nscd not caching

Stefan Esser se at freebsd.org
Mon Aug 18 07:52:53 UTC 2014

Am 17.08.2014 um 18:10 schrieb Adam McDougall:
> On 08/17/2014 09:09, Eggert, Lars wrote:
>> Nobody using nscd? Really?
> I would test for you, but we retired our NIS infrastructure at least a
> year ago.  I did have it working on a test client at some point, but I
> didn't push it into production because I found a couple issues (below).
> The two main problems I recall were nscd making java crash, and nscd
> holding on to negative cache lookups too long, causing failures while
> installing ports that depend on adding users/groups for a following file
> permission change.  I can't remember if the latter issue was fixed at
> some point.  I also can't remember if I was receiving perfectly accurate
> results from the cache either.

I added the "negative-confidence-threshold" option to nscd, a few
years ago.  If set to a number > 1 (the default), then that number
of failures are required to cause a negative cache entry.  Setting
this value to 3 should allow for 2 probes for the presence of a UID
or username, before the cache returns a failure without bothering
to re-check the source.  The value should be low enough to prevent
flooding of a remote source with requests, if an entry really does
not exist.

The default was left unchanged - you need to increase the value to
see any effect of this threshold.  3 might be a reasonable default
for the user database.  But I never bothered to suggest and discuss
an increased default value on the mail-lists ...

> I dabbled with nscd a bit after we switched from NIS to LDAP.  I think I
> recall lookups being slightly slower WITH the cache, plus I would get
> some duplicated group entries returned on all but the first getent
> group.  The short version is we in no way seem to benefit or require a
> cache of LDAP with our site size, so I'm just not using nscd.  I didn't
> make bug reports for these issues, I had to prioritize towards more
> pressing issues.  I'm trying to do better about reporting bugs.

I also found that there were glitches, when I tested the extension
to cache only the nth negative reply. The code is not easy to read
and change (IMHO), and I did not succeed when I tried to reproduce
and debug these glitches.

Regards, STefan

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