Initializing a Linux Data Disk (fdisk)

Scenarios

This section uses CentOS 7.4 64bit to describe how to initialize a data disk attached to a server running Linux and use fdisk to partition the data disk.

The maximum partition size that MBR supports is 2 TiB and that GPT supports is 18 EiB. If the disk size you need to partition is greater than 2 TiB, partition the disk using GPT.

The fdisk partitioning tool is suitable only for MBR partitions, and the parted partitioning tool is suitable for both MBR and GPT partitions. For more information, see Scenarios and Disk Partitions.

The method for initializing a disk varies slightly depending on the OS running on the server. This document is used for reference only. For the detailed operations and differences, see the product documents of the corresponding OS.

Notice

When using a disk for the first time, if you have not initialized it, including creating partitions and file systems, the additional space added to this disk in an expansion later may not be normally used.

Prerequisites

A data disk has been attached to a server and has not been initialized.
You have logged in to the server.
- For how to log in to an ECS, see the Elastic Cloud Server User Guide.

Creating and Mounting a Partition

The following example shows you how a new primary partition can be created on a new data disk that has been attached to a server. The primary partition will be created using fdisk, and MBR will be used. Furthermore, the partition will be formatted using the ext4 file system, mounted on /mnt/sdc, and configured to mount automatically at startup.

Query what block devices are available on the server.

fdisk -l

Information similar to the following is displayed:

[root@ecs-test-0001 ~]# fdisk -l

Disk /dev/vda: 42.9 GiB, 42949672960 bytes, 83886080 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x000bcb4e

   Device Boot      Start         End      Blocks   Id  System
/dev/vda1   *        2048    83886079    41942016   83  Linux

Disk /dev/vdb: 107.4 GiB, 107374182400 bytes, 209715200 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

In the command output, this server contains two disks: /dev/vda and /dev/vdb. /dev/vda is the system disk, and /dev/vdb is the new data disk.

Launch fdisk to partition the new data disk.

fdisk New data disk

In this example, run the following command:

fdisk /dev/vdb

Information similar to the following is displayed:

[root@ecs-test-0001 ~]# fdisk /dev/vdb
Welcome to fdisk (util-linux 2.23.2).

Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Device does not contain a recognized partition table
Building a new DOS disklabel with disk identifier 0x38717fc1.

Command (m for help):

Enter n and press Enter to create a new partition.
Information similar to the following is displayed:
```
Command (m for help): n
Partition type:
   p   primary (0 primary, 0 extended, 4 free)
   e   extended
```
There are two types of disk partitions:
- Choosing p creates a primary partition.
- Choosing e creates an extended partition.
Note
If MBR is used, a maximum of four primary partitions, or three primary partitions plus one extended partition can be created. The extended partition must be divided into logical partitions before use.

Disk partitions created using GPT are not categorized.
Enter p and press Enter to create a primary partition in this example.
Information similar to the following is displayed:
```
Select (default p): p
Partition number (1-4, default 1): 
```
Partition number indicates the serial number of the primary partition. The value ranges from 1 to 4.
Enter the serial number of the primary partition and press Enter. Primary partition number 1 is used in this example. One usually starts with partition number 1 when partitioning an empty disk.
Information similar to the following is displayed:
```
Partition number (1-4, default 1): 1
First sector (2048-209715199, default 2048):
```
First sector indicates the start sector. The value ranges from 2048 to 209715199, and the default value is 2048.
Select the default start sector 2048 and press Enter.
The system displays the start and end sectors of the partition's available space. You can customize the value within this range or use the default value. The start sector must be smaller than the partition's end sector.

Information similar to the following is displayed:
```
First sector (2048-209715199, default 2048):
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-209715199, default 209715199):
```
Last sector indicates the end sector. The value ranges from 2048 to 209715199, and the default value is 209715199.
Select the default end sector 209715199 and press Enter.
The system displays the start and end sectors of the partition's available space. You can customize the value within this range or use the default value. The start sector must be smaller than the partition's end sector.

Information similar to the following is displayed:
```
Last sector, +sectors or +size{K,M,G} (2048-209715199, default 209715199):
Using default value 209715199
Partition 1 of type Linux and of size 100 GiB is set

Command (m for help):
```
A primary partition has been created for the new data disk.

Enter p and press Enter to print the partition details.

Information similar to the following is displayed:

Command (m for help): p

Disk /dev/vdb: 107.4 GiB, 107374182400 bytes, 209715200 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x38717fc1

   Device Boot      Start         End      Blocks   Id  System
/dev/vdb1            2048   209715199   104856576   83  Linux

Command (m for help):

Details about the /dev/vdb1 partition are displayed.

Enter w and press Enter to write the changes to the partition table.
Information similar to the following is displayed:
```
Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.
```
The partition is created.

