Bug#702256: EnhanceIO redundant with bcache?
On Sat, 4 May 2013 11:50:16 Ben Hutchings wrote:
> > 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.
>
> But it won't require a modified kernel in future. That's my point.
>
Of course. I understand. But that's for the future...
> > 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.
> [...]
>
> Bcache can apparently do that as well, but as you noted the backing
> device must have been prepared with a bcache superblock. Apparently
> the reasoning for this is to prevent accidental access that bypasses
> the cache. It seems like this could be relaxed if the cache is
> configured for write-through behaviour, though.
Even if bcache were already in 3.8 kernel it would still be worthless
unless it could be possible to attach "write-through" or
"write-around" (AKA "read-only") cache to existing partition with
data. If it can be done then we still need to wait for kernel with
bcache support. Meanwhile EnhanceIO could fill the gap.
I with you on your concerns for the future. I hope eventually we will
have an elegant SSD-caching solution with native in-kernel support.
Especially if it can work effectively under heavy load.
However today EnhanceIO have no alternative yet. At the moment I'm
only concerned about its quality as upstream just broke it to unusable
state. I wonder if it will mature before support for bcache will
arrive...
All the best,
Dmitry Smirnov
GPG key : 4096R/53968D1B
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-- Winston Churchill
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