RAID 5 vs RAID 0+1


M

Mark Hartwig

Hello. I am a small business owner and my main computer
currently has only a single 80GB drive. I need to both
upgrade the size of that drive, but also add fault
tolerance to it. Because the operating system and boot
partitions are already on that first drive, can I upgrade
the system to a software RAID 5 setup? If so, will the
boot and operating system partitions also be backed up in
case of a single drive failure?
Also, if I were to upgrade instead to a software RAID
0+1, could I use three drives (existing 80GB + new 160GB
striped together with a new 250GB mirroring)? Thanks for
your help
 
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C

Colon Terminus

First things first.
Avoid software RAID like the plague it is. Just say no. IDE RAID cards are
inexpensive.

That said, lemme answer the remainder of your questions.
IDE RAID 5 is NOT a viable option, forget it. That leaves RAID 0, RAID 1 or
RAID 0+1. You're looking for fault tolerance, so really we're left with just
RAID 1 or RAID 0+1.

RAID 0+1 requires 4 hard disks of equal size. You create two RAID 0s then
mirror one to the other. Not a bad solution if you can squeeze 4 hard drives
into your case and you have the power supply and case cooling to handle it.
4 120GB drives would give you a 240GB mirrored RAID 0.

RAID 1 requires 2 hard disks of equal size. Probably your best solution. 2
250GB drives would be required.

In either case, you're gonna have to clone your existing 80GB hard drive
onto another drive, either a 240GB RAID 0 or the 250GB primary drive of the
mirror set. This is not a trivial task and if not done properly can easily
wind up with an unusable system. The dreaded "Inaccessable Boot Device" blue
screen.
 
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V

*Vanguard*

"Mark Hartwig" said in news:[email protected]:
Hello. I am a small business owner and my main computer
currently has only a single 80GB drive. I need to both
upgrade the size of that drive, but also add fault
tolerance to it. Because the operating system and boot
partitions are already on that first drive, can I upgrade
the system to a software RAID 5 setup? If so, will the
boot and operating system partitions also be backed up in
case of a single drive failure?
Also, if I were to upgrade instead to a software RAID
0+1, could I use three drives (existing 80GB + new 160GB
striped together with a new 250GB mirroring)? Thanks for
your help


Software RAID will not allow you to include the partition holding the
operating system since it obviously needs to load before it can even
load the drivers and to manage the disks.

RAID 0 = striping, needs 2 disks minimum, provides increased bandwidth
(does NOT shorten access time).

RAID 1 = mirroring, needs 2 disks minimum, provides instant disaster
recovery (does NOT enhance bandwidth but will actually slow performance
due to the required duplicate writes).

If you want to use RAID 0+1 then you need 2 disks to do the striping
(for performance) and another 2 disks to mirror those 2 disks that are
striped, for a total of 4 disks.

RAID 5 = striping with parity section on each disk, needs 3 disks
minimum, provides non-instaneous disaster recovery along with increased
bandwidth.

RAID 5 gives you the striping (for performance) along with the means to
recover from a single disk failure (the parity section on the other 2
drives is used to rebuild the data that was on the 3rd drive). There is
no mirroring for instant recovery to keep the box up continuously; i.e.,
when a single drive fails, you will have to wait until you replace the
drive to rebuild the data that was on it from the parity info on the
other drives, or you'll have to destroy your RAID set to rebuild the
data in a single volume and then use the drives separately without RAID.

If you use an 80GB, 160GB, and 250GB drive, only 80GB can be used on
each drive within a RAID set. RAID requires volumes of equal size when
striping them. That means with the 3 disks that you will only get a RAID
set that is 240GB in size (3 times 80GB). The remainder of the larger
disks (80GB from the 160GB drive and 170GB from the 250GB) can be used
but in their own partitions as regular non-raided drives. I don't know
if you could then define another 80GB RAID set using the leftovers from
the larger drives (for a 160GB RAID set using the 80GB leftover on the
160GB disk and 80GB from the 170GB leftover on the 250GB disk which
still leaves 90GB by itself as a non-RAID partition). I would think
having multiple RAID sets sharing the same IDE/SATA channel would defeat
the purpose of enhancing bandwidth by having them on separate channels.
It would probably be much easier to just get two more 80GB disks and
RAID 5 them into a 240GB volume.

Getting the OS into the RAID set can be tricky. From what I've seen, you
have to install the OS when that partition is *not* in a RAID set. Then
you add or include it in a RAID set. However, some RAID controllers will
automatically wipe out anything in a partition when you meld a partition
into a RAID set (i.e., the original contents are lost when the RAID
controller lays down the stripes) while some, I think, will let you meld
in existing files on a partition when adding it into a RAID set. You
would need to check documentation for the RAID you are using. For those
that destroy the contents when adding a partition to a RAID set, you
probably can save that partition to a disk image (on CDs or a backup
hard drive), like using Powerquest's DriveImage, add the partition to
RAID set (which destroys its contents as the RAID controller builds the
stripe sets), and then restore the disk image to that RAID set. Just be
sure to install the RAID driver *BEFORE* saving the disk image and
before you create the RAID set as the OS when restored to the RAID set
will then need to have the RAID driver already installed. I was told by
Powerquest that DriveImage will handle a RAID 0 set. I don't know if it
handles restoring a disk image to a RAID 5 set.

This is my experience using onboard RAID controllers. RAID cards
probably have more robust features and you may even be able to create
the RAID set using the RAID card's BIOS screens and start the OS install
within and existing RAID set (because no driver is actually required in
the OS since the hardware is handling it all). You need to read the
docs for the RAID controller or card to see if it destroys a partition's
contents when it gets added to a RAID set and, for a RAID card, if it
can be setup initially as RAID without first requiring a RAID driver in
the OS so you could pre-create the RAID set and then install Windows
there.
 

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