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

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Date:   Wed, 28 Nov 2018 20:41:40 +0800
From:   Bob Liu <bob.liu@...cle.com>
To:     Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <david@...morbit.com>
Cc:     Allison Henderson <allison.henderson@...cle.com>,
        linux-block@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        martin.petersen@...cle.com, shirley.ma@...cle.com
Subject: Re: [PATCH v1 5/7] xfs: Add device retry

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