On Thu, Nov 29, 2018 at 03:35:07PM +0800, Coly Li wrote: > On 11/29/18 11:16 AM, fangchen sun wrote: > > hello guys, > > > > I want to change writeback_percent in bcache, but it seems that the > > value is hardcoded in the bcache drive code. Why is writeback_percent > > capped at 40 ？ Can I set it 60 or higher percent? What are risks of > > higher percent ? > > I just posted a 5 patches series titled "writeback performance tuning > options", which may permit people to set writeback percent to 70 at most > for research purpose. > > Generally I don't suggest to set a higher writeback percent, a lower > writeback percent number may start writeback thread earlier and have a > more smooth I/O latency number. Too much dirty data on cache device may > have negative impact for most of work load. > > You may try the patch set with a higher writeback percent, but unless > you have a strong reason, I will always to use the default values firstly. One thing I'm wondering about: The mode (writeback/writethrough) of the device is persistent, while the writable percentage and other tunables need to be reset every boot (registration). It'd be very nice if all the parameters of the bcache device were persistent. -- Vojtech Pavlik Director SUSE Labs
