On Wed, Feb 12, 2020 at 01:47:53AM +0000, Damien Le Moal wrote: > On 2020/02/12 4:01, Tim Walker wrote: > > On Tue, Feb 11, 2020 at 7:28 AM Ming Lei <ming.lei@xxxxxxxxxx> wrote: > >> > >> 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://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. > > Exposing an HDD through multiple-queues each with a high queue depth is > simply asking for troubles. Commands will end up spending so much time > sitting in the queues that they will timeout. This can already be observed > with the smartpqi SAS HBA which exposes single drives as multiqueue block > devices with high queue depth. Exercising these drives heavily leads to > thousands of commands being queued and to timeouts. It is fairly easy to > trigger this without a manual change to the QD. This is on my to-do list of > fixes for some time now (lacking time to do it). Just wondering why smartpqi SAS won't set one proper .cmd_per_lun for avoiding the issue, looks the driver simply assigns .can_queue to it, then it isn't strange to see the timeout issue. If .can_queue is a bit big, HDD. is easily saturated too long. > > NVMe HDDs need to have an interface setup that match their speed, that is, > something like a SAS interface: *single* queue pair with a max QD of 256 or > less depending on what the drive can take. Their is no TASK_SET_FULL > notification on NVMe, so throttling has to come from the max QD of the SQ, > which the drive will advertise to the host. > > > 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? > > As far as I know, there is no merging going on once requests are passed to > the driver and added to an SQ. So this is beside the point. > The current default block scheduler for NVMe SSDs is "none". This is > decided based on the number of queues of the device. For NVMe drives that > have only a single queue *AND* the QUEUE_FLAG_NONROT flag cleared in their > request queue will can fallback to the default spinning rust mq-deadline > elevator. That will achieve command merging and LBA ordering needed for > good performance on HDDs. mq-deadline basically won't merge IO if STS_RESOURCE isn't returned from .queue_rq(), or blk_mq_get_dispatch_budget always return true. NVMe's .queue_rq() basically always returns STS_OK. Thanks, Ming
