large ldap server recommendation

[ram <ram@netcore.co.in>]

Hi,

  I am using ldap for authentication & addressbook  for a large
mailserver setup with around 300k users  ( this will grow to 500k )

The ldap server is a 8GB Ram box with RHEL-5 with
openldap-servers-2.3.27-5

I am confused what database type to use ldbm or bdb. Currently I have
the users on bdb with lot of problems. The ldap server dies all of a
sudden and I have to recover the data to get it started 


my DB_CONFIG file is 

------



# Note: most DB_CONFIG settings will take effect only upon rebuilding
# the DB environment.

set_cachesize   0      524288000        0
set_lg_regionmax        1048576
set_lg_max              10485760
set_lg_bsize            20485760
set_tmp_dir             /tmp


# Note: special DB_CONFIG flags are no longer needed for "quick"
# slapadd(8) or slapindex(8) access (see their -q option).

set_flags DB_LOG_AUTOREMOVE
set_flags DB_LOG_INMEMORY
set_flags DB_TXN_NOSYNC

-------------




Can someone help me 


Thanks
Ram