Re: measurable slower result time with large subtree

[Quanah Gibson-Mount]

--On Wednesday, August 24, 2022 12:27 PM +0200 Norbert 
<openl...@freakix.de> wrote:

Hi,

with OpenLDAP 2.4.47 (running on Debian 10) but also with 2.5.13 from
ltb-project.org (running on same Debian 10) I can observe the following:

given following rough idea of a tree

o=base
   |- bn=subtree1
   |    - handful of entries
   |- bn=subtree2
   |    - millions of entries
   |- bn=subtree3
   |    |- bn=sub-subtree1
   |    |- bn=sub-subtree2
   |    |- some entries
   |- bn=subtree4

ou is an indexed attribute with pres,eq,sub.

Two comments:

a) pres indices are almost always worthless.  They are only needed if 
you're going to be presence checking on an attribute that exists in < 50% 
of the entries.  Do you really do presence checks on ou?  See also 
<https://www.openldap.org/doc/admin26/tuning.html#Presence%20indexing>

Is there something which can be improved by changing the configuration?

You probably want to increase the IDL range if you truely have millions of 
entries.  See also:

<https://www.openldap.org/doc/admin26/tuning.html#Indexes>

and

<https://www.openldap.org/software/man.cgi?query=slapd-mdb&apropos=0&sektion=0&manpath=OpenLDAP+2.6-Release&arch=default&format=html>

Specifically the section on "idlexp".

In my current production environment, I have:

dn: olcBackend={0}mdb,cn=config
objectClass: olcBackendConfig
objectClass: olcMdbBkConfig
olcBackend: {0}mdb
olcBkMdbIdlExp: 18


Note that olcBkMdbIdlExp is type "backend" and not "database", and applies 
to *all* mdb databases in the configuration.  You will have to re-index the 
database after setting this, I'd suggest doing that offline with slapindex 
-q.

Regards,
Quanah