Logic in Disk Defragmantation & Disk Check

Question

What is the logic behind disk defragmentation and Disk Check in Windows? Can I do it using C# coding?

Answer 1

Look at Defragmenting Files at msdn for possible API helpers.

You should carefully think about using C# for this task, as it may introduce some undesired overhead for marshaling into native Win32.

Answer 2

If you don't know the logic for defragmentation, and if you didn't write the file system yourself so you can't authoritatively check it for errors, why not just start new processes running 'defrag' and 'chkdsk'?

Answer 3

Despite its importance, the file system is no more than a data structure that maps file names into lists of disk blocks. And keeps track of meta-information such as the actual length of the file and special files that keep lists of files (e.g., directories). A disk checker verifies that the data structure is consistent. That is, every disk block must either be free for allocation to a file or belong to a single file. It can also check for certain cases where a set of disk blocks appears to be a file that should be in a directory but is not for some reason.

Defragmentation is about looking at the lists of disk blocks assigned to each file. Files will generally load faster if they use a contiguous set of blocks rather than ones scattered all over the disk. And generally the entire file system will perform best if all the disk blocks in use confine themselves to a single congtiguous range of the disk. Thus the trick is moving disk blocks around safely to achieve this end while not destroying the file system.

The major difficulty here is running these application while a disk is in use. It is possible but one has to be very, very, very careful not to make some kind of obvious or extremely subtle error and destroy most or all of the files. It is easier to work on a file system offline.

The other difficulty is dealing with the complexities of the file system. For example, you'd be much better off building something that supports FAT32 rather than NTFS because the former is a much, much simpler file system.

As long as you have low-level block access and some sensible way for dealing with concurrency problems (best handled by working on the file system when it is not in use) you can do this in C#, perl or any language you like.

BUT BE VERY CAREFUL. Early versions of the program will destroy entire file systems. Later versions will do so but only under obscure circumstances. And users get extremely angry and litigious if you destroy their data.

Answer 4

Mark Russinovich wrote an article Inside Windows NT Disk Defragmentation a while ago which gives in-depth details. If you really want to do this I would really advise you to use the built-in facilities for defragmenting. More so, on recent OSes I have never seen a need as a user to even care about defragmenting; it will be done automatically on a schedule and the NTFS folks at MS are definitely smarter at that stuff than you (sorry, but they do this for some time now, you don't).

Answer 5

For completeness sake, here's a C# API wrapper for defragmentation:

http://blogs.msdn.com/jeffrey%5Fwall/archive/2004/09/13/229137.aspx

Defragmentation with these APIs is (supposed to be) very safe nowadays. You shouldn't be able to corrupt the file system even if you wanted to.

Commercial defragmentation programs use the same APIs.