Maximum NVMe IO command size > 1MB?

[Xuehua Chen]

Hi, Keith, 

I wonder whether this could be caused by BIO_MAX_PAGES defined as 256, which means 1MB at most.
What do you think?

Xuehua

________________________________________
From: Linux-nvme [linux-nvme-bounces at lists.infradead.org] on behalf of Xuehua Chen [xuehua at marvell.com]
Sent: Wednesday, January 6, 2016 11:51 AM
To: Keith Busch
Cc: linux-nvme at lists.infradead.org
Subject: RE: Maximum NVMe IO command size > 1MB?

The value is 2048, which seems to be 2MB.


________________________________________
From: Keith Busch [keith.busch at intel.com]
Sent: Wednesday, January 6, 2016 11:31 AM
To: Xuehua Chen
Cc: linux-nvme at lists.infradead.org
Subject: Re: Maximum NVMe IO command size > 1MB?

On Wed, Jan 06, 2016 at 07:23:53PM +0000, Xuehua Chen wrote:
> It seems to me kernel 4.3 supports NVMe IO command size > 512k after the following is added.
>
> blk_queue_max_segments(ns->queue,
>        ((dev->max_hw_sectors << 9) / dev->page_size) + 1);
>
> If I run the fllowing,
> fio --name=iotest --filename=/dev/nvme0n1 --iodepth=1 --ioengine=libaio --direct=1 --size=1M --bs=1M --rw=read
>
> I can see one read with data transfer size 1MB is sent to device.
>
> But if I increase the bs to 2M as below, I still see two 1MB commands are sent out instead of one 2MB read command
> fio --name=iotest --filename=/dev/nvme0n1 --iodepth=1 --ioengine=libaio --direct=1 --size=2M --bs=2M --rw=read
>
> Is there any other settings in kernel that make it split a 2M command into two 1M commands?

Is the device actually capable of 2MB transfers? You can confirm with:

  # cat /sys/block/nvme0n1/queue/max_hw_sectors_kb

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