[ZODB-Dev] Server-side caching

[Jim Fulton]

On Fri, Feb 10, 2012 at 4:49 AM, Pedro Ferreira
<jose.pedro.ferreira at cern.ch> wrote:
> Hello all,
>
> A (possibly silly) question: does ZEO have some kind of server-side cache? I
> mean, each time an oid is requested by one of the clients is it retrieved
> from the DB file directly, or are some of the objects kept in memory? From
> what I see in the code, the latter doesn't seem to happen.

No -- and yes. :)

The OS' file-system cache acts as a storage server cache.  The storage
server does (essentially) no processing to data read from disk, so an
application-level cache would add nothing over the disk cache provided by
the storage server.

Also note that, for better or worse, FileStorage uses an in-memory index
of current record positions, so no disk access is needed to find current data.

> I know there are client-side caches, but in a multiple client/server context
> I wonder if it's not faster to ask the DB for an oid that is already in
> memory instead of retrieving it from the client cache?

In general, I'd say no.  It can depend on lots of details, including:

- database size
- active set size
- network speed
- memory and disk speeds on clients and servers
- ...

Jim

-- 
Jim Fulton
http://www.linkedin.com/in/jimfulton