On Sat, 4 May 2013 04:45:00 Ben Hutchings wrote:
> According to <https://lwn.net/Articles/548348/>, bcache is going into
> Linux 3.10 (though it now looks like it has missed that). With
> dm-cache and bcache in the kernel, would there still be a need for
> this third option?
Good question. :)
I didn't have a chance to examine dm-cache yet but I tried others.
To me bcache is the most worthless of all because it requires
specially prepared (formatted) data partition. This makes it difficult
(if possible) to attach cache to existing partition with data as one
would need 200% capacity and to perform long data-moving in order to
activate/deactivate caching. I don't know if this fundamental
show-stopper changed since I tried bcache. Besides bcache also
requires modified kernel which is limiting its "field of endeavour"
even further.
We packaged flashcache when there were no other feasible alternatives.
To use flashcache it is necessary to create a virtual device-mapper
device that combines data and cache devices. Assuming that data
partition already have data it is only needed to mount it through
associated flashcache device.
The brilliance of EnhanceIO is that it doesn't need intermediate
device at all and can be attached to any block device on-the-fly even
when device is already mounted. Another super-cool thing is that you
can attach EnhanceIO cache not just to partition but to partitioned
block device to cache all its partitions at once. Just like flashcache
enchanceio modules are built with DKMS and can be used with older
kernels.
So among the above caching solutions I like EnhanceIO the most, for its
elegance and usefulness. IMHO EnhanceIO's unique features qualifies
for packaging at least for now until in-kernel solution(s) catch-up.
Unfortunately all three solutions are not too effective due to
"overcaching" effect, see
http://raid6.com.au/posts/SSD_caching_problems/
Regards,
Dmitry Smirnov.
---
What can be asserted without proof can be dismissed without proof.
-- Christopher Hitchens, 2004
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