
> As long as the kernel announces drives as disappeared but md still
> holds a lock, replugging drives will map to other major:minor number
> no matter what I try in userspace.
> 
> Static device nodes will therefore not help me select the drive that
> was unplugged/plugged per se.
> 
> Persistent naming using udev works OK (I used /dev/bay0 through
> /dev/bay3 to pinpoint the drive bays) but these disappear upon
> unplugging, while md keeps a lock to the major:minor, so replugging
> will move it to different major:minor numbers.
> 
> So the question remains: How will hotplug and md work together?
> 
> How does md and hotplug work together for current hotplug devices?

I have the same questions.

How does this work in a pure SCSI environment? (has it been tested?)
If something should change, should those changes be in the MD layer?
Or can this *really* all be done nicely from userspace?  How?

I've got to fix some problems related to this, for a couple of clients,
and would like to Do It Right, or as close to Right as reality permits.

Cheers
-- 
Mark Lord
Real-Time Remedies Inc.
mlord@pobox.com