On a relatively new system with a
speedy processor and plenty of physical memory, hard disk performance
is the single biggest bottleneck in everyday operation. Even with a
zippy hard disk, it takes time to load large data files into memory so
that you can work with them. The problem is especially noticeable with
movies, video clips, DVD-burning projects, databases, ISO image files,
and virtual hard disks, which can easily take up multiple gigabytes,
sometimes in a single file.
On a freshly
formatted disk, files load fairly quickly, but over time, performance
can degrade because of disk fragmentation.
To understand how fragmentation
works, it helps to understand the basic structure of a hard disk. The process of
formatting a disk divides it into sectors, each of which contains space for 512 bytes of
data. The file system combines groups of sectors into clusters, which are
the smallest units of space available for holding a single file or part
of a file.
On any NTFS volume greater
than 2 GB in size, the cluster size is 4 KB. Thus, when you save a
200-MB video clip, Windows divides the file into roughly 50,000 pieces.
When you save this file for the first time on a freshly formatted,
completely empty hard disk, Windows writes it in contiguous clusters.
Because all the clusters that hold individual pieces of the file are
physically adjacent to one another, the mechanical components of the
hard disk can work very efficiently, scooping up data in one smooth
operation. As a bonus, the hard disk's onboard cache and the Windows
disk cache are able to anticipate the need for data and fetch nearby
clusters that are likely to contain other parts of the file, which can
then be retrieved from fast cached memory rather than from the
relatively slow disk.
Unfortunately,
hard disks don't stay neatly organized for long. When you add data to an
existing file, the file system has to allocate more clusters for
storage, typically in a different physical location on the disk. As you
delete files, you create gaps in the once-tidy arrangement of
contiguously stored files. As you save new files, especially large ones,
the file system uses all these bits of free space, scattering the new
files over the hard disk in many noncontiguous pieces. The resulting
inefficiency in storage is called fragmentation; each time you open or save a file on a badly
fragmented disk, disk performance suffers, sometimes dramatically,
because the disk heads have to spend extra time moving from cluster to
cluster before they can begin reading or writing data.
The Disk Defragmenter in Windows 7 improves on
earlier versions in many ways, not the least of which is you shouldn't
need to do anything to benefit from it. Disk Defragmenter runs as a
low-priority background task that kicks off once a week, in the middle
of the night, without requiring any attention from you.
21.4.1. Using Disk
Defragmenter
The Disk Defragmenter
utility improves performance by physically rearranging files so that
they're stored in contiguous clusters. In addition to consolidating
files and folders, the utility also consolidates free space, making it
less likely that new files will be fragmented when you save them. The
Disk Defragmenter process starts according to a schedule that you can
adjust. To view the current settings, click the Disk Defragmenter
shortcut (in the System Tools subfolder of the Accessories folder on the
All Programs menu), or right-click any drive icon in the Computer
window and click Defragment Now on the Tools tab.
Figure 1 shows the simple Disk
Defragmenter interface. The Schedule section of the dialog box shows
whether scheduled defragmentation is on or off and when the next run is
to occur. The Current Status section shows the date and time of each
disk's most recent defragmentation. Buttons let you reconfigure the
schedule, analyze a selected disk to see how fragmented it might be, and
perform an immediate defragmentation.
Click Configure
Schedule to change when Disk Defragmenter runs automatically. By
default, the utility runs weekly, at 1:00 A.M. each Wednesday. You can
schedule operation to be daily, weekly (you pick the day of the week),
or monthly (you pick the date), and you can choose the time of day
(round numbers only), as shown in Figure
2.
If your computer
is off at the appointed time, Disk Defragmenter will run at the first
idle time after it's back up again. If your computer is nearly always
either off or in use at 1 A.M., you might want to reconfigure the
schedule. Choose a time when the machine is usually on but not in use—a
regular lunch break, for example.
If your computer has more
than one hard disk (more precisely, more than one volume, because each
hard disk can be partitioned into multiple volumes), you can specify which ones you
want Disk Defragmenter to act upon. Click Select Disks to display the
dialog box shown in Figure 3, in which you can remove the check
mark from any volumes you don't want to defragment.
