On Mon, Jul 24, 2017 at 09:15:48PM +0200, Pavel Machek wrote: > > > > Am 24.07.2017 um 20:57 schrieb Pavel Machek: > > >Would it be feasible to run bcache (write-through) with existing ext4 > > >filesystem? > > > > > >I have 400GB of data I'd rather not move, and SSD I could use for > > >caching. Ok, SSD is connecte over USB2, but I guess it is still way > > >faster then seeking harddrive on random access > > > > i doubt that seriously - USB2 has a terrible latency > > Well.. if that's too slow, I can get SSD M.2; plus bcache docs says > that combination works. > > And... if you ever tried to do git diff while git checkout is running > on spinning rust... spinning rust has awful parameters when idle, and > it only gets worse when loaded :-(.
So some hard numbers. Max throughput of USB 2.0 is 53 MiB/s[1]. In actual practice the max throughput you will see out of the USB 2.0 interface is 30-40 MiB/s. In contrast, a HDD doing sequential reads can easily do much more than that. [1] https://superuser.com/questions/317217/whats-the-maximum-typical-speed-possible-with-a-usb2-0-drive So a lot is going to depend on how bcache works. If you can get large sequential reads and writes to *bypass* the cache device, then I think there's a good cache that bcache on a USB 2.0 device won't hurt. It might not help as much as you like, but that's a function of the overhead of populating the cache and whether the cache can keep the useful bits in the cache device. Cheers, - Ted
