From mboxrd@z Thu Jan 1 00:00:00 1970 From: keith.busch@intel.com (Keith Busch) Date: Wed, 6 Jan 2016 19:31:28 +0000 Subject: Maximum NVMe IO command size > 1MB? In-Reply-To: References: Message-ID: <20160106193127.GA20732@localhost.localdomain> On Wed, Jan 06, 2016@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