I think one of the problems here is that there is no consistent requirements or figure of merit for performance.
You've argued against changes because of A) perceived impact to front-end I/O latency, B) writeback rate at idle, C) peak instantaneous writeback rate, D) time to writeback the entire amount. These are all, to some extent, contradictory. I was trying to make changes in writeback to improve **efficiency**-- the amount of writeback that is done per the amount of time the backing device is tied up. We could have added tunables for A/B/C to pick any operating along there. In addition, the changes I was trying to make were **on the path to further improvement**-- submitting sequential I/O together with plugging is known to improve merge. Users on #bcache IRC on oftc.net constantly talk about writeback performance and writeback use cases-- it's probably the most common topic of discussion about bcache. These changes would have improved that and laid the groundwork for further improvement. I've retired, but a year ago I was running a bcache environment with about 40 computers running development environments and continuous integration test rigs; we would provision and tear down hundreds of VMs per day; machines had 256-512GB of RAM, 1TB SATA RAID1 cache volumes and RAID10 sets 6-8 drives wide. The use case was enterprise DB-- so I think I know a thing or two about enterprise use cases ;) Writeback did obviously bad things in that environment, which is why I wanted to fix it. I also see obviously bad behavior on my small home test rig of RAID1 480GB NVMe + RAID1 6TB disks. Mike
