On Fri, Dec 20, 2019 at 2:24 AM Lennart Poettering <mzerqung@xxxxxxxxxxx> wrote: > So, if this is desirable, why doesn't the kernel do this on its own? A relevant recent article on the subject: Issues around discard https://lwn.net/Articles/787272/ There is a huge range of device firmware flash translation layer behaviors. There's no mechanism for that behavior to be announced to the file system or block layer, so that the kernel could adapt accordingly. For example the FTL can change behavior as the SSD fills up. It leads to users reporting different problems for the same workload, with the same SSD make/model/firmware, differing only in how full the SSD is. And also the file systems aren't even all on the same page. Nor is the SSD industry. > Why do we need a userspace component that just gets an event from the > kernel and then tells the kernel to do something? If this is generally > desirable, why is something as trivial as that not a kernel > functionality anyway? Second attempt at answering this. As it relates to online discards (by using the filesystem 'discard' mount option): adding a delay for issuing discards means the kernel having to track possibly thousands of delayed discards. And what happens when it expires? An even bigger set of discards, decently likely causing a more noticeable device hang, than without this hypothetical feature. Drives don't offer any kind of rate limiting for discard Conversely, the batch discard method (e.g. fstrim.timer->fstrim command) causes the file system to search for unused blocks. And yes that one time per week batch could be huge. Hence doing it at midnight on a Monday; which if that's missed, it happens at some point during the first boot on Monday. Free space fragmentation, as a result of file system aging, affects the duration of the batch discard. Combined with some (perhaps many) SSDs that have slow discard behavior, can cause the batch discard to take either a second, or minutes. For the same SSD. Literally one idea kernel developers have, is to enable 'discard' mount option (online discard) by default. The idea is that inevitably irate end users will compel vendors to fix their shit via ridicule. I find this hilarious because the most civil "go fuck yourself" invented in the modern era is the expired warranty. I expect this idea would just lead to a lot of finger pointing, and they really should have just done this 9 years ago, hindsight being 20/20. So yeah. *shrug* -- Chris Murphy _______________________________________________ devel mailing list -- devel@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to devel-leave@xxxxxxxxxxxxxxxxxxxxxxx Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/devel@xxxxxxxxxxxxxxxxxxxxxxx
