F20 System Wide Change: SSD cache

[Conrad Meyer]

On Tue, 16 Jul 2013 10:36:55 +0200
Florian Weimer <fweimer at redhat.com> wrote:

> On 07/15/2013 09:25 PM, Conrad Meyer wrote:
> 
> > By default, bcache runs a write-through cache -- it only
> > caches clean data. If the caching SSD dies, the bcache
> > layer can just forward requests to spinning drive. No
> > data is lost.
> >
> > (Bcache has a writeback mode where data loss is possible.
> > I do not recommend this mode.)
> 
> What's the benefit of bcache, compared to just sticking
> more RAM in the machine?  That you can get more cache,
> especially on systems that are short on memory sockets?  Or
> that the cache persits across reboots (something that can
> be tricky because it requires synchronizing writes to the
> cache and the disk)?

From 5 minutes of research:
  - 512 GB SSD on newegg -- $390.
  - 512 GB of RAM on newegg -- $4200*.
* Doesn't include the cost of a server board that has 32 ram
  slots.

So bcache is a more cost-effective way (than RAM) to expand
the working set of disk you can access very quickly.

bcache in write-back mode must persist or else you suffer
data loss on any power failure. So, I think that answers that
question. Getting the syncing right isn't actually that hard.

Regards,
Conrad