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Date: Wed, 17 Jan 2024 16:16:56 +0000
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v2 00/16] block atomic writes
To: Christoph Hellwig <hch@lst.de>
Cc: "Darrick J. Wong" <djwong@kernel.org>, axboe@kernel.dk, kbusch@kernel.org,
        sagi@grimberg.me, jejb@linux.ibm.com, martin.petersen@oracle.com,
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        jack@suse.cz, linux-block@vger.kernel.org,
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From: John Garry <john.g.garry@oracle.com>
Organization: Oracle Corporation
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On 17/01/2024 15:02, Christoph Hellwig wrote:
> On Tue, Jan 16, 2024 at 11:35:47AM +0000, John Garry wrote:
>> As such, we then need to set atomic write unit max = min(queue max
>> segments, BIO_MAX_VECS) * LBS. That would mean atomic write unit max 256 *
>> 512 = 128K (for 512B LBS). For a DMA controller of max segments 64, for
>> example, then we would have 32K. These seem too low.
> 
> I don't see how this would work if support multiple sectors.
> 
>>
>> Alternative I'm thinking that we should just limit to 1x iovec always, and
>> then atomic write unit max = (min(queue max segments, BIO_MAX_VECS) - 1) *
>> PAGE_SIZE [ignoring first/last iovec contents]. It also makes support for
>> non-enterprise NVMe drives more straightforward. If someone wants, they can
>> introduce support for multi-iovec later, but it would prob require some
>> more iovec length/alignment rules.
> 
> Supporting just a single iovec initially is fine with me, as extending
> that is pretty easy.  Just talk to your potential users that they can
> live with it.

Yeah, any porting I know about has just been using aio with 
IO_CMD_PWRITE, so would be ok

> 
> I'd probably still advertise the limits even if it currently always is 1.
> 

I suppose that we don't need any special rule until we support > 1, as 
we cannot break anyone already using > 1 :)

Thanks,
John