:Troubleshoot the difference between cloud disk size and purchased capacity

Problem description

You purchase a 200-GiB cloud disk for a Linux ECS instance and initialize the disk. When you run the df -h command to query the disk capacity, 197 GiB is returned instead of 200 GiB.

1. The output of the df -h command displays a total capacity (Size) that is different from the capacity that you purchased for the cloud disk in the ECS console.

2. The sum of used space (Used) and available space (Avail) is different from the total capacity (Size).

Causes

Issue 1: Total capacity (Size) in the df -h output is different from the purchased disk capacity in the ECS console

File system overhead

If you purchased a 200-GiB cloud disk, formatted the disk, and then attached the disk to the Linux ECS instance, you may see that the total capacity is only about 197 GiB. The issue occurs because additional data structure is generated when file systems are created. The data structure occupies disk space. Even if the data structure occupies a small amount of disk space, the data structure does affect the total capacity.

Issue 2: The sum of used space (Used) and available space (Avail) is different from the total capacity (Size)

In most cases, the issue occurs because inodes occupy disk space or an amount of disk space is reserved by the system. For information about how to calculate the disk space occupied by inodes or the space reserved by the system, see the Troubleshooting methods section of this topic.

Inode consumption

A file system allocates an inode to each file to record file attributes, such as the permissions and owner. The inodes occupy an amount of disk space. The disk space occupied by inodes is not counted by the df -h command.

System reservation

In addition to inodes, Linux reserves an amount of disk space to prevent the root directory from being completely filled, which ensures the stability and security of the system. The reserved disk space is not included in the sum of the used space (Used) and the available space (Avail) in the output of the df -h command.

Troubleshooting methods

sudo tune2fs -l /dev/[$Partition] | grep -E "count|size:"

Calculate the disk space occupied by inodes

Run the following command to obtain the total amount of disk space occupied by inodes:

sudo tune2fs -l /dev/[$Partition] | awk '/Inode count:/{c=$3} /Inode size:/{split($0,a,":"); s=a[2]} END{if(c && s) printf "Inode space: %.2f GiB\n", c*strtonum(s)/(1024^3); else print "Error: Could not find Inode information."}'

The following command output is returned.

You can also use the following formula to calculate the disk space occupied by inodes:

$$\text{Disk space occupied by inodes}=\frac{\text{Inode count}\times \text{Inode size}}{1024^3}=\frac{\text{13107200}\times \text{256}}{1024^3}=\text{3.12 GiB}$$

Calculate the disk space reserved by the system

Run the following command to obtain the total amount of disk space reserved by the system:

sudo tune2fs -l /dev/[$Partition] | awk '/Reserved block count:/{r=$4} /Block size:/{b=$3} END{if(r && b) printf "Reserved space: %.2f GiB\n", r*b/(1024^3); else print "Error: Could not find reserved block or block size information."}'

The following command output is returned.

You can also use the following formula to calculate the disk space reserved by the system:

$$\text{System reserved disk space}=\frac{\text{Reserved block count}\times \text{Block size}}{1024^3}=\frac{\text{2199325}\times \text{4096}}{1024^3}=\text{8.39 GiB}$$