linux-kernel - Re: Problems with the block-layer timeouts

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Date:	Sun, 2 Nov 2008 12:35:42 -0800
From:	Mike Anderson <andmike@...ux.vnet.ibm.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	Jens Axboe <axboe@...nel.dk>,
	SCSI development list <linux-scsi@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Problems with the block-layer timeouts

Alan Stern <stern@...land.harvard.edu> wrote:
> I spent most of the day yesterday debugging some tricky problems in the
> new block-layer timeout scheme.  Clearly it is in need of more work.
> 
> A major reason for these problems was that there doesn't seem to be a 
> clear a idea of when the timeout period should begin.  In 
> blk_add_timer() a comment says:

> How should this be fixed?  It would help to call scsi_dev_queue_ready()  
> before elv_next_request(), but that's not sufficient.  
> scsi_times_out() needs to recognize that a timeout for a non-running
> request can be handled by directly returning BLK_EH_HANDLED.  Right?
> 
> 

Tejun described a similar issue here.
http://thread.gmane.org/gmane.linux.ide/35603

And a fix to address the issue here.
http://thread.gmane.org/gmane.linux.ide/35725

Does the patch posted by Tejun address your issue?

> While I'm on the subject, there are a few related items that could be 
> improved.  In my tests, I was generating I/O requests simply by doing
> 
> 	dd if=/dev/sda ...
> 
> I don't know where the timeouts for these requests are determined, but
> they were set to 60 seconds.  That seems much too long.
> 

It is set by a udev rule and the value is historical.
http://thread.gmane.org/gmane.linux.scsi/45631/focus=45646


-andmike
--
Michael Anderson
andmike@...ux.vnet.ibm.com
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