linux-ext4 - Re: Weird I/O errors with USB hard drive not remounting filesystem readonly

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Message-ID: <20091127094339.GA9047@roll>
Date:	Fri, 27 Nov 2009 04:43:39 -0500
From:	tmhikaru@...il.com
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Jan Kara <jack@...e.cz>, tmhikaru@...il.com,
	Boaz Harrosh <bharrosh@...asas.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 Wed, Nov 25, 2009 at 11:10:48AM -0500, Alan Stern wrote:
> On Wed, 25 Nov 2009, Jan Kara wrote:
> 
> > > > > > Okay, very good.  There remains the question of the disturbing error
> > > > > > messages in the system log.  Should they be supressed for FAILFAST
> > > > > > requests?
> > > > >   I think it's useful they are there because ultimately, something really
> > > > > went wrong and you should better investigate. BTW, "end_request: I/O error"
> > > > > messages are in the log even for requests where we retried and succeeded...
> 
> That isn't true.  Take a look at the dmesg log accompanying Tim's 
> usbmon log.  Although there were 5 read errors in the usbmon log, there 
> were only 2 I/O error messages in dmesg, corresponding to the 2 reads 
> that weren't retried successfully.
> 
> Personally, I think it makes little sense to print error messages in 
> the system log for commands where retries are disallowed.  Unless we go 
> ahead and print error messages for _all_ failures, including those 
> which are retried successfully.
> 
> Perhaps a good compromise would be to set the REQ_QUIET flag in 
> req->cmd_flags for readaheads.  That would suppress the error messages 
> coming from the SCSI core.
> 
> >   Yeah, we might make it more obvious that read failed and whether or not
> > we are going to retry. Just technically it's not so simple because a
> > different layer prints messages about errors (generic block layer) and
> > different (scsi disk driver) decides what to do (retry, don't retry, ...).
> 
> Actually the retry decisions (or many of them) are made by the SCSI 
> core, and that's also where some of those error messages come from.
> 
> > > 	I should have asked since I'm here at the moment - do you need any
> > > more information out of the buggy USB enclosure at the moment, or can I work
> > > on trying to fix/replace it now?
> >   No, feel free to do anything with it :). Thanks for your help with
> > debugging this.
> 
> To clarify, the enclosure isn't really very buggy.  It _should_ have
> carried out the failed commands, or if it had a valid reason for not
> doing so then it _should_ have reported the reason.  Regardless, the
> errors that occurred were harmless because they went away when the
> commands were retried.  (Although if they weren't harmless, you
> wouldn't be able to tell just from reading the system log...)
> 
> Alan Stern

Okay. Okay. Back up a moment here - Clarify a little. I have the filesystem
set to remount readonly on errors. I have not seen any filesystem
corruption or file corruption I could find. The filesystem *was* remounting
readonly under 2.6.31.5, but has not since .6 came out. (and I reformatted
and redid the entire backup under 2.6.31.6 without errors)

How do I know when it has generated an actual failure that was not
corrected?

How do I know when errors have been detected but they were corrected?

I'm guessing in the former, it'll remount ro, and in the latter it won't. Am
I correct?

I would like to save some money and not trash the usb enclosure... At the
same time, I don't want to use an enclosure that's trashing my data.

It is important to me to know exactly how the failure path operates. Please
explain to me what I will see happen. - Not knowing is driving me nuts.

Thank you,
Tim McGrath
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