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
