OpenLdap: Why slapd tell me there's no DB_CONFIG when there is one

When I run slapcat or restart slapd on Debian 6, OpenLdap 2.4.23-7.32.4.23-7.3, I see this warning:

hdb_db_open: warning - no DB_CONFIG file found in directory /var/lib/ldap: (14).
Expect poor performance for suffix "dc=i-tracing,dc=lan".

But when I take a look:

[root@xxx-ldap:/etc/ldap]# ls -lh /var/lib/ldap
total 5,8M
[...]
-rw-r--r-- 1 openldap openldap 3,1K  8 nov.  12:00 DB_CONFIG
[...]

And

ps aux | grep openldap
openldap 22919  4.4  1.5  87204  5860 ?        SLsl 12:20   0:02 /usr/sbin/slapd -h ldaps://xxx-ldap.xxx.lan:636/ ldap://127.0.0.1:389 -g openldap -u openldap -f /etc/ldap/slapd.conf

Do you know what can I do in order to show Slapd that DB_CONFIG is really here, in /var/lib/ldap ?

NB: Even when doing this:

service slapd stop
cp /tmp/DB_CONFIG /var/lib/ldap/
chown openldap:openldap /var/lib/ldap/DB_CONFIG
service slapd start

It keeps telling that there's no DB_CONFIG file in /var/lib/ldap !?!

Maybe others person may hit the same problem. For me, the version of openldap:

openldap.x86_64 2.4.40-13.el7 @base openldap-clients.x86_64 2.4.40-13.el7 @base openldap-servers.x86_64 2.4.40-13.el7 @base

I found the DB_CONFIG file in /usr/share/openldap-servers/DB_CONFIG.example, so just cp the file to the directory of ldap. After fix the problem, the result of slaptest is:

599ade3a bdb_monitor_db_open: monitoring disabled; configure monitor database to enable config file testing succeeded

I have finally found the root cause, it was hidden in the creation time of DB_CONFIG:

-rw-r--r-- 1 openldap openldap 3,1K  8 nov.  12:00 DB_CONFIG

It was exactly the time of last slapd restart.

I have fixed it with those commands:

service slapd stop
cp /tmp/DB_CONFIG /var/lib/ldap/
chown openldap:openldap /var/lib/ldap/DB_CONFIG
service slapd start