From: Theodore Ts'o Subject: Re: bcache with existing ext4 filesystem Date: Mon, 24 Jul 2017 15:27:18 -0400 Message-ID: <20170724192718.t7n5zgualz5lillg@thunk.org> References: <20170724185703.GA31422@amd> <64c810cf-a95c-f862-f25a-ebd7419b2632@thelounge.net> <20170724191548.GA32425@amd> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Reindl Harald , linux-ext4@vger.kernel.org, kernel list , kent.overstreet@gmail.com, linux-bcache@vger.kernel.org To: Pavel Machek Return-path: Content-Disposition: inline In-Reply-To: <20170724191548.GA32425@amd> Sender: linux-bcache-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org 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