It's 240GB (half of a Samsung 850) in front of a 1TB 5400RPM disk. The size isn't critical. 1/8 is chosen to exceed 10% (default writeback dirty data thresh), it might need to be 1/6 on really big environments. It needs to be big enough that it takes more than 100 seconds to write back, but less than 40% of the cache[1]. I am also running this on "mountain", which is my virtualization server. It has 2x Samsung 950 PRO 512GB NVMe disks (MD raid1) in front of 2x Seagate Ironwolf 6TB (MD raid1) drivers. I can't run benchmarks on it, but it runs nightly builds and is otherwise lightly loaded in the middle of night. It does writeback for about 75 minutes on average after a set of nightly builds of my repositories (last 5 days 75, 79, 77, 72, 76); without this patch set it was more like 80 (77, 80, 74, 85, 84, 76, 82, 84). It has 128GB of RAM and doesn't do many reads-- most of its hottest working set hits the page cache-- bcache serves to accelerate the random writes its workloads generate. This is the workload that I sought to improve with the writeback changes. When you ran the tests, did you compare 1-5 to 1-3? Because otherwise, another uncontrolled factor is the writeback tuning is drastically different with patch 2. (On most systems patch 2 makes writeback less aggressive overall by default). Mike [1]: If you don't think this is fair--- this would match how I've been suggesting enterprise users set up bcache for various workloads-- a writeback cache isn't going to be very effective at lowering write workload if the size of the most active write working set doesn't fit nicely in the cache. so e.g. if you deploy under a SQL database the cache should be double the size of the indices to get a real write aggregation benefit; if you deploy under an environment doing builds the cache should be double the size of the amount written during builds; if you deploy under an environment doing rapid VM provisioning the cache should be double the size of the images blitted per provisioning cycle, etc. Basically if this criterion isn't met, the writeback cache doesn't do very much to improve performance. I think the test scenarios you ran didn't satisfy this :/
