Date: 01-13-2022

The accesslog is a database internal to OpenLDAP primarily used to support “delta syncreple” which only replicates the changed parts of entries. It can also be used to provide a more detailed history of database activity than the standard slapd logs provide.

These examples set the accesslog database content to be retained for 3 days and then purged every day thereafter. The database maxsize is 8589934592 bytes (8 GB) and the database is only used for delta-sync replication. These settings are typical for performance in a standard operational setting.

Example Accesslog Overlay Config

# Access log - used for delta-syncrepl too overlay accesslog
logdb         cn=accesslog
logops        writes
logsuccess    TRUE< o:p> 
logpurge      3+00:00 01+00:00

Example Accesslog Database Config

### Accesslog Database Configuration ###
database      mdb
directory     /var/symas/openldap-data/accesslog
maxsize       8589934592
suffix        cn=accesslog
rootdn        cn=accesslog
index         entryCSN,objectClass,reqEnd,reqResult,reqStart eq 
overlay       syncprov
syncprov-nopresent       TRUE
syncprov-reloadhint      TRUE

How to tell how full the accesslog is

There are two indicators to track how "full" the accesslog database is. Both use the built-in mdb_stat function.

1. To get the actual size used, you multiply the number of pages used by the page size value. This will give you the number of bytes in use, which you can compare to the configured maxsize value to track how "full" the database is.

Caution: when deletes occur the pages are still flagged as used, but they are put on the freelist. As long as the rate of change and rate of deletion are near equal, it is unlikely you will see page usage grow and it will never shrink.

mdb_stat -e /var/symas/openldap-data/accesslog 
Environment Info
Map address: (nil)
Map size: 8589934592
Page size: 4096      <------------
Max pages: 262144
Number of pages used: 675      <------------
Last transaction ID: 4
Max readers: 126
Number of readers used: 1
4096 (page size) x 675 (used pages) = 2764800/8589934592 = 3.21% Full 

2. To know how many pages are actively being used, subtract the freelist pages from the pages used. Then follow instructions from step 1.

mdb_stat -ef /var/symas/openldap-data/accesslog Environment Info
Map address: (nil)
Map size: 8589934592
Page size: 4096
<-----------
Max pages: 262144
Number of pages used: 675      <-----------
Last transaction ID: 4
Max readers: 126
Number of readers used: 1
Freelist Status
Tree depth: 1
Branch pages: 0
Leaf pages: 1
Overflow pages: 0
Entries: 2
Free pages: 86      <-----------
4096 (page size) x 589 (675 (used pages) - 86 (free pages)) = 2412544 / 8589934592 = 2.81% Full

However, the former strategy is better when considering whether or not the DB is in danger of becoming "full", because the used pages value gives you information on your "upper" bound usage, (i.e. the most pages that have ever been in full use).

For example, let's say in a average day, 500 pages are the most that are ever used. But during a special circumstance there are an above average number of writes that occur, and the pages used jumps up to 1000. Then mdb_stat will report 1000 as the "pages used", even if 500 is the average usage. This indicates that generally on the upper end, 1000 is the most pages ever in use. The purpose is revealing what the usage is when things are more extreme, as opposed to normal operations.

Recommended delta-sync replication configuration

Configure delta-syncrepl configurations to purge the accesslog multiple times a day.

For example:

logpurge      03+00:00 00+04:00

It is problematic for some clients to have the master delay write operations in a busy environment while the accesslog is purged of a full day's worth of expired changes. Instead, break it down into 6 purges a day. This minimizes the delay during the purge phase, thus reducing the performance impact for write handling and replication.