7.1 Block Devices, fdisk & gdisk (104.1)

7.1 Block Devices, fdisk & gdisk

Quick Summary: In Linux, storage devices are exposed to userspace as special block device nodes within the /dev/ filesystem hierarchy. Storage drives are divided into distinct storage boundaries using either legacy Master Boot Record (MBR/DOS) partition tables or modern GUID Partition Tables (GPT). System administrators manage MBR partitions using fdisk and GPT partitions using gdisk. Because partitioning utilities initially stage alterations in volatile memory, changes must be written to disk and synchronized with the running kernel using tools like partprobe or partx before filesystems can be formatted.

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1. Linux Storage Architecture & Block Device Taxonomy

In Unix-like operating systems, the Linux kernel abstracts underlying storage hardware into block devices. Unlike character devices (which stream data sequentially byte-by-byte, such as serial ports or terminal lines), block devices transfer data in fixed-size blocks (typically 512 bytes or 4096 bytes) and support random-access read and write operations via hardware sector addressing.

Block device special files reside in /dev/ and are created dynamically by the udev daemon. You can distinguish block devices in directory listings by the leading b character in the file permission string:

$ ls -l /dev/sda /dev/sda1 /dev/tty0
brw-rw---- 1 root disk 8, 0 Aug 29 10:00 /dev/sda
brw-rw---- 1 root disk 8, 1 Aug 29 10:00 /dev/sda1
crw--w---- 1 root tty  4, 0 Aug 29 10:00 /dev/tty0

In the output above, the numbers 8, 0 and 8, 1 represent the major device number (identifying the device driver subsystem, where 8 corresponds to SCSI/SATA disk drivers) and the minor device number (identifying the unique physical drive unit or partition index).

Comprehensive Block Device Naming Conventions

Linux assigns device node names deterministically based on the underlying hardware controller bus and storage protocol:

⚠️ LPIC-1 Trap — NVMe & MMC Partition Syntax: Notice the partition numbering syntax difference: SCSI/SATA partitions append the integer directly to the drive letter (/dev/sdb1), whereas NVMe and MMC/SD device partitions require an explicit p separator before the partition index (/dev/nvme0n1p1, /dev/mmcblk0p1). Forgetting the p on NVMe device names is a frequent exam mistake.

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2. Master Boot Record (MBR / DOS) Partitioning Architecture

The Master Boot Record (MBR) partitioning structure was introduced by IBM in 1983. It is located in the very first 512-byte physical sector (Sector 0, LBA 0) of the storage drive.

MBR 512-Byte Sector Layout (LBA 0):
┌────────────────────────────────────────────────────────┬────────────────────────────────────────────┬─────────────┐
│            Bootstrap Code (446 Bytes)                  │       Partition Table (64 Bytes)           │ Magic Boot  │
│       Primary bootloader stage code                    │     4 entries × 16 bytes per entry         │  Signature  │
│                                                        │  [Part 1] [Part 2] [Part 3] [Part 4]       │ (0x55 0xAA) │
└────────────────────────────────────────────────────────┴────────────────────────────────────────────┴─────────────┘
 0                                                        446                                          510         512

MBR Structural Constraints

1. The 2 TiB Disk Limit: The MBR partition table allocates exactly 32 bits for the starting sector Logical Block Address (LBA) and 32 bits for the partition sector count. With standard 512-byte sector sizes, 2^32 × 512 bytes = 2,199,023,255,552 bytes = 2 TiB. MBR cannot address or partition disk space beyond 2.19 TB.
2. Four Primary Partitions Maximum: Because the MBR partition table is only 64 bytes wide and each partition entry requires 16 bytes (64 / 16 = 4), an MBR drive can hold a maximum of 4 Primary Partitions.
3. Extended & Logical Partitions: To bypass the 4-partition limitation, one primary partition entry can be designated as an Extended Partition (Hex code 0x05 or 0x0f). The extended partition acts as a container holding a linked list of Extended Boot Records (EBRs), which define an arbitrary number of Logical Partitions.

MBR Partition Numbering Rules for Linux

• Primary Partitions: Always assigned numbers 1 through 4 (e.g., /dev/sda1, /dev/sda2, /dev/sda3, /dev/sda4).
• Extended Partition: Consumes one of the primary numbers (1 through 4).
• Logical Partitions: Always start numbering at 5 (e.g., /dev/sda5, /dev/sda6, /dev/sda7), regardless of how many primary partitions actually exist on the disk!

💡 LPIC-1 Exam Fill-in-the-Blank Alert: If a SATA disk /dev/sda contains exactly one primary partition (/dev/sda1) and one extended partition (/dev/sda2), what device node name does Linux assign to the first logical partition created inside the extended partition? Answer: /dev/sda5

3. Interactive MBR Partitioning with fdisk

The fdisk utility (from the util-linux package) is the traditional, menu-driven command-line partitioning tool for MBR partition tables on Linux.

To partition a block device, execute fdisk followed by the device node path (e.g., fdisk /dev/sdb):

# Launch fdisk on secondary SATA drive
# fdisk /dev/sdb

Welcome to fdisk (util-linux 2.37.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Command (m for help): 

Essential fdisk Interactive Commands Reference

Practical fdisk Step-by-Step Partition Creation

When creating a partition with n, specify partition boundaries. Linux administrators almost never calculate raw sector numbers manually for the end boundary; instead, use the +size{K,M,G,T,P} notation:

Command (m for help): n
Partition type
   p   primary (0 primary, 0 extended, 4 free)
   e   extended (container for logical partitions)
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-41943039, default 2048): [Press Enter for 2048]
Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-41943039, default 41943039): +10G

Created a new partition 1 of type 'Linux' and of size 10 GiB.

