linux-kernel - Re: Weird I/O errors with USB hard drive not remounting filesystem readonly

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Message-ID: <20091124195607.GC16662@quack.suse.cz>
Date:	Tue, 24 Nov 2009 20:56:08 +0100
From:	Jan Kara <jack@...e.cz>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Jan Kara <jack@...e.cz>, Boaz Harrosh <bharrosh@...asas.com>,
	tmhikaru@...il.com,
	Kernel development list <linux-kernel@...r.kernel.org>,
	USB list <linux-usb@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	SCSI development list <linux-scsi@...r.kernel.org>,
	linux-ext4@...r.kernel.org
Subject: Re: Weird I/O errors with USB hard drive not remounting filesystem
 readonly

On Tue 24-11-09 14:28:35, Alan Stern wrote:
> On Tue, 24 Nov 2009, Jan Kara wrote:
> 
> >   My naive guess would be that those non-retried reads were actually
> > readahead. That's not retried if I remember correctly. Later, when we
> > really needed the data, we sent another read request...
> 
> That would be my guess too.  I don't know how to verify it, though.
> 
> If you're interested in pursuing this farther, I can show you how to 
> generate equivalent errors on demand using an emulated USB drive.  
> At this point it's not clear how much more one could learn by doing 
> this, however.
  After digging in block layer code, it's as we suspected:
In case of host error DID_ERROR (which is our case), scsi request is
retried iff it is not a FAILFAST request which is set if bio is doing
readahead... So this is explained and everything behaves as it should.
Thanks everybody involved :).

								Honza
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