The ZFS (Zettabyte File System) ARC (Adaptive Replacement Cache) is a memory cache used by the ZFS filesystem to improve performance. You can set the size of the ARC on a FreeBSD system by using the kern.vfs.zfs.arc_max sysctl variable.
To set the size of the ARC, you can use the sysctl command with the kern.vfs.zfs.arc_max variable. For example, to set the ARC size to 4 GB, you can use the following command:
sysctl kern.vfs.zfs.arc_max=4g
Replace 4g with the desired size of the ARC. You can specify the size in bytes, kilobytes (k), megabytes (m), or gigabytes (g).
Note that the ARC size is limited by the amount of available physical memory on the system. If you set the ARC size to a value that is larger than the available physical memory, the ARC size will be reduced to the size of the available memory.
To make the change permanent, you can add the kern.vfs.zfs.arc_max variable to the /etc/sysctl.conf file. For example:
kern.vfs.zfs.arc_max=4g
This will set the ARC size to 4 GB every time the system is booted.