From mboxrd@z Thu Jan 1 00:00:00 1970 References: <06b65c62-8ca2-0d29-b98d-1a1585141f81@redhat.com> <8ecbd3c8-4819-9ff2-40b1-315e6ae03c97@izyk.ru> <36f0f734-c49f-eaf9-7ff5-8aedfd7345b3@redhat.com> <707aafdc-cfd2-3bdd-13bb-1bfdc08c6ad9@assyoma.it> From: Zdenek Kabelac Message-ID: Date: Fri, 19 Oct 2018 15:08:55 +0200 MIME-Version: 1.0 In-Reply-To: <707aafdc-cfd2-3bdd-13bb-1bfdc08c6ad9@assyoma.it> Content-Language: en-US Content-Transfer-Encoding: 7bit Subject: Re: [linux-lvm] Why doesn't the lvmcache support the discard (trim) command? Reply-To: LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Gionatan Danti , LVM general discussion and development , Ilia Zykov Dne 19. 10. 18 v 14:45 Gionatan Danti napsal(a): > On 19/10/2018 12:58, Zdenek Kabelac wrote: >> Hi >> >> Writecache simply doesn't care about caching your reads at all. >> Your RAM with it's page caching mechanism keeps read data as long as there >> is free RAM for this - the less RAM goes to page cache - less read >> operations remains cached. > > Hi, does it mean that to have *both* fast write cache *and* read cache one > should use a dm-writeback target + a dm-cache writethrough target (possibly > pointing to different devices)? > > Can you quantify/explain why and how faster is dm-writeback for heavy write > workload? Hi It's rather about different workload takes benefit from different caching approaches. If your system is heavy on writes - dm-writecache is what you want, if you mostly reads - dm-cache will win. That's why there is dmstats to also help identify hotspots and overal logic. There is nothing to win always in all cases - so ATM 2 different targets are provided - NVDIMMs already seems to change game a lot... dm-writecache could be seen as 'extension' of your page-cache to held longer list of dirty-pages... Zdenek