I have been looking at the options for SSD caching for a bit now. Here is my 
take on the current options:

1) bcache - Seems to have lots of reliability issues mentioned on mailing list 
with little sign of improvement.

2) flashcache - Seems to be no longer (or minimally?) developed/maintained, 
instead folks are working on the fork enhanceio.

3) enhanceio - Fork of flashcache.  Dropped the ability to skip caching on 
sequential writes, which many folks have claimed is important for Ceph OSD 
caching performance. (see: https://github.com/stec-inc/EnhanceIO/issues/32)

4) LVM cache (dm-cache) - There is now a user friendly way to use dm-cache, 
through LVM.  Allows sequential writes to be skipped. You need a pretty recent 
kernel.

I am going to be trying out LVM cache on my own cluster in the next few weeks.  
I will share my results here on the mailing list.  If anyone else has tried it 
out I would love to hear about it.

-Brendan

> In a long term use I also had some issues with flashcache and enhanceio. I've 
> noticed frequent slow requests.
> 
> Andrei
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