On 12/11/2019 06:39, Christian Balzer wrote: >> From internal >> customers and external users, the feedback of maximum writeback rate is >> quite positive. This is the first time I realize not everyone wants it. >> > The full speed (1TB/s) rate will result in initially high speeds (up to > 280MBs) in most tests, but degrade (and cause load spikes -> alarms) later > on, often resulting in it taking LONGER than if it had stuck with the > 4MB/s minimum rate set. > So yes, in my case something like a 32MB/s maximum rate would probably be > perfect. I have some backup/archival targetted "drive-managed" SMR drives which include a non-SMR magnetic storage cache area which can cause this sort of behaviour. Sustained random writes make the drives fill their cache, and then performance falls off a cliff, since the drive must start making many read-modify-write passes in the SMR area. e.g. this latency result: https://www.storagereview.com/images/seagate_archive_8tb_sata_main_4kwrite_avglatency.png (taken from https://www.storagereview.com/node/4665) - which illustrates performance after the drive's non-SMR internal write cache area is full. There is somewhat similar behaviour from some SSDs (plus the additional potential problem of thermal throttling from sustained writes, and other internal house-keeping operations): https://www.tweaktown.com/image.php?image=images.tweaktown.com/content/8/8/8875_005_samsung-970-evo-plus-ssd-review-96-layer-refresh_full.png Perhaps bcache could monitor backing store write latency and back-off to avoid this condition? Tim.
