# Why Does the Disk Drive Letter Change After the ECS Is Restarted?

#### Symptom

For a Linux ECS, the drive letter may change after an EVS disk is detached and then attached again, or after an EVS disk is detached and then the ECS is restarted.

#### Root Cause

When an Linux ECS has multiple disks attached, it allocates drive letters in the attachment sequence and names the disks as /dev/vda1, /dev/vdb1, and /dev/vdc1, etc.

After a disk is detached and then attached again, or after a disk is detached and the ECS is restarted, the drive letter may change.

For example, an ECS has three disks attached: /dev/vda1, /dev/vdb1, and /dev/vdc1. The mounting parameters in /etc/fstab are as follows:

cat /etc/fstab

    UUID=b9a07b7b-9322-4e05-ab9b-14b8050bdc8a  /  ext4  defaults  0  1
    /dev/vdb1                       /data1   ext4  defaults  0  0
    /dev/vdc1                       /data2   ext4  defaults  0  0

After /dev/vdb1 is detached and the ECS is restarted, /dev/vdc1 becomes /dev/vdb1 and is mounted to /data. In such a case, no disk is mounted to /data2.

The change of drive letters can affect the running of applications. To solve this problem, you are advised to use the universally unique identifiers (UUIDs) to replace /dev/vdx because a UUID uniquely identifies a disk partition in the Linux OS.

#### Solution

1. Log in to the ECS.
2. Run the following command to obtain the partition UUID:

   blkid Disk partition

   In this example, run the following command to obtain the UUID of the /dev/vdb1 partition:

   blkid /dev/vdb1

   Information similar to the following is displayed:

       [root@ecs-test-0001 ~]# blkid /dev/vdb1
       /dev/vdb1: UUID="b9a07b7b-9322-4e05-ab9b-14b8050cd8cc" TYPE="ext4"

   The UUID of the /dev/vdb1 partition is displayed.

3. Run the following command to open the fstab file using the vi editor:

   vi /etc/fstab

4. Press i to enter the editing mode.
5. Move the cursor to the end of the file and press Enter. Then, add the following information:

       UUID=b9a07b7b-9322-4e05-ab9b-14b8050cd8cc    /data1   ext4    defaults        0 0

   The parameters are defined as follows:

   - UUID=b9a07b7b-9322-4e05-ab9b-14b8050cd8cc: UUID of a disk partition.
   - /data1: directory on which the partition is mounted. You can run df -TH to query the directory.
   - ext4: File system format of the partition. You can run df -TH to query the format.
   - defaults: partition mount option. Normally, this parameter is set to defaults.
   - 0 (the first one): whether to use Linux dump backup.
     - 0: Linux dump backup is not used. Normally, dump backup is not used, and you can set this parameter to 0.
     - 1: Linux dump backup is used.
   - 0 (the second on): fsck option, that is, whether to use fsck to check disks during startup.
     - 0: fsck is not used.
     - If the mount point is the root partition (/), this parameter must be set to 1.

       When this parameter is set to 1 for the root partition, this parameter for other partitions must start with 2 so that the system checks the partitions in the ascending order of the values.

6. Repeat steps 2 to 5 to replace the UUID of /dev/vdc1.
7. Run the following command again to check the disk mounting parameters:

   cat /etc/fstab

   The following information is displayed:

       UUID=b9a07b7b-9322-4e05-ab9b-14b8050bdc8a  /  ext4  defaults  0  1
       UUID=b9a07b7b-9322-4e05-ab9b-14b8050cd8cc   /data1   ext4  defaults  0  0
       UUID=b9a07b7b-9322-4e05-ab9b-14b8050ab6bb   /data2   ext4  defaults  0  0

Parent topic: Disk Management