On Sunday 20 January 2008 18:51, Theodore Tso wrote:
quoted text
> On Sat, Jan 19, 2008 at 08:10:20PM -0800, Daniel Phillips wrote:
> > I can see value in preemptively loading indirect blocks into the
> > buffer cache, but is building a second-order extent tree really
> > worth the effort?  Probing the buffer cache is very fast.
>
> It's not that much effort, and for a big database (say, like a 50GB
> database file), the indirect blocks would take up 50 megabytes of
> memory.  Collapsing it into an extent tree would save that memory
> into a few kilobytes.  I suppose a database server would probably
> have 5-10GB's of memory, so the grand scheme of things it's not a
> vast amount of memory, but the trick is keeping the indirect blocks
> pinned so they don't get pushed out by some vast, gigunndo Java
> application running in the same server as the database.  If you have
> the indirect blocks encoded into the extent tree, then you don't have
> to worry about that.

Hi Ted,

OK I think you are right, because this is a nice step towards developing 
an on-disk extent format for Ext4 that avoids committing design 
mistakes to permanent storage.  The benefit can be proven using a pure 
cache, in order to justify the considerable work necessary to make it 
persistent.

Chris and Jens have an effort going to implement a physical disk extent 
cache for loop.c.  It is actually the same problem, and I smell a 
library here.

Issue: how do you propose to make this cache evictable?

Regards,

Daniel
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