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

[Nitesh Shetty <nj.shetty@samsung.com>]

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On Mon, Feb 06, 2023 at 08:49:15PM +0800, Ming Lei wrote:
> 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.
> 

Just curious, will this also involve running do_splice_direct*() in async style
like normal async read/write, instead of offloading to iowq context ?

Regards,
Nitesh Shetty

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