From mboxrd@z Thu Jan 1 00:00:00 1970 From: Lauri Niskanen Subject: dm-cache with a very slow device Date: Fri, 7 Aug 2015 08:39:21 +0300 Message-ID: <55C44489.2050101@ape3000.com> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: dm-devel@redhat.com List-Id: dm-devel.ids Hi, In my understanding dm-cache is usually used with fairly fast spinning hard drives as the "slow" device. Because of this the cache is not used very aggressively and sequential accesses skip the cache. Is it possible to configure dm-cache for a scenario where the fast disk is a state-of-the-art SSD, but the slow disk is very slow especially so that sequential reads and writes are significantly slower than on the fast device? I'd like it to work as a very aggressive writeback buffer, so that all or at least close to all writes are done to the fast device if there is at least some free non-dirty space left. And sequential reads should also be promoted fairly easily. In addition there should be aggressive demotions to make room for future promotions always keeping a sizable buffer available. So the goal would be to achieve the performance characteristics of the fast device for all kinds of use cases even when the backing slow device is at least a magnitude slower. -- Lauri Niskanen