On 19.10.2018 16:08, Zdenek Kabelac wrote: > 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 Sorry, but I don't understand too. What be if reboot happens between data writes from the fast cache to the slow device? After reboot what data will be reads? A new data from the fast cache or an old from the slow device? And what data will be read 'dd if=/dev/cached iflag=direct'? Thanks.
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