If you’re a Linux user, you’ve probably heard that you don’t need to defragment your Linux file systems. You’ll also notice that Linux distributions don’t come with disk-defragmenting utilities. But why is that?

To understand why Linux file systems don’t need defragmenting in normal use – and Windows ones do – you’ll need to understand why fragmentation occurs and how Linux and Windows file systems work differently from each other.

What Fragmentation Is

Many Windows users, even inexperienced ones, believe that regularly defragmenting their file systems will speed up their computer. What many people don’t know is why this is.

In short, a hard disk drive has a number of sectors on it, each of which can contain a small piece of data. Files, particularly large ones, must be stored across a number of different sectors. Let’s say you save a number of different files to your file system. Each of these files will be stored in a contiguous cluster of sectors. Later, you update one of the files you originally saved, increasing the file’s size. The file system will attempt to store the new parts of the file right next to the original parts. Unfortunately, if there’s not enough uninterrupted room, the file must be split into multiple pieces – this all happens transparently to you. When your hard disk reads the file, its heads must skip around between different physical locations on the hard drive to read each chunk of sectors — this slows things down.

Defragmenting is an intensive process that moves the bits of files around to reduce fragmentation, ensuring each file is contiguous on the drive.

Of course, this is different for solid state drives, which don’t have moving parts and shouldn’t be defragmented – defragmenting an SSD will actually reduce its life. And, on the latest versions of Windows, you don’t really need to worry about defragmenting your file systems – Windows does this automatically for you. For more information on best practices for defragmenting, read this article:

HTG Explains: Do You Really Need to Defrag Your PC?

How Windows File Systems Work

Microsoft’s old FAT file system – last seen by default on Windows 98 and ME, although it’s still in use on USB flash drives today – doesn’t attempt to arrange files intelligently. When you save a file to a FAT file system, it saves it as close to the start of the disk as possible. When you save a second file, it saves it right after the first file – and so on. When the original files grow in size, they will always become fragmented. There’s no nearby room for them to grow into.

Microsoft’s newer NTFS file system, which made its way onto consumer PCs with Windows XP and 2000, tries to be a bit smarter. It allocates more “buffer” free space around files on the drive, although, as any Windows user can tell you, NTFS file systems still become fragmented over time.

Because of the way these file systems work, they need to be defragmented to stay at peak performance. Microsoft has alleviated this problem by running the defragmentation process in the background on the latest versions of Windows.

How Linux File Systems Work

Linux’s ext2, ext3, and ext4 file systems – ext4 being the file system used by Ubuntu and most other current Linux distributions – allocates files in a more intelligent way. Instead of placing multiple files near each other on the hard disk, Linux file systems scatter different files all over the disk, leaving a large amount of free space between them. When a file is edited and needs to grow, there’s usually plenty of free space for the file to grow into. If fragmentation does occur, the file system will attempt to move the files around to reduce fragmentation in normal use, without the need for a defragmentation utility.

Because of the way this approach works, you will start to see fragmentation if your file system fills up. If it’s 95% (or even 80%) full, you’ll start to see some fragmentation. However, the file system is designed to avoid fragmentation in normal use.

If you do have problems with fragmentation on Linux, you probably need a larger hard disk. If you actually need to defragment a file system, the simplest way is probably the most reliable: Copy all the files off the partition, erase the files from the partition, then copy the files back onto the partition. The file system will intelligently allocate the files as you copy them back onto the disk.

中文翻译版本：http://my.oschina.net/liux/blog/61636

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