Re: removed disk && md-device

[Bernd Schubert <bs@q-leap.de>]

On Friday 11 May 2007 10:51:40 Michael Tokarev wrote:
> Neil Brown wrote:
> []
>
> >> But joggling a usb stick (similar to your use case) would probably be OK
> >> since it would be hot-removed and then hot-added.
> >
> > This still needs user-space interaction.
> > If the USB layer detects a removal and a re-insert, sdb may well come
> > back a something different (sdp?) - though I'm not completely familiar
> > with how USB storage works.
>
> This is in fact an.. interesting issue.
>
> Suppose I pulled the USB cable of sdb -- the WRONG one -- by a mistake.
> I noticed this immediately (since the led on the disk stopped lighting),
> and plugged the cable back again.  There was no write requests to the
> array during this time, there was no ANY requests to it at all, it was
> completely idle.
>
> But.
>
> The unplug immediately triggers USB device removal.  But md subsystem still
> holds a reference to (now orphan) sdb.  So upon plugging it back, since
> sdb is busy, scsi subsystem (which handles USB disks) grabs first available
> sdX device, let's say it'll be sdp.
>
> So we've orphan sdb which is "in use" by the array, and fresh new sdp,
> which is unused but contains the orphaned array component.
>
> And there's no way to hot-re-add sdp to the array (there's nothing to do
> to the array itself!) but.. to powercycle the machine!  Because on
> hot-remove, event count will be updated on the still-plugged-in device
> (sda let it be), and upon hot-add, md will start resyncing.  Oh well...
> (the only help from md subsystem here is in case if it is using bitmaps,
> but that's different issue.)

Yep, thats exactly what I'm talking about and its not only limited to usb, but 
happens with sata as well.


Bernd


-- 
Bernd Schubert
Q-Leap Networks GmbH

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