Re: [LSF/MM/BPF TOPIC] NVMe HDD

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

> 
> 
> 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://lore.kernel.org/linux-scsi/Pine.LNX.4.44L0.1909181213141.1507-100000@xxxxxxxxxxxxxxxxxxxx/
[2] https://lore.kernel.org/linux-scsi/20191226083706.GA17974@ming.t460p/


Thanks, 
Ming





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