linux-kernel - Re: QUEUE_FLAG

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Message-ID: <48F4539A.5000102@tuffmail.co.uk>
Date:	Tue, 14 Oct 2008 09:08:58 +0100
From:	Alan Jenkins <aj504@...dent.cs.york.ac.uk>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	jeff@...zik.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: QUEUE_FLAG_NONROT

Jens Axboe wrote:
> On Tue, Oct 14 2008, Pierre Ossman wrote:
>> Hi Jeff,
>>
>> I noticed you've added a new flag to indicate that the drive has no
>> seek costs and I figured it would be a good idea to use that on the
>> MMC/SD cards.
> 
> That was me, actually...
> 
>> Since the name isn't entirely clear in what is implied, I just wanted
>> to check that there are no plans to assume that there is negligable
>> request overhead for queues with this flag. I.e. the flag should
>> indicate that the elevator doesn't have to care about seeks, but it
>> should still try to merge requests to reduce the transaction overhead.
> 
> Sounds about right. The flag is just meant to indicate zero-seek cost,
> as devices will still have per-command overheads, merging is still
> applicable.
> 
> So yes, you want to set that flag for mmc/sd cards, definitely.

Is there a way for users to get / set it manually?  Can hdparm / sdparm / sg_inq tell me whether my device sets the flag... I think you said it was word 0x217 in a recent draft, but I don't know how I could query that as a user.

I'd like to know whether the SSD in my netbook provides the right flag - and if not, set it manually, instead of having to force the noop io scheduler.

It might also be possible to write a udev test program, which would be guaranteed exclusive access, to measure seek times and set the flag appropriately.  I assume we wouldn't be able to rely on USB flash drives having the right flag set.

Thanks
Alan
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