Question: Running Windows 8.0 Pro, chkdsk has a /scan option that Runs a online scan on the volume (sic). What is an online scan?
My guess is that it runs it while letting you use the drive, which is good if it’s your c: drive.
Here’s the full options:
CHKDSK [volume[[path]filename]]] [/F] [/V] [/R] [/X] [/I] [/C] [/L[:size]] [/B][/scan] [/spotfix] ?volume ?Specifies the drive letter (followed by a colon), ??mount point, or volume name. ?filename ?FAT/FAT32 only: Specifies the files to check for ??fragmentation. ?/F ??Fixes errors on the disk. ?/V ??On FAT/FAT32: Displays the full path and name of every ??file on the disk. ??On NTFS: Displays cleanup messages if any. ?/R ??Locates bad sectors and recovers readable information ??(implies /F, when /scan not specified). ?/L:size ?NTFS only: ?Changes the log file size to the specified ??number of kilobytes. ?If size is not specified, displays ??current size. ?/X ??Forces the volume to dismount first if necessary. ??All opened handles to the volume would then be invalid ??(implies /F). ?/I ??NTFS only: Performs a less vigorous check of index ??entries. ?/C ??NTFS only: Skips checking of cycles within the folder ??structure. ?/B ??NTFS only: Re-evaluates bad clusters on the volume ??(implies /R) ?/scan ?NTFS only: Runs a online scan on the volume ?/forceofflinefix ?NTFS only: (Must be used with “/scan”) ??Bypass all online repair; all defects found ??are queued for offline repair (i.e. “chkdsk /spotfix”). ?/perf ?NTFS only: (Must be used with “/scan”) ??Uses more system resources to complete a scan as fast as ??possible. This may have a negative performance impact on ??other tasks running on the system. ?/spotfix ?NTFS only: Runs spot fixing on the volume ?/sdcleanup ?NTFS only: Garbage collect unneeded security descriptor ??data (implies /F). ?/offlinescanandfix ?Runs an offline scan and fix on the volume.The /I or /C switch reduces the amount of time required to run Chkdsk byskipping certain checks of the volume.
Answer: An offline scan is done by dismounting the volume to fix file system corruption and bring the volume back to a healthy state. Generally it is only possible with non-system volumes, and system volumes (with the dirty bit set) are scanned during boot-up before the OS is up and running completely.
Self-healing is a feature built into NTFS that fixes certain classes of corruptions encountered during normal operation, and can make these fixes while the volume is still online. If all issues that are detected are self-healed online then obviously there is no need for an offline repair. In Windows 8 the number of issues that can be handled online has been increased and hence the need for running an offline scan using chkdsk is further reduced. All this is part of the new NTFS health model:
You can also run an on-demand online scan with chkdsk /scan to identify and spot-fix file system corruption, although for major problems a full offline scan is still required.
Source: Redesigning chkdsk and the new NTFS health model