RAM 

1-2 GB for evaluation purposes 

At least 4 GB and probably more for production servers 

Local disk space 

300 MB disk space for binaries. By default, binaries installed from native packages are placed in /opt on UNIX systems. For evaluation purposes, an additional 2 GB local disk space for server software might be sufficient.

If you are using Directory Server, consider that entries stored in Directory Server use local disk space. Directory Server does not support logs and databases installed on NFS-mounted file systems. Sufficient space should be provided for the database on a local file system in, for example, /var/opt or /local. For a typical production deployment with a maximum of 250,000 entries and no binary attributes such as photos, 4 GB might be sufficient.