[Ham-Computers] RE: To RAID or not to RAID, that is the question

[Hsu, Aaron (NBC Universal)]

>>> You wrote:
I read the sites you referenced me to (after putting the "ml" in the
address of the one).

>>> My reply:
Hmmm, looks like the plain-text parser put a line-feed after the "ht"
... the "ml" shows up on the next line.  One of the intricacies
(headaches) of e-mail - oh well.  I was actually surprised the URL's
were so short!


>>> You wrote:
1.  With the two 80GB drives set as RAID-1, when I go into "My
Computer", will I see the two drives independently (like Drive C: and D:
) that contain the same information (mirrored), or will I see just one
drive shown for the pair?

>>> My reply:
This is a little tricky...it depends on the RAID controller and whether
the proper drivers are installed.  Some background...

There are three ways to handle RAID - Software, Hardware-Assisted, and
"true" or "full" Hardware based.  The terms "hardware assisted" and
"hardware" are my own terms...I believe there are "official" terms, but
I don't remember what they are at this time.  Regardless, the principles
are the same.

In software RAID, the operating system handles all the RAID functions.
This is generally slow, but effective for those who can't afford a
hardware based RAID controller and relatively simple to setup.  Windows
XP Pro can do RAID-1 via software, but not on the OS partition as the
RAID facility isn't available until after Windows XP boots.

In Hardware-Assisted RAID, the RAID controller includes support to
enhance software-RAID functionality.  For example, with RAID-1, it might
handle the actual "write" function to both drives so that the OS doesn't
need to perform two separate writes (one to each drive) - the software
performs one write and the hardware writes the data to both drives.  In
RAID-5, hardware-assisted RAID controllers include a "XOR" register to
perform the parity bit calculation.  This eases the burden on OS and
MoBo CPU.

In a fully Hardware based RAID controller, the controller itself handles
all drive access (and parity calculations) and the OS never really
"sees" the physical disks - it only sees a "volume" available for it to
use.  All of the low-level functionality of the RAID set is handled by
the controller.  Therefore, the OS only needs to do basic "read/write"
functions and the RAID controller handles the rest.  Many hardware-based
RAID controllers also include a RAM cache to enhance performance.


Now, back to your question.  Will both drives show up?  Maybe...

If you have a software based RAID system, then both drives will show up
in the Partition Manager, but the Windows Explorer will only present you
with one drive letter.  Windows is presenting you with the one "logical"
volume available after you initialize the RAID-1 array, but it also
shows you 2 physical drives in the Partition Manager (and the Device
Manager) as it is also directly handling these devices.

If you have a hardware-assisted RAID controller (most common for MoBo
"on-board" controllers), then you might initially see two drives and two
drive letters until you've installed the drivers for the controller.  If
you setup the controller configuration properly (usually ROM based) and
install the appropriate drivers during the initial WinXP OS setup (via
the "F6" key), then you should only see one drive and one volume in the
Device and Partition managers.  Same is true if you install the drivers
*after* installing the controller if adding the controller to an
existing WinXP installation.

If you have a true Hardware-based RAID controller, then the controller
handles all of the drive functionality and presents the OS with one
logical drive to work with.  All of the RAID setup is initially done via
a boot disk or via a ROM-based setup.  Once configured, the controller
handles all low-level read/write functions and the OS only sees one
drive from the get-go.  You can actually run the system this way, but it
will be slow as all disk requests will be via INT13 calls (legacy mode).
Installing the drivers for the controller card will allow direct 32-bit
access to the controller - much like installing native drivers for video
cards makes video faster and "prettier" in Windows.  As with
hardware-assisted RAID, you'll need to use the "F6" installation method
to install Windows XP.  Regardless of whether you install the drivers or
not, the Device and Partition managers in Windows will only show 1 drive
(as this is all the RAID controller "presents").  The exception is if
you partitioned the RAID set into multiple volumes, in which case you'll
see as many volumes as you created.


>>> You wrote:
2.  When I "install" a program (other than the OS), will it
automatically know to install to the RAID pair (I assume it would), or
how does one insure programs are installed correctly?

>>> My reply:
The RAID "set" should always be presented as a single volume (or
individual volumes, if configured).  Therefore, it's just another drive
letter in Windows.  When you install a program, it usually gives you the
option to install to a specific location.  If you choose the default
(usually "C:"), then the program will install the "C:" drive.  For
simplicity, I generally advise to install all programs on the "C:" drive
in Windows.


>>> You wrote:
3.  When the drives are "powered down" under the windows power control,
(after 30 minutes idle time on this computer) will there be a problem
"waking them up", or is the automatic power-down a bad idea under RAID.
Ditto for waking up from Hibernation and/or Standby?

>>> My reply:
Shouldn't be a problem.  In fact, the RAID controller may even ignore
power-savings commands from Windows.  To be safe, I would disable
Windows' power saving feature for the hard disk drives.  Hibernation and
Standby should not be a problem - just make sure the proper RAID drivers
are installed so the controller and Windows communicate effectively.  On
desktops, I would trust Hibernation a bit more than Standby as
Hibernation is handled by Windows XP natively and doesn't rely on any
hardware components...the contents of RAM is "dumped" to the hard disk
and the motherboard is instructed to turn off - as far as the hardware
is concerned, Windows is shutdown.  When you reboot, the hardware does
it's thing (POST test) stats the boot process.  Windows takes over
booting, but instead of doing a normal "fresh" boot, it reloads the RAM
"dump" and drops you back to where you where.  In Standby, windows goes
to sleep, but the hardware must stay "on" (or supply power) so that RAM
is constantly refreshed.  Should something go wrong while in Standby,
it's the same as if you turned off the power while Windows was
running...not a good thing.


Hope I've answered your questions!

73,

  - Aaron Hsu, NN6O