What is defragmenting a hard drive?

Answer 1

The operating system divides the hard drive into blocks, and a file may large enough to need several of these blocks for storage.

As the hard drive becomes full, and after several deletions of old files, there may not be enough consecutive free blocks to store a file, so it is instead stored in several blocks far apart, for example a few blocks at the start of the disk, a few blocks at the middle and the rest at the end of the disk.

This is called fragmentation and it slows down file reading, because it a lot slower for a hard drive to move between two different locations on the drive than it is to keep reading at the location it already is. The more a file is scattered over different locations on the disk, the slower it becomes to read it.

Enter the Defragmenter, which moves the blocks around on the disk so that all blocks storing the same file are put together, and thus quicker to read again.

Fragmenting is mostly a Windows issue, Linux (and Mac) uses a different method to store files so fragmenting is rarely a concern. The files on you hard drive are not arranged in order and they're scattered all over the place. Some files are even split and set, say, a quarter of the disk apart. Now, when you try and access that file, the computer has to remember where the halves are then go and fetch those fragments. Might just take a few milliseconds more but when you think about all those system files which drive your operating system, it slows down your whole computer.
The Disk Defragmenter analyses your hard drive and moves around all the fragments so that your computer doesn't have to do much. Keeps the computer nice and happy (lazy thing ain't it?) and keeps you nice and happy (yay! fast computer).

Answer 2

Most modern operating systems use a file directory system to keep track of the physical locations of files. When you delete a file, its physical address is considered by the directory system to be unoccupied. New files being copied to the same drive will take up those unoccupied spaces. If a file is larger than the available unoccupied space it is being written to, the directory system will put the rest of the file in another unoccupied space and keep track of where the pieces are physically located. As this process of deletion and filling in is repeated throughout normal user activities, the files on the drive will become very "fragmented" overall. The physical arms and discs of the hard drive will have to jump around to more locations in order to read the files. A defragmenter analyzes this situation and re-arranges the physical locations of files so that they are continuous instead of broken up.

Answer 3

The short answer is that it detangles files.

Hard drive background and what fragmentation is
Hard drives are made up of blocks or allocation units. In mechanical drives, the allocation units are in a certain order. Files load the fastest when all of their pieces are in the same group of blocks. The reason for this is because it takes time to move the drive heads to new locations. If a file is split into multiple pieces, that means that the hard drive heads have to move all over the place to access them all. If the data is in the same "cylinder" (the group of tracks on all platters at the same head location), then the heads don't have to move at all, and if they are in neighboring cylinders, then the heads won't have to travel far. So fragmentation is when files are not all stored in contiguous(neighboring, touching) groups of allocation units.

The cause of fragmentation
There are a number of factors that cause fragmentation. The easiest model of how fragmentation begins is when you delete a handful of smaller files and you copy larger files back to the drive. Depending on how the file system works and the operating system's strategy, new files might be stored starting in the first block available, then the next block available, with no regard to the number of free blocks in a given location. So if you have ten single block gaps in ten places, and you copy in a file that needs ten blocks, then it could be broken up into ten pieces. Then the hard drive has to read, seek, read, seek, etc.

The basis of defrag programs
Defrag programs move files and their individual blocks around as well as update their filesystem entries to match their new locations. Their exact strategies vary per program. Windows has basic APIs (application programming interfaces) that the programs need to be able to defragment hard drives. The programs decide where things will go. There are certain places where the blocks cannot be moved to, and the operating system likely won't give it permission to put them in those places. It cannot overwrite the partition table or GPT, the swap file, files that are in use, the boot records, the FAT tables or MFT, MBR, nor anything marked as reserved. If a file is mostly contiguous, the program will move any small files or file pieces that are in the way of the larger file. If the fragmentation is caused by an even larger file, then the smaller of the two will likely be moved. Some defrag programs add extra features such as grouping files by directories, placing large files that are seldom used last, defragmenting the folders themselves (which contain the file names and locations), among other things. You can watch a defrag program work to get an idea of what it is doing.

Answer 4

It provides more space if your drive is fragmented.

It's like cleaning up your room, everything is neatly organised leaving more free space.