Disk Defragmentation – Background and Engineering the Windows 7 Improvements

In this blog, we focus on disk defragmentation in Windows 7. Before we discuss the changes introduced in Windows 7, let’s chat a bit about what fragmentation is, and its applicability.

Within the storage and memory hierarchy comprising the hardware pipeline between the hard disk and CPU, hard disks are relatively slower and have relatively higher latency. Read/write times from and to a hard disk are measured in milliseconds (typically, 2-5 ms) – which sounds quite fast until compared to a 2GHz CPU that can compute data in less than 10 nanoseconds (on average), once the data is in the L1 memory cache of the processor.

This performance gap has only been increasing over the past 2 decades – the figures below are noteworthy.

In short, the figures illustrate that while disk capacities are increasing, their ability to transfer data or write new data is not increasing at an equivalent rate – so disks contain more data that takes longer to read or write. Consequently, fast CPUs are relatively idle, waiting for data to do work on.

Significant research in Computer Science has focused on improving overall system I/O performance, which has lead to two principles that the operating system tries to follow:

1. Perform less I/O, i.e. try and minimize the number of times a disk read or write request is issued.
2. When I/O is issued, transfer data in relatively large chunks, i.e. read or write in bulk.

Both rules have reasonably simply understood rationale:

1. Each time an I/O is issued by the CPU, multiple software and hardware components have to do work to satisfy the request. This contributes toward increased latency, i.e., the amount of time until the request is satisfied. This latency is often directly experienced by users when reading data and leads to increased user frustration if expectations are not met.
2. Movement of mechanical parts contributes substantially to incurred latency. For hard disks, the “rotational time” (time taken for the disk platter to rotate in order to get the right portion of the disk positioned under the disk head) and the “seek time” (time taken by the head to move so that it is positioned to be able to read/write the targeted track) are the two major culprits. By reading or writing in large chunks, the incurred costs are amortized over the larger amount of data that is transferred – in other words, the “per unit” data transfer costs decrease.

So how does defragmentation help? In essence, defragmentation helps by moving data around so that it is once again placed more optimally on the hard disk, providing the following benefits:

1. Any logically related content that was fragmented can be placed adjacently
2. Free space can be coalesced so that new content written to the disk can be done so efficiently

The following diagram will help illustrate what we’re discussing. The first illustration represents an ideal state of a disk – there are 3 files, A, B, and C, and all are stored in contiguous locations; there is no fragmentation. The second illustration represents a fragmented disk – a portion of data associated with File A is now located in a non-contiguous location (due to growth of the file). The third illustration shows how data on the disk would look like once the disk was defragmented.

Nearly all modern file systems support defragmentation – the differences generally are in the defragmentation mechanism, whether, as in Windows, it’s a separate, schedulable task or, whether the mechanism is more implicitly managed and internal to the file system. The design decisions simply reflect the particular design goals of the system and the necessary tradeoffs. Furthermore, it’s unlikely that a general-purpose file system could be designed such that fragmentation never occurred.

In Windows Vista, we had removed all of the UI that would provide detailed defragmentation status. We received feedback that you didn’t like this decision, so we listened, evaluated the various tradeoffs, and have built a new GUI for defrag! As a result, in Windows 7, you can monitor status more easily and intuitively. Further, defragmentation can be safely terminated any time during the process and on all volumes very simply (if required). The two screenshots below illustrate the ease-of-monitoring:

In Windows XP, defragmentation had to be a user-initiated (manual) activity i.e. it could not be scheduled. Windows Vista added the capability to schedule defragmentation – however, only one volume could be defragmented at any given time. Windows 7 removes this restriction – multiple volumes can now be defragmented in parallel with no more waiting for one volume to be defragmented before initiating the same operation on some other volume! The screen shot below shows how defragmentation can be concurrently scheduled on multiple volumes:

Among the other changes under the hood in Windows 7 are the following:

• Defragmentation in Windows 7 is more comprehensive – many files that could not be re-located in Windows Vista or earlier versions can now be optimally re-placed. In particular, a lot of work was done to make various NTFS metadata files movable. This ability to relocate NTFS metadata files also benefits volume shrink, since it enables the system to pack all files and file system metadata more closely and free up space “at the end” which can be reclaimed if required.
• If solid-state media is detected, Windows disables defragmentation on that disk. The physical nature of solid-state media is such that defragmentation is not needed and in fact, could decrease overall media lifetime in certain cases.
• By default, defragmentation is disabled on Windows Server 2008 R2 (the Windows 7 server release). Given the variability of server workloads, defragmentation should be enabled and scheduled only by an administrator who understands those workloads.

Best practices for using defragmentation in Windows 7 are simple – you do not need to do anything! Defragmentation is scheduled to automatically run periodically and in the background with minimal impact to foreground activity. This ensures that data on your hard disk drives is efficiently placed so the system can provide optimal responsiveness and I can continue to enjoy glitch free listening to the Eagles :-).