Re: [SPDK] Read request exceeding max_hw_sectors_kb

[Daniel Verkamp <daniel.verkamp at intel.com>]

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On 06/13/2018 04:47 AM, Christoph Hellwig wrote:
> On Wed, Jun 13, 2018 at 04:11:56PM +0530, Jitendra Bhivare wrote:
>> So something like resolves the issue:
>> static void nvme_set_chunk_size(struct nvme_ns *ns)
>> {
>> 	u32 chunk_size = (((u32)ns->noiob) << (ns->lba_shift - 9));
>>
>> 	chunk_size = rounddown_pow_of_two(chunk_size);
>> 	chunk_size = min(ns->ctrl->max_hw_sectors, chunk_size);
>> 	blk_queue_chunk_sectors(ns->queue,
>> rounddown_pow_of_two(chunk_size));
>> }
>>
>> Please do let me know if this is the right approach or SPDK should set
>> noiob appropriately.
> 
> I think the answer is both:  The kernel code should be more robust
> in handling bogus noiob sizes, so please submit a patch per your above
> analysis.  spdk should be fixed to not expose a bogus noiob size as well.

Hi Christoph,

By my understanding, this is what the spec is trying to say about NOIOB
(though unfortunately not very clearly): for best performance, read and
write I/Os should not cross an LBA that is a multiple of NOIOB.  For
example, if NOIOB is 64KB, an I/O of LBA=48KB size=32KB should be split
into two smaller I/Os, LBA=48KB size=16KB and LBA=64KB size=16KB, since
it crosses a 64KB LBA boundary.

Based on the current wording in the spec, I don't believe there is any
requirement that NOIOB be less than or equal to MDTS; it's purely about
avoiding I/O overlapping any boundary where LBA = NOIOB * n.

That said, we could consider some kind of workaround on the target side
if this kernel host code is in wide use already, like clamping NOIOB to
MDTS (accounting for unit conversion) or reporting NOIOB = 0 if NOIOB
would be greater than MDTS.  I'd like to avoid this route if possible,
though.

Based on a quick glance at the kernel history, it looks like the host
code has been using NOIOB since Linux 4.12 (commit 6b8190d61a622), so
I'm surprised we haven't had more reports about this failing.  Did
something change in the kernel block stack more recently than that to
cause chunks larger than the maximum data transfer size, or has this
always been an issue since the introduction of the NOIOB host code?

Thanks,
-- Daniel