InformationThe auditing daemon, auditd, stores log data in the /var/log/audit directory.
The reasoning for mounting /var/log/audit on a separate partition is as follow.
Protection from resource exhaustion
The default installation only creates a single / partition. Since the /var/log/audit directory contain the audit.log file that can grow quite large, there is a risk of resource exhaustion. It will essentially have the whole disk available to fill up and impact the system as a whole. In addition, other operations on the system could fill up the disk unrelated to /var/log/audit and cause auditd to trigger it's space_left_action as the disk is full. See man auditd.conf for details.
Fine grained control over the mount
Configuring /var/log/audit as its own file system allows an administrator to set additional mount options such as noexec/nosuid/nodev. These options limits an attackers ability to create exploits on the system. Other options allow for specific behavior. See man mount for exact details regarding filesystem-independent and filesystem-specific options.
Protection of audit data
As /var/log/audit contains audit logs, care should be taken to ensure the security and integrity of the data and mount point.
Resizing filesystems is a common activity in cloud-hosted servers. Separate filesystem partitions may prevent successful resizing, or may require the installation of additional tools solely for the purpose of resizing operations. The use of these additional tools may introduce their own security considerations.
SolutionFor new installations, during installation create a custom partition setup and specify a separate partition for /var/log/audit.
For systems that were previously installed, create a new partition and configure /etc/fstab as appropriate.