I recently installed Windows XP on a system with two hard drives, intending to use the drives as mirrors. I then found out, much to my surprise, that mirroring is not enabled in Windows XP, not even in the so-called "professional" (?) version.
Fortunately, it appears that this is one of those features that are actually present in the operating system, only disabled. Thanks to a page I found with Google, from Tom's Hardware Guide, I knew exactly how to enable it.
The bottom line is that you need to hand-edit three binary system files and then install them even if Windows insists on "repairing" them by replacing your copies with the original versions.
Files to edit
The three files to edit are dmadmin.exe, dmconfig.dll, and dmboot.sys. The first two of these are in the system32 directory, while dmboot.sys is in system32\drivers. Here are the changes that need to be made (I use the elvis editor under Linux, which allows me to edit binary files).
| dmadmin.exe | |
| Before | 00001c30 73 65 72 76 65 72 6e 74 00 00 00 00 6c 61 6e 6d servernt....lanm 00001c40 61 6e 6e 74 00 00 00 00 50 72 6f 64 75 63 74 54 annt....ProductT |
| After | 00001c30 77 69 6e 6e 74 00 00 00 00 00 00 00 6c 61 6e 6d winnt.......lanm 00001c40 61 6e 6e 74 00 00 00 00 50 72 6f 64 75 63 74 54 annt....ProductT |
| dmconfig.dll | |
| Before | 00005140 4c 41 4e 4d 41 4e 4e 54 00 00 00 00 53 45 52 56 LANMANNT....SERV 00005150 45 52 4e 54 00 00 00 00 57 49 4e 4e 54 00 00 00 ERNT....WINNT... |
| After | 00005140 4c 41 4e 4d 41 4e 4e 54 00 00 00 00 57 49 4e 4e LANMANNT....WINN 00005150 54 00 00 00 00 00 00 00 53 45 52 56 45 52 4e 54 T.......SERVERNT |
| dmboot.sys | |
| Before | 00011070 54 00 79 00 70 00 65 00 00 00 57 49 4e 4e 54 00 T.y.p.e...WINNT. 00011080 00 00 53 45 52 56 45 52 4e 54 00 00 00 00 4c 41 ..SERVERNT....LA |
| After | 00011070 54 00 79 00 70 00 65 00 00 00 53 45 52 56 45 52 T.y.p.e...SERVER 00011080 4e 54 57 49 4e 4e 54 00 00 00 00 00 00 00 4c 41 NTWINNT.......LA |
Note that these are slightly different from what is in Tom's Hardware Guide; I suspect the driver has been changed by Microsoft since that page has been last edited. Yes, there's a slight concern that possible future driver changes by Microsoft may render unbootable a system that was modified this way and set to boot from a RAID volume.
Replacing system files
Simply copying the edited version over the original will not work. Windows XP has a feature where it protects sensitive system files from accidental overwrites; if you copy, say, your modified version of dmboot.sys over the original, you will notice that after a few seconds, it'll revert back to the original version, even if you do the copying from the command line.
One way to overcome this difficulty is by booting from the Windows XP CD-ROM to the Windows Recovery Console, which is a DOS-like command-line environment that lets you, among other things, replace protected system files.
Establishing the mirror
From this point on you proceed as you would with Windows 2003 Server: Go into Administrative Tools, Computer Management, Disk Management, and go nuts with your dynamic volumes.
Legal and practical implications
"Hacking" a device with intentionally disabled features has been a long established practice. Is it legal? I am no expert, but I doubt that Microsoft would go after anyone with a legally obtained Windows XP license who made this modification. Just to be on the safe side, it doesn't hurt to have a valid Windows 2003 Server license (I do, through MSDN) so you could argue that you just "downgraded" from that to the "modified" version of Windows XP.
On the practical side, running a hacked version of any operating system in a production environment can be very problematic. However simple the hack is, it probably hasn't been tested for regressions the same way as real operating system components are. It may be incompatible with future hotfixes released by the vendor. Or, the vendor may release a hotfix for the hacked component, causing it to revert to an unhacked version, rendering the system unbootable.
Update (August 25, 2012): a bug prevents Windows from creating mirrors 1 TB or greater in size. This bug was fixed for Windows Server 2003 (KB827913) but the same patch was not issued for Windows XP for obvious reasons. The file to be patched is dmio.sys; the patch removes an unnecessary sign test of an unsigned quantity (the sector number).
