Re: IOCache vs. block cache

On 2010-06-17 at 20:23:55 [+0200], Rene Gollent <[email protected]> wrote:
> On Thu, Jun 17, 2010 at 8:51 AM, Ingo Weinhold <[email protected]> wrote:
> > I think eventually the block cache should be rewritten to be VMCache based
> > with on-demand mapping of hot blocks, but for the time being a simpler
> > solution would be possible: The IOCache could be extended to make it
> > possible to lay it under the IOSchedulerSimple. This would give us a
> > VMCache based cache close to the device. The block cache could then work
> > with a very small, limited (!) set of unused blocks.
> 
> A question - what cases would the block cache actually used in? It
> seems as if the file / IO caches cover the cases of actual file data,
> which would seem to only leave things like FS metadata blocks going
> directly through the block cache.  And even that seems somewhat
> redundant if the IOCache is also at the device level. Am I missing
> something?

It is FS dependent whether file and/or block cache are used. Unlike the 
IOCache, which is a fully transparent write-through cache, both provide a 
public API for the FS. The block cache is transactional. Blocks of committed 
transactions are written with delay.

CU, Ingo

------------------------------------------------------------------------------
ThinkGeek and WIRED's GeekDad team up for the Ultimate 
GeekDad Father's Day Giveaway. ONE MASSIVE PRIZE to the 
lucky parental unit.  See the prize list and enter to win: 
http://p.sf.net/sfu/thinkgeek-promo