Disks and Storage
Storage is a four-layer stack: the physical disk is a block device, partitions carve it into regions, a filesystem structures each region, and mount grafts it into the tree. This chapter covers each layer — the tools that see them (lsblk, fdisk), the ones that shape them (mkfs, parted), the /etc/fstab that makes mounts permanent, and the recovery work for when disks go bad.
Seeing the disks
lsblk # the tree — disks, partitions, mounts
lsblk -f # + filesystems and UUIDs
fdisk -l # partition tables, every disk
blkid # UUIDs + fs types — fstab's input
df -hT # mounted usage + types
findmnt # the mount tree
smartctl -a /dev/sda # the disk's own health report (SMART)
hdparm -I /dev/sda # hardware infolsblk is the first command — the visual map; blkid gives the UUIDs fstab prefers over /dev/sdX names (names shift; UUIDs don't).
Partitioning — the three tools
fdisk /dev/sda # MBR + GPT — the classic
gdisk /dev/sda # GPT specialist — the UEFI path
parted /dev/sda # scriptable — `parted -l` lists all
cfdisk /dev/sda # fdisk with menus — the kind oneMBR vs GPT: MBR — 4 primaries (or extended), BIOS-era, 2TB ceiling. GPT — 128 partitions, GUIDs everywhere, UEFI's partner, a backup table at the disk's end. Partition types carry numbers: 8300 Linux filesystem, 8200 swap, EF00 the EFI System Partition, 8E00 LVM.
Filesystems — mkfs and friends
mkfs.ext4 /dev/sda3 # the default choice
mkfs.xfs /dev/sda3 # large-scale
mkfs.btrfs /dev/sda3 # CoW + snapshots
mkfs.vfat -F32 /dev/sda1 # the ESP / USB sticks
mkfs.exfat /dev/sda3 # cross-platform big files
mkswap /dev/sda2 # swap
swapon /dev/sda2 # activate swap
tune2fs -l /dev/sda3 # ext4's tunables
mkfs.ext4 -L rootfs /dev/sda3 # a labelext4 = boring correct. btrfs = snapshots/send. xfs = throughput at scale. The FS choice is the disk's personality.
Mounting — graft and persist
mount /dev/sda3 /mnt # temporary
mount -o ro /dev/sda3 /mnt # read-only
mount -o remount,rw / # remount live — the rescue move
umount /mnt # release
umount -l /mnt # lazy — "device is busy" escape/etc/fstab makes it permanent — six fields:
/dev/sda3 / ext4 defaults 0 1
UUID=xxx /home ext4 defaults 0 2
/dev/sda2 none swap sw 0 0
tmpfs /tmp tmpfs defaults 0 0mount -a re-reads fstab — test fstab with it before rebooting; a broken fstab drops to emergency mode. Options worth knowing: noatime (skip access-time writes), ro, noexec/nosuid/nodev (hardening), _netdev (wait for network), nofail (don't die if absent), x-systemd.automount (lazy-mount).
The partition layouts — the schemes that matter
The classic simple: The UEFI scheme: The LFS scheme:
/dev/sda1 /boot ext4 /dev/sda1 ESP vfat /dev/sda1 ESP vfat
/dev/sda2 swap swap /dev/sda2 swap swap /dev/sda2 swap swap
/dev/sda3 / ext4 /dev/sda3 / ext4 /dev/sda3 / ext4
The separated: The LVM scheme: The encrypted:
sda1 ESP · sda2 / sda1 ESP · sda2 /boot sda1 ESP · sda2 /boot
sda3 /home · sda4 /var sda3 LUKS→LVM sda3 LUKS→dm-crypt→LVM→/+/home
→root · home · swap LVs →root · home · swapWhy separate /home: reinstalls wipe / but keep /home — the standard trick. /var separated keeps logs/spool from filling the root. /boot separated for encryption or exotic filesystems.
The filesystem comparison — choosing
| FS | Snapshots | CoW | Speed | Stability | The job |
|---|---|---|---|---|---|
| ext4 | no | no | fast | bulletproof | The default — boring correct |
| xfs | no | no | fastest at scale | proven | Huge files/filesystems |
| btrfs | yes | yes | good | maturing | Snapshots, subvolumes, send/recv |
| zfs | yes | yes | heavy | proven | The everything-fs — out-of-tree |
| f2fs | no | no | flash-tuned | good | SSD/flash-native |
| tmpfs | n/a | n/a | RAM | — | /run, /tmp |
| vfat | no | no | — | — | ESP + interop |
| exfat | no | no | — | — | USB + cross-platform |
| iso9660/udf/squashfs | no | no | — | — | CDs/live images |
The deeper layers — LVM, RAID, dm-crypt
pvcreate /dev/sda3 # physical volume
vgcreate vg0 /dev/sda3 # volume group
lvcreate -L 10G -n data vg0 # logical volume
mkfs.ext4 /dev/vg0/data # treat it like a partition
lvextend -L +5G /dev/vg0/data # grow it — resize2fs afterLVM stacks virtual partitions over physical — resizable, snapshot-able, movable across disks. mdadm does software RAID (mirror/stripe). dm-crypt/LUKS encrypts a whole partition — cryptsetup luksFormat + luksOpen → a /dev/mapper/ decrypted view. The modern stack: disk → LUKS → LVM → filesystem → mount — each layer a /dev/ node.
The rescue work — when disks go bad
fsck -n /dev/sda3 # check, don't fix — NEVER on a mounted fs
fsck.ext4 -y /dev/sda3 # fix — unmounted only!
e2fsck -b 32768 /dev/sda3 # the backup superblock
badblocks /dev/sda # surface scan
ddrescue /dev/sda img logfile # the dying-disk imager — rescue first
testdisk /dev/sda # partition recovery
photorec img # file carving from an image
mount -o loop img /mnt # the rescued image browsedfsck on a mounted filesystem destroys it — always unmount first. For dying hardware, ddrescue images what still reads before the disk dies fully.
Where storage goes wrong
| Symptom | Cause |
|---|---|
| fstab boots to emergency | Typo'd UUID/device — mount -a would've caught it |
| "device or resource busy" on umount | A process holds it — lsof /mnt, umount -l |
| fsck won't run | FS mounted — unmount first |
| Wrong partition got mkfs'd | lsblk first — always |
| Disk full but files fit | Inodes — df -i |
| Mount point empty after reboot | fstab missing the entry |
| SMART warnings | The disk is dying — image it now, not later |
Exercises
lsblk -f— map your real disks' layers.- Partition a scratch disk,
mkfs.ext4, mount, fstab it,mount -a, reboot-verify. blkid— UUIDs vs /dev names; write both fstab forms.mount -o remount,ro /on a scratch VM — feel the read-only world.smartctl -ayour disk — read its self-assessment.