Re: [PATCH v4 17/25] ibnbd: client: main functionality

[Bart Van Assche <bvanassche@acm.org>]

On 9/18/19 12:14 AM, Danil Kipnis wrote:
> I'm not familiar with dm code, but don't they need to deal with the
> same situation: if I configure 100 logical volumes on top of a single
> NVME drive with X hardware queues, each queue_depth deep, then each dm
> block device would need to advertise X hardware queues in order to
> achieve highest performance in case only this one volume is accessed,
> while in fact those X physical queues have to be shared among all 100
> logical volumes, if they are accessed in parallel?

Combining multiple queues (a) into a single queue (b) that is smaller 
than the combined source queues without sacrificing performance is 
tricky. We already have one such implementation in the block layer core 
and it took considerable time to get that implementation right. See e.g. 
blk_mq_sched_mark_restart_hctx() and blk_mq_sched_restart().

dm drivers are expected to return DM_MAPIO_REQUEUE or 
DM_MAPIO_DELAY_REQUEUE if the queue (b) is full. It turned out to be 
difficult to get this right in the dm-mpath driver and at the same time 
to achieve good performance.

The ibnbd driver introduces a third implementation of code that combines 
multiple (per-cpu) queues into one queue per CPU. It is considered 
important in the Linux kernel to avoid code duplication. Hence my 
question whether ibnbd can reuse the block layer infrastructure for 
sharing tag sets.

Thanks,

Bart.