On Tue, Feb 11, 2020 at 7:28 AM Ming Lei [off-list ref] wrote:

On Mon, Feb 10, 2020 at 02:20:10PM -0500, Tim Walker wrote:

quoted

Background:

NVMe specification has hardened over the decade and now NVMe devices
are well integrated into our customers’ systems. As we look forward,
moving HDDs to the NVMe command set eliminates the SAS IOC and driver
stack, consolidating on a single access method for rotational and
static storage technologies. PCIe-NVMe offers near-SATA interface
costs, features and performance suitable for high-cap HDDs, and
optimal interoperability for storage automation, tiering, and
management. We will share some early conceptual results and proposed
salient design goals and challenges surrounding an NVMe HDD.

HDD. performance is very sensitive to IO order. Could you provide some
background info about NVMe HDD? Such as:

- number of hw queues
- hw queue depth
- will NVMe sort/merge IO among all SQs or not?

quoted

Discussion Proposal:

We’d like to share our views and solicit input on:

-What Linux storage stack assumptions do we need to be aware of as we
develop these devices with drastically different performance
characteristics than traditional NAND? For example, what schedular or
device driver level changes will be needed to integrate NVMe HDDs?

IO merge is often important for HDD. IO merge is usually triggered when
.queue_rq() returns STS_RESOURCE, so far this condition won't be
triggered for NVMe SSD.

Also blk-mq kills BDI queue congestion and ioc batching, and causes
writeback performance regression[1][2].

What I am thinking is that if we need to switch to use independent IO
path for handling SSD and HDD. IO, given the two mediums are so
different from performance viewpoint.

[1] https://urldefense.proofpoint.com/v2/url?u=https-3A__lore.kernel.org_linux-2Dscsi_Pine.LNX.4.44L0.1909181213141.1507-2D100000-40iolanthe.rowland.org_&d=DwIFaQ&c=IGDlg0lD0b-nebmJJ0Kp8A&r=NW1X0yRHNNEluZ8sOGXBxCbQJZPWcIkPT0Uy3ynVsFU&m=pSnHpt_uQQ73JV4VIQg1C_PVAcLvqBBtmyxQHwWjGSM&s=tsnFP8bQIAq7G66B75LTe3vo4K14HbL9JJKsxl_LPAw&e=
[2] https://urldefense.proofpoint.com/v2/url?u=https-3A__lore.kernel.org_linux-2Dscsi_20191226083706.GA17974-40ming.t460p_&d=DwIFaQ&c=IGDlg0lD0b-nebmJJ0Kp8A&r=NW1X0yRHNNEluZ8sOGXBxCbQJZPWcIkPT0Uy3ynVsFU&m=pSnHpt_uQQ73JV4VIQg1C_PVAcLvqBBtmyxQHwWjGSM&s=GJwSxXtc_qZHKnrTqSbytUjuRrrQgZpvV3bxZYFDHe4&e=


Thanks,
Ming

I would expect the drive would support a reasonable number of queues
and a relatively deep queue depth, more in line with NVMe practices
than SAS HDD's typical 128. But it probably doesn't make sense to
queue up thousands of commands on something as slow as an HDD, and
many customers keep queues < 32 for latency management.

Merge and elevator are important to HDD performance. I don't believe
NVMe should attempt to merge/sort across SQs. Can NVMe merge/sort
within a SQ without driving large differences between SSD & HDD data
paths?

Thanks,
-Tim

-- 
Tim Walker
Product Design Systems Engineering, Seagate Technology
(303) 775-3770