[linux-lvm] Why doesn't the lvmcache support the discard (trim) command?

Zdenek Kabelac zkabelac at redhat.com
Fri Oct 19 13:08:55 UTC 2018

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?


It's rather about different workload takes benefit from different caching 

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...


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