s3-passdb: Keep caches coherent

[Volker Lendecke]

On Mon, Aug 22, 2011 at 09:10:51AM +1000, Andrew Bartlett wrote:
> On Sun, 2011-08-21 at 16:40 +0200, Simo Sorce wrote:
> >     via  d713f9e s3-passdb: Only delete 1 entry from memcache.
> >        via  99bb3ee s3-passdb: Remove always the user from getpwsid
> > cache.
> >        via  1152aa8 s3-passdb: Keep caches coherent 
> 
> Andreas and Simo,
> 
> It seems you have also tripped across the passdb cache - I noticed it
> while working on the py_passdb layer, and wondered if you or anyone on
> the list knew the full background?
> 
> In particular, I wondered if anyone had any numbers on how much the
> passdb cache helps us, and if there is another way we could get that
> benefit?  As you have noticed, keeping the cache coherent takes quite a
> bit of effort.
> 
> In particular, we don't seem to have inter-processes cache coherency,
> except for the pdb_delete_user() call. 
> 
> The only call that uses the cache, as far as I can tell, is
> pdb_getsampwsid(), and it is only populated on pdb_getsampwnam()
> 
> I dug into the history of this cache, which has been moved around and
> re-factored many times, but never managed to find it's original
> incarnation.  Does anyone remember what it was added for?

Please keep up that particular cache or replace it
with something that makes us query the passdb backend a LOT
less frequently. Before you remove that cache, please
test a full domain logon including a range of applications
that do their own name lookups while running from autostart.
This was the scenario I unfortunately had to support and
thus had to add that particular cache.

I have a customer with a huge user database in a proprietary
LDAP directory that is unfortunately very slow. The customer
has already spent a LOT of money with the company providing
that LDAP server, but it remains very, very slow compared to
OpenLDAP. We can certainly stop supporting that kind of
environment, but this would as certainly create customer
specific patches that distros will have to support.

Alternatively, you may have hints how to tune those
proprietary DSAs for decent performance. If so, please
update me on that.

With best regards,

Volker Lendecke

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