Monthly Archives: July 2018

Basic Concepts: Linux Disks

Disk Interface

  • IDE (ATA): Bandwidth is 133 Mbps. IOPS is ~100. The interface can connect maximum 2 disks.
  • SCSI: IOPS is ~ 150. It can connects 8 or 16 disks
    • Ultrascsi320 – 320 MB/s
    • Ultrascsi640 – 640 MB/s
  • SATA: Bandwidth is 6 Gbps. IOPS is ~150. It can connects to 8 or 16 disks.
  • SAS: Bandwidth is 6 Gbps. IOPS is ~200. It can connects to 8 or 16 disks.
  • USB: Bandwidth is 480 MB/s. IOPS is vary.

Linux Disks

Disk (Device) Types

Block: Can be accessed randomly. Unit is “block”.

Character: Can be accessed sequentially. Unit is “character”.

Disk Files (FHS)

Files are under ‘/dev/’. Every disk (device) is file on Linux environment.

Device ID:

Major: Primary device ID. To identify device type for proper drivers.

Minor: Secondary device ID. It’s the entree of specific device of same type of device.

Create new device:

# mknode

[root@centos] mknode /dev/usbtest b 100 231 
[root@centos] ll | grep test 
brw-r--r--. 1 root root 100, 231 Jul 1 11:02 usbtest 

Disk Name

It’s assigned by ICANN.

IDE: /dev/hd[a-z]

SCSI/SATA/USB/SAS: /dev/sa[a-z][0-9]

There are 3 identifications below. You also can identify disks by go to /dev/disk/by-*.

  • Device file name
  • Volume labels
  • UUIDs

Disk Partition

MBR: Master Boot Record. It stars on sector 0. Size is 512 bytes. It consist of 3 parts:

  • Bootloader and boot applications. Size is 446 bytes.
  • Partition tables. Maximum 4 tables. 16 bytes each. Size is 64 bytes.
  • MBR availability mark. ’55AA’ means “active”. Size is 2 bytes.

The maximum ‘Primary‘ partition tables are 4. If you need more than 4 partitions, you need to create ‘Extend‘ partition as the last partition of the 4, then create ‘Logical partitions‘ on top of ‘Extend partition‘. Labels of the 4 are start from 1 to 4. ‘Logical partitions’ always starts from 5.