In Windows NT version 4.0 Service Pack 4 (SP4) and Windows 2000, two new switches have been added to Chkdsk.exe. These switches enable users to better manage downtime incurred by running CHKDSK or AUTOCHK.
The switches that are added in Windows NT 4.0 SP4 and Windows 2000 are /C and /I, and are only valid when the target drive has the NTFS format. Each switch directs the CHKDSK routine to bypass certain actions it would otherwise take to validate the integrity of NTFS data structures.
Microsoft does not recommend interrupting the CHKDSK process when it is used with the /f switch, and Microsoft does not guarantee the integrity of the disk if the CHKDSK program is interrupted.
Chkdsk.exe is the command-line interface for a program that verifies
the logical integrity of a file system on Windows. When CHKDSK encounters logical inconsistencies it takes actions to repair file system data, provided it is not in read-only mode.
The code that actually performs the verification when CHKDSK is run
online resides in utility DLLs such as Untfs.dll and Ufat.dll. The verification routines invoked by Chkdsk.exe are the same ones invoked when a volume is verified through the graphical user interface provided by the Windows Explorer or Disk Administrator. When CHKDSK is scheduled to run at reboot, on the other hand, the binary module that contains the verification code is Autochk.exe. Autochk.exe is a native Windows application that runs early enough in the system boot sequence that it does not have the benefit of Virtual Memory or other Win32 services. Autochk.exe generates the same kind of textual output that the utility DLLs invoked by Chkdsk.exe does. But in addition to displaying this output on the screen during the boot process, Autochk.exe also logs an event to the Application Event Log for the system containing as much of the textual output as can fit into the event log's data buffer.
Because Autochk.exe and the verification code in the utility DLLs used
by Chkdsk.exe are based on the same source code, both will be referred to generically as "CHKDSK" throughout the remainder of this article. Likewise, as this article is concerned only with changes in CHKDSK behavior with respect to NTFS volumes, it should be understood that, by saying, "CHKDSK does such-and-such," the following is meant: "CHKDSK does such-and-such when run on an NTFS volume".
Because the use of the /C and /I switches can result in a volume remaining
corrupted even after CHKDSK completes, the use of these switches is not
recommended except in situations where system downtime must be kept to a
minimum. These switches are intended to be used by users with exceptionally
large volumes and who require flexibility in managing the downtime that is
incurred when CHKDSK must be run on such volumes.
To understand when it might be appropriate to use these switches, it is
important to have a basic understanding of some of the internal NTFS data
structures, the kinds of corruption that can take place, what actions
CHKDSK takes when it verifies a volume, and what the potential consequences
are in circumventing CHKDSK's usual verification steps.
CHKDSK's activity is split into three major "stages" during which it
examines all the "metadata" on the volume and an optional fourth stage.
Metadata is "data about data." It is the file system overhead, so to speak,
that is used to keep track of everything about all of the files on the
volume. Metadata tells what allocation units make up the data for a given
file, what allocation units are free, what allocation units contain bad
sectors, and so on. The "contents" of a file, on the other hand, is termed
"user data." NTFS protects its metadata through the use of a transaction
log. User data is not so protected.
During its first stage, CHKDSK displays a message on the screen saying that
it is verifying files and counts from 0 to 100 percent complete. During
this phase, CHKDSK examines each file record segment (FRS) in the volume's
master file table (MFT). Every file and directory on an NTFS volume is
uniquely identified by a specific FRS in the MFT and the percent complete
that CHKDSK displays during this phase is the percent of the MFT that has
been verified. During this stage, CHKDSK examines each FRS for internal
consistency and builds two bitmaps, one representing what FRSs are in use,
and the other representing what clusters on the volume are in use. At the
end of this phase, CHKDSK knows what space is in use and what space is
available both within the MFT and on the volume as a whole. NTFS keeps
track of this information in bitmaps of its own that are stored on the disk
allowing CHKDSK to compare its results with NTFS's stored bitmaps. If there
are discrepancies, they are noted in CHKDSK's output. For example, if an
FRS that had been in use is found to be corrupted, the disk clusters
formerly associated with that FRS will end up being marked as available in
CHKDSK's bitmap, but will be marked as being "in use" according to NTFS's
During its second stage, CHKDSK displays a message on the screen saying that
it is verifying indexes and counts from 0 to 100 percent complete a second
time. During this phase, CHKDSK examines each of the indexes on the volume.
