Learn OpenBSD - Filesystem, Mount & Disks

#Introduction

In episode 6 you managed services with the rc system. But all those services write data somewhere — to the filesystem. This episode covers the storage foundation of OpenBSD: how data is organized, how disks are partitioned and formatted, how mounting works, and how to keep data healthy.

OpenBSD uses FFS2 as its default filesystem, laid out on partitions created by disklabel, mounted via /etc/fstab, and maintained with fsck. All of this is fully controllable from the command line — there's no GUI hiding anything.

#Filesystems on OpenBSD

#FFS2: The Default Filesystem

FFS2 (Fast File System 2) is an improved successor to UFS. Its important features:

• Support for large files and large volumes.
• Soft updates for metadata consistency without full journaling.
• Filesystem snapshots for consistent backups (covered in episode 11).
• Special mount options like softdep, noatime, and wxallowed.

Most system partitions — root, /var, /home — use FFS2.

#tmpfs and mfs

Besides FFS2, OpenBSD provides memory-based filesystems:

tmpfs is popular for reducing disk I/O: moving /tmp to tmpfs speeds up temporary operations, but its contents vanish on reboot. Mounting /tmp with the wxallowed option on OpenBSD allows executing binaries there (this option is considered risky, so it must be explicit).

#Mount and Umount

Mounting connects a filesystem to a directory. Without arguments, mount displays all active filesystems:

mount
mount /dev/sd0a /mnt
umount /mnt

Manual mount is used for repairs and recovery. For permanent mounts, you write an entry in /etc/fstab.

#Disklabel and newfs

#Reading and Changing Disk Labels

disklabel manipulates disk partitions. To view the layout:

disklabel sd0
disklabel -p wd0

Each partition is given a letter. You can also use disklabel -E for interactive editing. Remember: don't mess with partitions currently in use without a backup.

#Creating a New Filesystem

After a partition is created, format it with newfs:

newfs sd0d
newfs -O 2 /dev/rsd0d

The -O 2 option explicitly sets the FFS2 version. This operation destroys data on the partition — only do it for new disks/partitions.

#/etc/fstab: The Permanent Mount Blueprint

The /etc/fstab file determines what is mounted at boot. Its format: device, mount point, filesystem, options, dump frequency, and fsck pass:

/dev/sd0a / ffs rw,softdep 1 1
/dev/sd0b none swap sw 0 0
/dev/sd0d /tmp ffs rw,nodev,nosuid,softdep 1 2
tmpfs /var/run tmpfs rw,nodev,nosuid,mode=0755 0 0

Note the security options: nodev and nosuid prevent devices and setuid binaries from being mounted at points where they shouldn't be. softdep enables soft updates. The last column (1 2) determines the fsck pass order.

#Maintenance with fsck

fsck checks and repairs filesystem consistency. Before checking, the filesystem must be unmounted:

umount /dev/sd0d
fsck /dev/sd0d
mount /dev/sd0d

#Swap: Partitions and Swap Files

OpenBSD uses a swap partition (b) by default. But swap files are also supported — useful when you need more swap without repartitioning:

dd if=/dev/zero of=/var/swapfile bs=1m count=2048
chmod 600 /var/swapfile
chown 0:0 /var/swapfile
swapctl -a /var/swapfile

To make it permanent, add it to /etc/fstab as a swapfile entry. To view swap status:

swapctl -l

#Managing Disk Space

An administrator must know exactly what is consuming space:

df -h
du -h /var

df -h shows the usage of every filesystem; du walks through directory usage. The combination of both is the first step every time a disk fills up — and it will be used often in episode 11 when discussing backups.

#Closing

In episode 7 you managed OpenBSD storage: getting to know FFS2 as the main filesystem plus tmpfs and mfs, performing mount and umount, shaping partitions with disklabel and newfs, arranging the mount blueprint in /etc/fstab, maintaining health with fsck, and adding a swap file.

Key takeaways:

• FFS2 is the default filesystem; tmpfs is used for RAM-based temporary data.
• /etc/fstab determines permanent mounts with security options like nodev and nosuid.
• fsck is only run on unmounted filesystems; use bsd.rd for a damaged root.
• Swap can be a partition (b) or a file via swapctl.

In the next episode, episode 8, we'll build networking and network configuration — arranging /etc/hostname.if, using ifconfig and route, configuring DNS in resolv.conf, and getting to know aliases, VLAN, trunk, and bridge as a bridge toward the pf firewall.