[DRBD-user] drbd + flashcache

Florian Haas florian at hastexo.com
Sun Feb 12 17:23:26 CET 2012


On Sat, Feb 11, 2012 at 7:30 PM, Pascal BERTON <pascal.berton3 at free.fr> wrote:
> Florian,
>
> I've watched your video, very interesting indeed... Unfortunately for you it
> raised some more questions to me :-)
> First, just to be sure I correctly understood : flashcache will only (But
> significantly) improve read operations, no impact or so on writes right ?

It's a write-back capable device, so if you configure it in that mode,
of course it will have an impact on writes.

> Then, a year or 2 ago I had the opportunity to play around with Ming Zhao's
> dm-cache (completely apart from DRBD by the way, it was a standalone
> system), and to what I remember it roughly looked like this flashcache thing
> I didn't know of, except it makes use of a ramdisk instead of an SSD drive
> (And it has an impact on write operations, as far as I remember). I
> immediately see one advantage compared to flash-cache : it would (should?)
> be more performant, being memory-based instead of IO-based. However,
> inconvenient, it would be certainly smaller.
> Have you ever had the chance to add that dm-cache feature to DRBD the way
> you did it with flashcache ? Do you know/think it would work efficiently
> with DRBD ? And, lastly, do you think it would make sense to think of
> combining dm-cache and flashcache together to get kind of an L1+L2 storage
> cache level pair ?

As far as I'm informed there are currently three generic block-layer
caching devices available on Linux: flashcache, dm-cache and bcache.
Of those, only bcache seemed to be seriously interested in a push
towards mainline until recently (though that may be changing). As for
myself I have zero experience with bcache and dm-cache, so flashcache
is the only one I can comment on.

Hope this helps. What's your intended use case?

Cheers,
Florian

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