Note
In case that you want to discard the changes made before, you can exit fdisk by entering q.
Synchronize the new partition table to the OS.
partprobe

Format the new partition with a desired file system format.

mkfs -t File system format /dev/vdb1

In this example, the ext4 format is used for the new partition.

mkfs -t ext4 /dev/vdb1

Information similar to the following is displayed:

[root@ecs-test-0001 ~]# mkfs -t ext4 /dev/vdb1
mke2fs 1.42.9 (28-Dec-2013)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
6553600 inodes, 26214144 blocks
1310707 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=2174746624
800 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
        4096000, 7962624, 11239424, 20480000, 23887872

Allocating group tables: done
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done

The formatting takes a period of time. Observe the system running status and do not exit.

Notice

The partition sizes supported by file systems vary. Choose an appropriate file system format based on your service requirements.

Create a mount point.
mkdir Mount point

In this example, the /mnt/sdc mount point is created.

mkdir /mnt/sdc

Note
The /mnt directory exists on all Linux systems. If the mount point cannot be created, it may be that the /mnt directory has been accidentally deleted. You can run mkdir -p /mnt/sdc to create the mount point.
Mount the new partition on the created mount point.
mount Disk partition Mount point

In this example, the /dev/vdb1 partition is mounted on /mnt/sdc.

mount /dev/vdb1 /mnt/sdc

Check the mount result.

df -TH

Information similar to the following is displayed:

[root@ecs-test-0001 ~]# df -TH
Filesystem     Type      Size  Used Avail Use% Mounted on
/dev/vda1      ext4       43G  1.9G   39G   5% /
devtmpfs       devtmpfs  2.0G     0  2.0G   0% /dev
tmpfs          tmpfs     2.0G     0  2.0G   0% /dev/shm
tmpfs          tmpfs     2.0G  9.1M  2.0G   1% /run
tmpfs          tmpfs     2.0G     0  2.0G   0% /sys/fs/cgroup
tmpfs          tmpfs     398M     0  398M   0% /run/user/0
/dev/vdb1      ext4      106G   63M  101G   1% /mnt/sdc

You should now see that partition /dev/vdb1 is mounted on /mnt/sdc.

Note

After the server is restarted, the disk will not be automatically mounted. You can modify the /etc/fstab file to configure automount at startup. For details, see Configuring Automatic Mounting at System Start.

Configuring Automatic Mounting at System Start

The fstab file controls what disks are automatically mounted at startup. You can use fstab to configure your data disks to mount automatically. This operation will not affect the existing data.

The example here uses UUIDs to identify disks in the fstab file. You are advised not to use device names to identify disks in the file because device names are assigned dynamically and may change (for example, from /dev/vdb1 to /dev/vdb2) after a server stop or start. This can even prevent the server from booting up.

Note

UUIDs are the unique character strings for identifying partitions in Linux.

Query the partition UUID.
blkid Disk partition

In this example, the UUID of the /dev/vdb1 partition is queried.

blkid /dev/vdb1

Information similar to the following is displayed:
```
[root@ecs-test-0001 ~]# blkid /dev/vdb1
/dev/vdb1: UUID="0b3040e2-1367-4abb-841d-ddb0b92693df" TYPE="ext4"
```
Carefully record the UUID, as you will need it for the following step.
Open the fstab file using the vi editor.
vi /etc/fstab
Press i to enter editing mode.
Move the cursor to the end of the file and press Enter. Then, add the following information:
```
UUID=0b3040e2-1367-4abb-841d-ddb0b92693df /mnt/sdc                ext4    defaults        0 2
```
In this example, the line starting with "UUID" is the information added. Edit this line to match the following format:
- UUID: The UUID obtained in 1.
- Mount point: The directory on which the partition is mounted. You can query the mount point using df -TH.
- Filesystem: The file system format of the partition. You can query the file system format using df -TH.
- Mount option: The partition mount option. Usually, this parameter is set to defaults.
- Dump: The Linux dump backup option.
  - 0: Linux dump backup is not used. Usually, dump backup is not used, and you can set this parameter to 0.
  - 1: Linux dump backup is used.
- fsck: The fsck option, which means whether to use fsck to check the disk during startup.
  - 0: The fsck option is not used.
  - If the mount point is the root partition (/), this parameter must be set to 1.
    If this parameter is set to 1 for the root partition, this parameter for other partitions must start with 2 because the system checks the partitions in the ascending order of the values.
Press Esc, enter :wq, and press Enter.
The system saves the configurations and exits the vi editor.
Verify that the disk is auto-mounted at startup.
1. Unmount the partition.
  umount Disk partition
  
  In this example, run the following command:
  
  umount /dev/vdb1
2. Reload all the content in the /etc/fstab file.
  mount -a
3. Query the file system mounting information.
  mount | grep Mount point
  
  In this example, run the following command:
  
  mount | grep /mnt/sdc
  
  If information similar to the following is displayed, automatic mounting has been configured:
```
root@ecs-test-0001 ~]# mount | grep /mnt/sdc
/dev/vdb1 on /mnt/sdc type ext4 (rw,relatime,data=ordered)
```

Parent topic: Initializing EVS Data Disks

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