Indexes are essentially NTFS directories and the percent complete that
CHKDSK displays during this phase is the percent of the total number of
directories on the volume that have to be checked. During this stage, CHKDSK
examines each directory on the volume for internal consistency and also
verifies that every file and directory represented by an FRS in the MFT is
referenced by at least one directory. It also confirms that every file or
subdirectory referenced in each directory actually exists as a valid FRS in
the MFT and checks for circular directory references. Finally, it confirms
that the various time stamps and file size information associated with
files are all up-to-date in the directory listings for those files. At the
end of this phase, CHKDSK has ensured that there are no "orphaned" files
and that all the directory listings are for legitimate files. An orphaned
file is one for which a legitimate FRS exists, but which is not listed in
any directory. When an orphaned file is found, it can often be restored to
its rightful directory, provided that directory is still around. If the
directory that should hold the file no longer exists, CHKDSK will create a
directory in the root directory and place the file there. If directory
listings are found that reference FRSs that are no longer in use or that
are in use but do not correspond to the file listed in the directory, the
directory entry is simply removed.
During its third stage, CHKDSK displays a message on the screen saying that
it is verifying security descriptors and counts from 0 to 100 percent
complete a third time. During this phase, CHKDSK examines each of the
security descriptors associated with each of the files and directories on
the volume. Security descriptors contain information regarding the owner of
the file or directory, NTFS permission for the file or directory, and
auditing information for the file or directory. The percent complete in
this case is the percent of the number of files and directories on the
volume. CHKDSK verifies that each security descriptor structure is well
formed and internally consistent. It does not verify that the listed users
or groups actually exist or that the permissions granted are in any way
The fourth stage of CHKDSK is only invoked if the /R switch is used. /R is
used to locate bad sectors in the volume's free space. When /R is used,
CHKDSK attempts to read every sector on the volume to confirm that the
sector is usable. Sectors associated with metadata are read during the
natural course of running CHKDSK even when /R is not used. Sectors
associated with user data are read during earlier phases of CHKDSK provided
/R is specified. When an unreadable sector is located, NTFS will add the
cluster containing that sector to its list of bad clusters and, if the
cluster was in use, allocate a new cluster to do the job of the old. If a
fault tolerant disk driver is being used, data is recovered and written to
the newly allocated cluster. Otherwise, the new cluster is filled with a
pattern of 0xFF bytes. When NTFS encounters unreadable sectors during the
course of normal operation, it will also remap them in the same way. Thus,
the /R switch is usually not essential, but it can be used as a convenient
mechanism for scanning the entire volume if a disk is suspected of having
The preceding paragraphs give only the broadest outline of what CHKDSK is
actually doing to verify the integrity of an NTFS volume. There are many
specific checks made during each stage and several quick checks between
stages that have not been mentioned. Instead, this is simply an outline to
the more important facets of CHKDSK activity as a basis for the following
discussion regarding the time required to run CHKDSK and the impact of the
new switches provided in SP4.
During the first and third phases of CHKDSK, the percent complete indicator
advances relatively smoothly. There can be some unevenness in the rate at
which these phases progress. FRSs that are not in use require less time to
process than do those that are in use. Larger security descriptors take
more time to process than do smaller ones, and so on. But, overall, the
percent complete displayed is a fairly accurate representation of the
actual time required for that phase.
The same is not necessarily true for the second phase of CHKDSK. The amount
of time required to process a directory is closely tied to the number of
files or subdirectories listed in that directory. But the percent complete
listed during this phase is the percent of the number of directories to be
examined without regard for the fact that some directories might take much
longer than others to process. For example, on a volume with many small
directories and one very large one, the percent complete might progress
rapidly from 0 to 10 percent complete and then appear to get stuck for a
long period of time before rapidly progressing from 10 to 100 percent
complete. Therefore, unless you know for certain that the directories on a
volume are highly uniform with respect to the number of files they contain,
the displayed "percent complete" during this phase cannot be considered a
reliable representation of the actual time remaining for this phase.
To make matters worse for anyone caught in the middle of an unexpected
CHKDSK, the second phase of CHKDSK is the one that typically takes the
longest to run.
By now, it should be clear that many factors having to do with the state of
a volume play a roll in how long CHKDSK will take to run. A formula to
predict the time required to run CHKDSK on a given volume would have to
take into account such factors as the number of files and directories, the
degree of fragmentation of the volume in general as well as of the master
file table in particular, whether files have both long names and 8.3
formatted names, and how much corruption actually needs to be fixed. And
that is to say nothing of hardware issues such as amount of system memory,
the speed of the CPU, the speed of the disk or disks, and so on.
Rather than attempt to predict how long CHKDSK will take to run for a given
volume on a given hardware platform, suffice to say that it can take
anywhere from a few seconds to several days -- depending on your specific
situation. Unless /R is used, for a given hardware platform the biggest
concern is the number of files and directories rather than the absolute
size of the volume. That is, a 50 GB volume with one or two large database
files will only take seconds for CHKDSK to run provided /R is not
specified. If /R is specified, CHKDSK will have to read verify every sector
on the volume, and that clearly adds significantly for large volumes. On
the other hand, even a relatively small volume might take hours to run
CHKDSK if it has hundreds of thousands or millions of small files --
whether or not /R is specified.
