Re: [linux-usb-devel] Dealing with flaky USB storage devices and rootfs

[Dan Aloni <da-x@monatomic.org>]

On Tue, May 29, 2007 at 05:50:49PM -0400, Alan Stern wrote:
> On Tue, 29 May 2007, Dan Aloni wrote:
> 
> > Hello,
> > 
> > We have a system where the rootfs is a partition on a USB device,
> > and I've noticed upon a few rare cases where the USB controller 
> > loses the connection to the USB device after some uptime (days,
> > weeks...), and the USB device reappears a very short time later.
> 
> That failure mode is pretty uncommon.  More often what happens is the
> connection remains intact but communication/protocol/firmware/???  
> errors cause the device to stop working.  It never disappears but it
> can't be used again without unplugging or power-cycling.

Yes this is also what I assume happening. i.e. more likely a USB 
flash disk firmware bug than a controller bug (there are lots of 
crappy USB flash drives out there).
 
> > So, I gave some thoughts about this, I and came up with two 
> > main solutions:
> > 
> > 1) Improve the USB storage error handling - bind the already 
> > existing SCSI host to the USB port that has the device, e.g., 
> > if host2 got created for usb 5-3 then keep it that way for the 
> > sake of EH. /dev/sda1 should come to life when the USB device 
> > recovers, unless a few seconds have passed or some attributes 
> > (such as manufactor id or serial) have changed.
> 
> The difficulty here is that this "error" is indistinguishable from
> normal activity -- someone simply unplugs the device and then later on
> another connection is made.  It might be the same device as before or
> it might be a different one.  In other words, it isn't really an error.  
> You would solve this by relying on "a few seconds" timeout.
[...]
> 
> It also goes against the USB specification.  And it is potentially 
> unsafe, in that it is possible for users to change media or make other 
> alterations that the kernel cannot detect.  The same would be true of 
> your proposal, assuming that somebody was quick enough to unplug one 
> device and plug in another (or swap memory cards) in the span of a few 
> seconds.

The specific use case I refer to is with a flash drive embedded 
inside a locked and closed chassis of a dedicated server. So, 
anyone repluging it must know what they are doing anyway.

-- 
Dan Aloni
XIV LTD, http://www.xivstorage.com
da-x (at) monatomic.org, dan (at) xiv.co.il
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