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(ITS#7657) Alias dereferencing with MDB slow compared with BDB

Full_Name: Mark Cairney
Version: 2.4.35-RE24
OS: SL 6.4 64-bit
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (

Having migrated the database of our production service running 2.4.35 from BDB
to MDB alias de-referencing stopped working completely (this has already been
reported in ITS# 7557).

Upgrading to the RE 24 now has alias derefencing working but it is considerably
slower than BDB was for the same search. 

To confirm this I downgraded one of our nodes (we run 4 nodes in multi-master)
and on that node the search results are once more acceptable:

With MDB:
time ldapsearch -H ldaps://alder.authorise.is.ed.ac.uk:636 -b
'ou=staff,ou=law,ou=hss,dc=authorise,dc=ed,dc=ac,dc=uk' -a always

# numResponses: 425
# numEntries: 424

real	0m26.490s
user	0m0.028s
sys	0m0.032s

with BDB:

time ldapsearch -H ldaps://oak.authorise.is.ed.ac.uk:636 -b
'ou=staff,ou=law,ou=hss,dc=authorise,dc=ed,dc=ac,dc=uk' -a always

real	0m2.605s
user	0m0.054s
sys	0m0.006s

If there's anything additional I can supply (log files, strace output) let me

Our OpenLDAP DB currently has around 400,000 user accounts if that's relevant.