Hi! On Mon 2017-07-24 15:27:18, Theodore Ts'o wrote: > 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. Another useful number is that spinning rust does less than 3MB/sec on common operations done by git. (Yes, probably because a lot of seeking). So... USB device should be able to help. Question for you was... Is the first 1KiB of each ext4 filesystem still free and "reserved for a bootloader"? If I needed more for bcache superblock (8KiB, IIRC), would that be easy to accomplish on existing filesystem? Thanks, Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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