Re: support for external persistent cache

[Cory Coager <ccoager@gmail.com>]

On Wed, Jan 19, 2011 at 03:15:40PM -0200, Roberto Spadim wrote:
> hummm....
> thinking again.....
> 
> what?s the today raid1 problem?
> it?s based on minimal disk size
> for example...
> /dev/sda 1gb
> /dev/sdb 1tb
> we can have a 1gb raid only
> 
> what you want is:
> 1tb raid0
> with a automatic /dev/sda to /dev/sdb write
> 
> 
> could we call this raid0-cache?
> i think we could implement it in raid....
> 
> check this fuse (filesystem) implementation: http://www.furquim.org/chironfs/
> it?s based on filesystem (ok it?s a raid1, not a cache like, but?s
> limited to max filesystem size, not the minimal filesystem size... ok
> it?s not secure to use it since small filesystem don?t have all
> information, but?s nice)
> 
> anyone want to make raid0-cache implementation?

I have two comments on this.

I don't think the cache device should be the same size as your
disk storage.  It should be a small cache size similar to
hardware RAID.

Also, we'd need a way to flush the data from the cache device
on bootup.