The best way to predict how long CHKDSK will take to run on a given volume
is to actually do a trial run in read-only mode during a period of low
system usage. Care must be taken using this technique, however, for three
- Read-only CHKDSK will abort before it completes all three phases if it
encounters errors in earlier phases and is prone to falsely reporting
errors when in read-only mode. That is, CHKDSK may report that a disk is
corrupted even when there is no real corruption present. This can happen
if NTFS happens to modify areas of the disk on behalf of some program
activity that CHKDSK is examining at the same time. To verify a volume
correctly, the volume must be in a static state, and the only way to
guarantee that state is to lock the volume. CHKDSK only locks the volume
when /F or /R (which implies "F") is specified. Thus, you may need to
run CHKDSK more than once to get it to complete all stages in read-only
- System load and whether CHKDSK is running online or during the Windows
NT boot sequence can impact the time required to run CHKDSK. CHKDSK is
both CPU and disk intensive. Which factor becomes the bottleneck will
depend on the specific hardware scenario, but, if heavy disk I/O or high
CPU usage is going on concurrent with a read-only CHKDSK, inflated times
will result. Also, Autochk.exe runs in a different environment than
Chkdsk.exe. While running CHKDSK through Autochk.exe affords exclusive
use of CPU and I/O resources to CHKDSK, it also deprives CHKDSK of the
benefit of virtual memory. Thus, while Autochk.exe would usually be
expected to run faster than Chkdsk.exe, systems with relatively low
amounts of RAM may see longer times for Autochk.exe than for Chkdsk.EXE.
- Fixing corruption adds to the time required. A read-only CHKDSK can
complete only if no significant corruption is found. If a disk suffers
only minor corruption, the time to fix the problems will be only
slightly longer than that required for read-only CHKDSK. But if there is
major damage, as might result from a serious head-crash or other major
hardware failure, the time required to run CHKDSK can increase in
proportion to the number of files damaged. In extreme cases, this could
more than double the time required for CHKDSK.
Introducing the /C and /I Switches
The /C switch directs CHKDSK to skip the checks that detect cycles in the
directory structure. Cycles are a very rare form of corruption in which a
subdirectory has itself for an ancestor. Using the /C switch can speed
CHKDSK by about 1 to 2 percent. Using /C can also leave directory "loops"
on an NTFS volume. Such loops may be inaccessible from the rest of the
directory tree and could result in some number of files being orphaned in
the sense that they cannot be seen by any Win32 applications -- including
The /I switch directs CHKDSK to skip checks that compare directory entries
to the FRSs that correspond to those entries. Thus, while the directory
entries are still checked to be sure that they are self-consistent, they
are not necessarily consistent with the data stored in their corresponding
FRSs even after CHKDSK has run with this switch in effect. Using the /I
switch typically results in CHKDSK times being reduced by 50 to 70 percent.
Exactly how much faster CHKDSK is with this switch will depend on factors
such as the ratio of files to directories, as well as on the relative speed
of disk I/O versus CPU speed, and is, therefore, difficult to predict in
advance. The use of the /I switch can result in directory entries remaining
that refer to incorrect FRSs or in FRSs remaining that are not referenced
by any directory entry. The later case is another form of orphaning. The
file represented by the FRS may be intact in all ways except for the fact
that it is invisible to all Win32 applications-including backup
applications. In the former case, files may appear to exist; yet
applications encounter errors when attempting to access them.
When disk corruption is detected on a volume, you have three basic choices
- Do nothing. For a mission critical server that is expected to be online
24 hours a day, this is often the choice of necessity. The drawback to
this option is that relatively minor corruption can "snowball" into
major corruption if it is not repaired as soon as possible after it is
detected. Therefore, this option should only be considered when keeping
a system up is more important that the integrity of the data stored on
the corrupted volume because all data on the corrupted volume should be
considered "at risk" until CHKDSK is run.
- Run a full CHKDSK. This option repairs all file system data, restoring
all user data that can be recovered by means of an automated process.
The drawback to this option is that a full CHKDSK can require several
hours of downtime for a mission critical server at an inopportune time.
- Run an abbreviated CHKDSK using some combination of the /C and /I
switches. This option repairs the kinds of corruption that can
"snowball" into bigger problems in much less time than a full CHKDSK
would require, but does not repair all the corruption that might exist.
A full CHKDSK will still be required at some future time to guarantee
that all the data that can be recovered has been recovered.
It should be pointed out that NTFS does not guarantee the integrity of user
data following an instance of disk corruption -- even when a full CHKDSK is
run immediately after corruption has been detected. Thus, there may be
files that CHKDSK cannot recover. Also, files that are recovered may be
internally corrupted even after CHKDSK has been run. It, therefore, remains
vitally important that mission critical data be protected by means of a
regimen of periodic backups or other robust disaster recovery methodology.