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 - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
