Turning off the cloud: Why the Lastlog file keeps getting bigger

Article Title: The reason why Lastlog files keep getting bigger. Linux is a technology channel of the IT lab in China. Includes basic categories such as desktop applications, Linux system management, kernel research, embedded systems, and open source.

The Lastlog file records the last logon information of all users. The algorithm of this file is as follows:

Logged user login information size = UID * 256 byte

For example, if the nfsnobody user on a 64-bit system has a UID of 4294967294 or 2 ^ 32-2, this is the last UID on the system.

Therefore, the above algorithm shows how the file size is 1.2TB:

4294967294*256 = 1099511627264 bytes as shown in the preceding algorithm, 256 is the space occupied by each UID in the lostlog file. Therefore, this type of file is called a sparse (sparse) file: A sparse file contains a lot of free space in the file and will be used for future data insertion. These free spaces are occupied by the NULL characters of the ASCII code, and these spaces are quite large. This file is called a sparse file, but it does not allocate disk blocks. That is, the file system space is not actually occupied. So you don't have to worry about the file occupying 1. 2 TB of space. You can use du-h/var/log/lastlog to check the space occupied by the disk.