Re: [LSF/MM/BPF BoF]: A host FTL for zoned block devices using UBLK

[Ming Lei <ming.lei@redhat.com>]

On Mon, Feb 06, 2023 at 10:00:20AM +0000, Hans Holmberg wrote:
> I think we're missing a flexible way of routing random-ish
> write workloads on to zoned storage devices. Implementing a UBLK
> target for this would be a great way to provide zoned storage
> benefits to a range of use cases. Creating UBLK target would
> enable us experiment and move fast, and when we arrive
> at a common, reasonably stable, solution we could move this into
> the kernel.

Yeah, UBLK provides one easy way for fast prototype.

> 
> We do have dm-zoned [3]in the kernel, but it requires a bounce
> on conventional zones for non-sequential writes, resulting in a write
> amplification of 2x (which is not optimal for flash).
> 
> Fully random workloads make little sense to store on ZBDs as a
> host FTL could not be expected to do better than what conventional block
> devices do today. Fully sequential writes are also well taken care of
> by conventional block devices.
> 
> The interesting stuff is what lies in between those extremes.
> 
> I would like to discuss how we could use UBLK to implement a
> common FTL with the right knobs to cater for a wide range of workloads
> that utilize raw block devices. We had some knobs in  the now-dead pblk,
> a FTL for open channel devices, but I think we could do way better than that.
> 
> Pblk did not require bouncing writes and had knobs for over-provisioning and
> workload isolation which could be implemented. We could also add options
> for different garbage collection policies. In userspace it would also 
> be easy to support default block indirection sizes, reducing logical-physical
> translation table memory overhead.
> 
> Use cases for such an FTL includes SSD caching stores such as Apache
> traffic server [1] and CacheLib[2]. CacheLib's block cache and the apache
> traffic server storage workloads are *almost* zone block device compatible
> and would need little translation overhead to perform very well on e.g.
> ZNS SSDs.
> 
> There are probably more use cases that would benefit.
> 
> It would also be a great research vehicle for academia. We've used dm-zap
> for this [4] purpose the last couple of years, but that is not production-ready
> and cumbersome to improve and maintain as it is implemented as a out-of-tree
> device mapper.

Maybe it is one beginning for generic open-source userspace SSD FTL,
which could be useful for people curious in SSD internal. I have
google several times for such toolkit to see if it can be ported to
UBLK easily. SSD simulator isn't great, which isn't disk and can't handle
real data & workloads. With such project, SSD simulator could be less
useful, IMO.

> 
> ublk adds a bit of latency overhead, but I think this is acceptable at least
> until we have a great, proven solution, which could be turned into
> an in-kernel FTL.

We will keep improving ublk io path, and I am working on ublk
copy. Once it is done, big chunk IO latency could be reduced a lot.

 
Thanks,
Ming