Re: [PATCH v2 03/15] block: Support data lifetime in the I/O priority bitfield

From: Bart Van Assche <bvanassche@acm.org>
To: Damien Le Moal <dlemoal@kernel.org>, Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org, linux-scsi@vger.kernel.org,
	linux-fsdevel@vger.kernel.org,
	"Martin K . Petersen" <martin.petersen@oracle.com>,
	Christoph Hellwig <hch@lst.de>,
	Niklas Cassel <Niklas.Cassel@wdc.com>,
	Avri Altman <Avri.Altman@wdc.com>, Bean Huo <huobean@gmail.com>,
	Daejun Park <daejun7.park@samsung.com>,
	Hannes Reinecke <hare@suse.de>
Subject: Re: [PATCH v2 03/15] block: Support data lifetime in the I/O priority bitfield
Date: Wed, 11 Oct 2023 13:51:00 -0700	[thread overview]
Message-ID: <b0b015bf-0a27-4e89-950a-597b9fed20fb@acm.org> (raw)
In-Reply-To: <46c17c1b-29be-41a3-b799-79163851f972@acm.org>

On 10/6/23 11:07, Bart Van Assche wrote:
> On 10/6/23 01:19, Damien Le Moal wrote:
>> Your change seem to assume that it makes sense to be able to combine 
>> CDL with
>> lifetime hints. But does it really ? CDL is of dubious value for solid 
>> state
>> media and as far as I know, UFS world has not expressed interest. 
>> Conversely,
>> data lifetime hints do not make much sense for spin rust media where 
>> CDL is
>> important. So I would say that the combination of CDL and lifetime 
>> hints is of
>> dubious value.
>>
>> Given this, why not simply define the 64 possible lifetime values as 
>> plain hint
>> values (8 to 71, following 1 to 7 for CDL) ?
>>
>> The other question here if you really want to keep the bit separation 
>> approach
>> is: do we really need up to 64 different lifetime hints ? While the scsi
>> standard allows that much, does this many different lifetime make 
>> sense in
>> practice ? Can we ever think of a usecase that needs more than say 8 
>> different
>> liftimes (3 bits) ? If you limit the number of possible lifetime hints 
>> to 8,
>> then we can keep 4 bits unused in the hint field for future features.
> 
> Hi Damien,
> 
> Not supporting CDL for solid state media and supporting eight different
> lifetime values sounds good to me. Is this perhaps what you had in mind?
> 
> Thanks,
> 
> Bart.
> 
> --- a/include/uapi/linux/ioprio.h
> +++ b/include/uapi/linux/ioprio.h
> @@ -100,6 +100,14 @@ enum {
>          IOPRIO_HINT_DEV_DURATION_LIMIT_5 = 5,
>          IOPRIO_HINT_DEV_DURATION_LIMIT_6 = 6,
>          IOPRIO_HINT_DEV_DURATION_LIMIT_7 = 7,
> +       IOPRIO_HINT_DATA_LIFE_TIME_0 = 8,
> +       IOPRIO_HINT_DATA_LIFE_TIME_1 = 9,
> +       IOPRIO_HINT_DATA_LIFE_TIME_2 = 10,
> +       IOPRIO_HINT_DATA_LIFE_TIME_3 = 11,
> +       IOPRIO_HINT_DATA_LIFE_TIME_4 = 12,
> +       IOPRIO_HINT_DATA_LIFE_TIME_5 = 13,
> +       IOPRIO_HINT_DATA_LIFE_TIME_6 = 14,
> +       IOPRIO_HINT_DATA_LIFE_TIME_7 = 15,
>   };

(replying to my own e-mail)

Hi Damien,

Does the above look good to you?

Thanks,

Bart.