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The best way to avoid
disk fragmentation is to start with a completely clean slate. If you
routinely work with CD images, for instance, consider creating a
separate partition that's big enough to temporarily hold the files
you're working with. A 2-GB partition, for instance, is big enough to
hold a CD image and all temporary files associated with it. (You'll need
roughly 10 GB for a DVD-burning partition.) Keep that drive empty
except when you plan to create a CD, at which time you can copy files to
it for burning. Using this strategy, you can be certain that
fragmentation won't have a deleterious impact on your CD-burning
performance.
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2. Running
Disk Defragmenter from a Command Line
The
command-line version of Disk Defragmenter allows you to exercise
fine-grained control over the defragmentation process, and it uses the
exact same program code as the scheduled version. To use this command
for a specific drive, type defrag
d: in an
elevated Command Prompt window, where d is the drive letter or mount point of an
existing volume. To see the full range of the
Defrag utility's capabilities, type defrag
/?. Among the more useful switches are the following:
/c Defragments all volumes on the computer. Use
this switch without specifying a specific drive letter or mount point.
/a
Analyzes the specified volume, and displays a summary of the analysis
report.
/f Consolidates the free space on the
specified volume, reducing the likelihood that large new files will be
fragmented.
/r Defragments multiple volumes in parallel. If
your volumes are on physically separate disks, you might save a bit of
time by using this switch.
/v Displays complete
(verbose) reports. When used in combination with /a, this switch
displays only the analysis report. When used alone, it displays both the
analysis and defragmentation reports.
In addition to the
documented switches listed, the command-line Defrag
utility includes this useful but undocumented switch:
The command-line Disk Defragmenter does not provide any
progress indicator except for a blinking cursor. To interrupt the
defragmentation process, click in the Command Prompt window and press
Ctrl+C.
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The Disk
Defragmenter utility does not fully defragment the drive
A volume must have at
least 15 percent free
space before Disk Defragmenter can completely defragment the volume. If
you have less free space available, the operation will run, but only
partial defragmentation will result. From a Command Prompt window, run
Defrag with the –a switch to see statistics (including free space)
regarding the specified volume.
You cannot defragment a
volume that Windows has marked as possibly containing errors. To troubleshoot
this possibility, type chkdsk
d: /f at any
command prompt, substituting the letter of the drive in question. Chkdsk
will report and repair any file-system errors it finds (after
restarting, in the case of a system or boot volume).
Disk
Defragmenter does not defragment files in the Recycle Bin. Empty the
Recycle Bin before defragmenting.
Additionally, Disk
Defragmenter does not defragment the following files: Bootsect.dos,
Safeboot.fs, Safeboot.csv, Safeboot.rsv, Hiberfil.sys, and Memory.dmp.
In addition, the Windows page file is never defragmented. (See the
"Defragmenting Particular Files" sidebar to learn how to work around
this issue.)
Disk Defragmenter ignores
fragments that are more than 64 MB in size, both in its analytical
reports and in operation. According to Microsoft's benchmarks, fragments
of this size (which already consist of at least 16,000 contiguous
clusters) have a negligible impact on performance. Thanks to disk
latency, a large file divided into 10 fragments of 64 MB or greater in
size will not load measurably slower than the same file in a single
unfragmented location; under those circumstances, it's best to leave the
fragments alone.
Disk Defragmenter will
pass over any files that are currently in use. For best results, shut
down all running programs before running the utility. For even better
results, log off and log back on (using an account in the Administrators
group) before continuing.
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Do you still want to
defragment files larger than 64 MB, despite Microsoft's assurance that
those files don't need defragmenting? Mark Russinovich's Contig utility,
a free download from Microsoft's Sysinternals website (w7io.com/2105) will do the job. Contig is a file-specific
defragmenter. You can use it to analyze and defragment individual files
or groups of files meeting wildcard specifications.
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3.
Defragmenting Solid-State Media
Because flash disks
and other solid-state media don't employ moving parts to save and
retrieve data, file fragmentation on these drives is likely to impose a
smaller performance penalty than it does on rotating media. For this
reason, as well as to avoid decreasing performance lifespan, Disk
Defragmenter does not perform scheduled defragmentation of solid-state
drives. You can still defragment a solid-state drive if you choose, but
only on an ad hoc basis. To do this, run Disk Defragmenter, select the
disk you want to defragment, and then click either Analyze Disk or
Defragment Disk.