Command (m for help): t
Selected partition 1
Hex code or alias (type L to list all): 8e
Changed type of partition 'Linux' to 'Linux LVM'.

Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

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4. Essential MBR Partition Type Hex Codes

Every MBR partition entry includes a 1-byte system ID (hexadecimal) designating what filesystem or subsystem owns the partition. The LPIC-1 exam requires exact recall of these key codes:

💡 LPIC-1 Exam Fill-in-the-Blank Alert: What hexadecimal code must be entered in fdisk to set a partition's type to Linux LVM? Answer: 8e

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5. GUID Partition Table (GPT) & gdisk Management

Modern hardware with Unified Extensible Firmware Interface (UEFI) firmware replaces legacy MBR with the GUID Partition Table (GPT) standard (defined under the UEFI specification):

Key Architectural Features of GPT

1. Massive Capacity & 64-Bit LBAs: GPT utilizes 64-bit logical block addresses, supporting maximum partition and disk capacities up to 8 Zebibytes (8 ZiB = 9.44 x 10^21 bytes).
2. Arbitrary Partition Count: The standard GPT specification reserves 33 sectors for partition entries. With each entry sized at 128 bytes, GPT natively supports 128 primary partitions by default without requiring extended or logical containers.
3. Redundant Backup Headers: GPT writes a Primary GPT Header at LBA 1 and duplicates the entire table as a Secondary (Backup) GPT Header at the final physical sectors of the disk, protecting against header corruption.
4. Protective MBR (LBA 0): GPT places a legacy MBR structure at LBA 0 with a single partition of type 0xee covering the entire disk. This prevents legacy MBR-only tools from misidentifying the disk as blank and overwriting data.
5. Globally Unique Identifiers (GUIDs): Every GPT partition is assigned a unique 128-bit GUID (e.g., UUID="a1b2c3d4-..."), allowing persistent identification regardless of hardware bus reordering.

The gdisk (GPT fdisk) Utility

gdisk provides an interactive, text-mode command interface specifically engineered for GPT disks, matching the keystroke semantics of fdisk:

# Launch gdisk on secondary drive
# gdisk /dev/sdb
GPT fdisk (gdisk) version 1.0.8

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): 

gdisk Commands & 4-Digit Hex Codes

While gdisk shares standard commands (p print, n new, d delete, w write, q quit, v verify), it introduces the x command to access the expert / advanced functionality menu (for partition table alignment, GUID alteration, and backup header recovery).

Furthermore, gdisk uses 4-digit hexadecimal type codes by taking the legacy 2-digit MBR code and appending 00:

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6. Partition Discovery & Kernel Table Synchronization

When you modify and write a partition table on an active system, the Linux kernel may still hold the old partition map in memory if any partition on that device is currently mounted. If the kernel cannot automatically reload the table, fdisk outputs the warning:

WARNING: Re-reading the partition table failed with error 16: Device or resource busy.
The kernel still uses the old table. The new table will be used at the next reboot.

To force the running Linux kernel to re-read the partition table without rebooting the server, administrators use partprobe or partx.

1. partprobe

Part of the GNU parted package, partprobe requests that the operating system kernel re-read the partition tables on all disks or a specific block device:

# Inform kernel of partition changes on all attached drives
# partprobe

# Inform kernel of partition changes on a specific device
# partprobe /dev/sdb

2. partx

Part of util-linux, partx tells the kernel driver about the presence and numbering of on-disk partitions:

# Update the kernel's partition table map for /dev/sdb
# partx -u /dev/sdb

# Add all partitions found on /dev/sdb to the kernel map
# partx -a /dev/sdb

# Delete partition 3 from the kernel's runtime device map
# partx -d --nr 3 /dev/sdb

3. lsblk and blkid for Partition Discovery

# List all block devices in a visual hierarchy with filesystem types and UUIDs
$ lsblk -f
NAME        FSTYPE FSVER LABEL  UUID                                 FSAVAIL FSUSE% MOUNTPOINTS
sda                                                                                 
├─sda1      vfat   FAT32 BOOT   5A21-098F                             480.2M     6% /boot/efi
├─sda2      ext4   1.0   ROOT   a28b12f4-7e11-42b8-a6d1-817290123456   38.4G    42% /
└─sda3      swap   1     SWAP   d1e02934-8c01-4478-b112-998877665544                [SWAP]
sdb                                                                                 
└─sdb1      xfs          DATA   7f31c201-3810-47b2-8431-a0b1c2d3e4f5  192.5G     3% /mnt/data

# Inspect block device attributes including UUID and filesystem TYPE
# blkid /dev/sda2
/dev/sda2: UUID="a28b12f4-7e11-42b8-a6d1-817290123456" BLOCK_SIZE="4096" TYPE="ext4" PARTUUID="00084321-02"

💡 LPIC-1 Exam Fill-in-the-Blank Alert: What utility informs the Linux operating system kernel of partition table changes without requiring a system reboot? Answer: partprobe (or partx)