Alignment Issue with Direct IO to NVMe Drive

[Jens Axboe]

On 2012-11-27 01:35, Laine Walker-Avina wrote:
> Hi all,
> 
> We are experiencing an issue with doing direct IO to a NVMe device I'm
> helping to develop. Every so often, the physical address given by
> sg_dma_address() is aligned to 0x800 instead of 0x1000 as specified by
> blk_queue_dma_alignement(queue, 4095) when the queue is initialized.
> The request is also split over multiple segments to make up for the
> missing space (eg: for a 4k IO it's split into two segments 2k in
> size, and for an 8k IO it's split into 3 segments--2k,4k,2k). Our
> design requires the physical segments given to the device be aligned
> to 4k boundaries and be multiples of 4k in size. When not doing direct
> IO the physical addresses appear to always be 4k aligned as expected.
> One possible issue is the kernel we're primarily testing against is
> 2.6.32-220 from CentOS, but we have observed similar behavior from a
> vanilla 3.3 kernel as well. Any help would be greatly appreciated.

I'm assuming you set the hardware sector size to 4k as well?

-- 
Jens Axboe