Re: [PATCH v1 5/7] xfs: Add device retry

From: Allison Henderson <allison.henderson@oracle.com>
To: Bob Liu <bob.liu@oracle.com>,
	Christoph Hellwig <hch@infradead.org>,
	Dave Chinner <david@fromorbit.com>
Cc: linux-block@vger.kernel.org, linux-xfs@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	martin.petersen@oracle.com, shirley.ma@oracle.com
Subject: Re: [PATCH v1 5/7] xfs: Add device retry
Date: Wed, 28 Nov 2018 09:47:13 -0700	[thread overview]
Message-ID: <b24bdf13-e5b3-3793-54fe-5eca5b10915a@oracle.com> (raw)
In-Reply-To: <f30fd190-28e5-938a-1cb9-3690559a60f0@oracle.com>

On 11/28/18 5:41 AM, Bob Liu wrote:
> On 11/28/18 3:35 PM, Christoph Hellwig wrote:
>> On Wed, Nov 28, 2018 at 04:08:50PM +1100, Dave Chinner wrote:
>>> So the first time through this loop the block layer devices what
>>> device to read from, then we iterate devices 1..n on error.
>>>
>>> Whihc means if device 0 is the only one with good information in it,
>>> we may not ever actually read from it.
>>>
>>> I'd suggest that a hint of "-1" (or equivalent max value) should be
>>> used for "device selects mirror leg" rather than 0, so we can
>>> actually read from the first device on command.
>>
>> Yes.  For one thing I think we really need to split this retry counter
>> of sorts from the write hints.  I.e. make both u8 types and keep them
>> separate.  Then start out with (u8)-1 as initialized by the block layer
>> for the first attempt.  The device then fills out which leg it used
>> (in the completion path, so that another underlying driver doesn't
>> override it!), and then the file system just preserves this value on
>> a resumit, leaving the driver to chose a new value when it gets a
>> non -1 value.
>>
> 
> Will update as suggested, thank you for all your feedback :)
> 
> -Bob
> 

Yes, thanks everyone for your feed back.  Maybe Bob and I can come up 
with some test cases that recreate the problem scenarios described here 
and see if we can work out a solution to the multi bio complexities. 
Thanks!

Allison