feature request: allow bigger cache poisoning

[jpsollie]

Could a parameter be added to allow bcachefs to delay writes to background target until a certain % of foreground cache is filled? this is mainly for power-saving reasons: if I have a foreground SSD for let's say 500G of the total area of the background, it may as well wait before spinning up the large background hardware raid 6 HDD (80TB) until it makes sense to do so, to avoid a case where the user uploads a 512k file, continues to work on it -> raid spins down, and after 1h a new version is stored, so the disk spins up again, simply to store 512k of data... Not sure whether a flush is required at fs unmount, that"s mostly esthetical

[mocksoul]

I guess you can remove/add background target by cron... :)