What Is Disk Fragmentation?

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Disk fragmentation is a side effect of how data is written to system memory on hard disk drives (HDDs) and some other types of storage. It occurs when a computer writes data to the first available block on the drive, splitting up contiguous files. Over time, fragmentation decreases the computer’s read speeds as it takes more time to locate files. To optimize your business’s hard drives and maintain high speeds, your storage team must plan to manage fragmentation consistently. This guide focuses on fragmentation’s effects on hard drives, the advantages and disadvantages of fragmentation, and how to manage it over time. 

What is Disk Fragmentation? 

Disk fragmentation happens over time as a machine writes data to the first available block on a disk drive. When a computer writes a file to its hard disk drive, the fastest way to write is to choose whatever storage block is first available. However, this splits up contiguous files on the drive. 

In short, proximity matters. An operating system will typically store application and file storage blocks in the next available location on a storage device. This might not necessarily be the storage block that is directly adjacent to the prior block in an application.

Fragmentation occurs in hard disk drives, but not in solid state drives. SSDs do not fragment files in the same way that hard drives do. Fragmentation can also occur in primary system memory (RAM), where application and system processes allocate and use memory in non-contiguous blocks as existing memory registers are used and re-used, but this article focuses on HDD fragmentation.

Disk fragmentation typically increases over time as read and write operations take place within an operating system. Applications and files are added and removed, so new storage leaves different blocks open in a non-contiguous manner. Over time, files become so fragmented that reading the data on the hard drive is a slow process. This is sometimes regarded as an inefficient use of file storage resources. On the other hand, the ability of an operating system to rapidly write files on any available storage block, without first re-allocating blocks so there is a sequential read, speeds up file writing operations.

Some degree of disk fragmentation is generally considered acceptable in exchange for increased performance and simplicity. However, over time it can result in slower drives and wasted disk space. To optimize storage space and speed, businesses must understand how disk fragmentation works and how to mitigate it.

Types of Fragmentation

The two most common types of fragmentation are internal and external. 

  • Internal fragmentation. Because of the way memory is divided into fixed partitions, sometimes a file is a poor fit. For example, if a particular process requires 4 MB but the storage system has allocated a 6 MB space, all six of those megabytes are used up even though the process only consumes four. When allocated memory exceeds what is needed for a write, you have wasted leftover space.
  • External fragmentation. When memory allocation processes are inefficient, they can leave fragments of space on the drive that are too small to be useful. If a process tries to run using that space, the process won’t be fulfilled because the empty space isn’t contiguous—it’s split up amongst unusable fragments on the drive.

To mitigate internal fragmentation, storage admins should use dynamic allocation software—often already existing in the operating system—to avoid allocating more memory than is needed to run a process. To solve external fragmentation issues, use defragmentation tools, which reposition fragments on hard drives so more contiguous space is available. 

Storage professionals recommend defragging a disk either when the drive is 10 percent fragmented or once a month. Either is fine, but if one particular drive is constantly writing and reading data, be sure to defrag it regularly.

Internal vs. external fragmentation

The following chart breaks down the differences between internal and external fragmentation and ways to mitigate their effects.  

Internal fragmentation External fragmentation
Why it happens  Internal fragmentation happens because memory and storage is typically partitioned into fixed-sized units. External fragmentation is a function of memory being partitioned into variable-sized blocks that are dynamically allocated by different processes.
Result When space is allocated that doesn’t completely fill a fixed-size unit, a leftover fragment remains. As processes use space on a drive and then release that space, a fragment can be left behind.
How to resolve Use dynamic allocation tools within the operating system to reduce wasted space. Use defragmentation software.

Advantages of Fragmentation

Benefits of disk fragmentation include faster write times, reduced write failures, and optimized storage space. Disk fragmentation isn’t always negative, but rather a compromise for storage systems that must  improve their write performance. 

Faster write times

Because data is written to the most immediately available block on the drive, the computer can write data more quickly. The system doesn’t try to reorganize storage to enable contiguous data writes, which improves speed during write processes. 

Fewer failures

In a system that doesn’t enable fragmentation, if there isn’t enough sequential space, the write will fail. But in a system with fragmentation, the write process won’t automatically fail just because there isn’t enough contiguous space. Files from the same application can be written in different locations on the disk. 

Storage optimization

By making use of every available storage block, a fragmented system can potentially make more use of a storage device. Optimizing disk space can also save money. High-quality hard drives can be expensive, and businesses must invest in many of them. Fragmentation helps HDDs use more of the existing drive space (though this benefit becomes less relevant once a disk has been highly fragmented, with very limited block space for processes to run).  

Disadvantages of Fragmentation

While fragmentation is initially beneficial for disk performance, over time it slows systems considerably if it is not monitored. It’s challenging because it slows data read times and requires regular maintenance by storage teams.  

Slow read speeds

Fragmentation can lead to read slowdowns because the files required to run an application may not all reside in the same place. The computer has to hunt through non-contiguous locations to find all those files, which can become time-consuming. While this might not be too apparent on a single consumer laptop, it will quickly reveal itself on business machines, particularly heavy-duty computers that run critical applications.  

Need for consistent defragmentation 

Fragmentation requires work to fix. For internal fragmentation, admins must use memory allocation tools to avoid wasting space on storage devices or within RAM. For external fragmentation, admins must use defragmentation tools to adjust remaining memory so new processes have sufficient space to run. 

Read more about the best defragmentation tools for hard drives

Bottom Line: Disk Fragmentation

While fragmentation is an inevitable component of hard drives, it can be manageable as well. Your IT or storage team should monitor frequently used storage devices for fragmentation issues. One potential indicator is slow read speeds. 

To manage internal fragmentation, use dynamic memory allocation tools, whether they already exist in your operating system or you add a third-party one. To manage external fragmentation, employ a highly-reviewed business defrag solution. Disk fragmentation is necessary for rapid data writes, but it requires consistent maintenance to make drives usable long-term. 

Read about the value of using hard drives in data centers next.

Sean Michael Kerner contributed to this report.

Jenna Phipps
Jenna Phipps
Jenna Phipps is a staff writer for Enterprise Storage Forum and eSecurity Planet, where she covers data storage, cybersecurity and the top software and hardware solutions in the storage industry. She’s also written about containerization and data management. Previously, she wrote for Webopedia. Jenna has a bachelor's degree in writing and lives in middle Tennessee.

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