Second, the administrator must inspect the current partition layout. The command gpart show ada0 (assuming ada0 is the target disk) displays partitions, their indices, start sectors, sizes, and, crucially, any free space. For a successful grow operation, there must be unallocated sectors immediately after the target partition. For a shrink operation, the partition must have enough internal free space within its file system to allow the truncation without data loss.
Another pitfall is sector alignment. Modern disks use 4K sectors, and gpart aligns partitions to 1 MB boundaries by default. When resizing manually with -s , specifying a size that does not respect alignment can degrade performance. Always use gpart show to view the current alignment and ensure new sizes are multiples of the alignment value. gpart resize partition
A key concept is that a partition is merely a defined range of sectors on a disk. Resizing involves either growing the partition into unallocated space immediately following it or shrinking the partition by moving its end boundary inward. The operation is only possible if the adjacent space is free. Moreover, gpart modifies the partition table, but it resize the file system inside the partition. Consequently, any resize operation must be paired with an appropriate file system resize command (e.g., growfs for UFS or zfs online expand for ZFS). Prerequisites and Critical Preparations Before executing any gpart resize command, a prudent administrator must take several preparatory steps. First and foremost is data backup . Although gpart is mature and reliable, an unexpected power loss or kernel panic during a partition table write can corrupt the partition layout, rendering data inaccessible. A full backup or, at minimum, a snapshot of critical data is non-negotiable. Second, the administrator must inspect the current partition