built-in IDE RAID vs. Linux-level RAID?

built-in IDE RAID vs. Linux-level RAID?

Post by Jeffrey J. Kosows » Mon, 14 Oct 2002 14:51:58



First I will put on my newbie disclaimer here...

Can someone explain the differences and advantage/disadvantages of
using the built-in hardware IDE RAID on some motherboards vs. using
the software level RAID in Linux?

- Which is faster/more-reliable?
- Can the hardwared IDE RAID support RAID for a dual-boot Windows XP
  and Linux system?

- What is the real scoop on the benefits/risks of using RAID 0?
  (i.e. faster vs danger that if one disk fails then both disks are
  screwed)

- Will hardware IDE RAID or Linux software RAID support RAID >0?
 (also, is it true that to go beyond RAID 0, you need more than two
 disks?)

Any pointers to good intros would also be helpful.

Thanks!

 
 
 

built-in IDE RAID vs. Linux-level RAID?

Post by dav.. » Mon, 14 Oct 2002 16:55:36



Quote:> Can someone explain the differences and advantage/disadvantages of
> using the built-in hardware IDE RAID on some motherboards vs. using
> the software level RAID in Linux?

The hardware version is implemented in hardware, so it does not bother
the CPU to split the information, in addition, for the machine the
writing/reading are one process, but they are splitted in the controller,
so the effective I/O is cut in half.

Quote:> - Can the hardwared IDE RAID support RAID for a dual-boot Windows XP
>   and Linux system?

Usually the Hardware RAID is seen as a single device from any OS (that has
a device driver for it of course)

Quote:> - What is the real scoop on the benefits/risks of using RAID 0?

Read some documentation about RAID. Google is your friend.

Quote:> - Will hardware IDE RAID or Linux software RAID support RAID >0?

Hardware raid-> read documentation of your device, software -> yes.

Quote:> (also, is it true that to go beyond RAID 0, you need more than two
> disks?)

Yes, minimum 3 disks.

Quote:> Any pointers to good intros would also be helpful.

As I said, google is your friend.

Davide

 
 
 

built-in IDE RAID vs. Linux-level RAID?

Post by Andrew Carso » Mon, 14 Oct 2002 18:01:39


<snip>
Quote:> Hardware raid-> read documentation of your device, software -> yes.

> > (also, is it true that to go beyond RAID 0, you need more than two
> > disks?)

> Yes, minimum 3 disks.

Actually, for RAID 1, (mirroring) only 2 disks are required.
You effectively only have half the total physical disk space, but you you
have redundancy.
Quote:

> > Any pointers to good intros would also be helpful.

> As I said, google is your friend.

> Davide

 
 
 

built-in IDE RAID vs. Linux-level RAID?

Post by Martin Fejrskov Pederse » Mon, 14 Oct 2002 18:16:33




>> Can someone explain the differences and advantage/disadvantages of
>> using the built-in hardware IDE RAID on some motherboards vs. using
>> the software level RAID in Linux?

> The hardware version is implemented in hardware, so it does not bother
> the CPU to split the information, in addition, for the machine the
> writing/reading are one process, but they are splitted in the controller,
> so the effective I/O is cut in half.

In the perfect world, this is true. Some modern IDE-RAID controllers
(Highpoint Technologies and Promise) do not have a full RAID implementation
in hardware. They actually rely on the CPU to do some of the dirty work.
This is sometimes referred to as "Software IDE RAID" whereas a "real"
RAID-implementation is called "Hardware IDE RAID". The RAID implemented in
the linux kernel is also known as "md RAID" or "Linux software RAID".

Quote:>> - Can the hardwared IDE RAID support RAID for a dual-boot Windows XP
>>   and Linux system?

> Usually the Hardware RAID is seen as a single device from any OS (that has
> a device driver for it of course)

Device drivers are precious: Many of them are only available in binary form,
that is precompiled for at specific kernel (that's the case for Highpoint
and Promise). To use these proprietary drivers, you need to be installing
the distribution, they're made for (often RedHat and one or two others).

An alternative exists: The linux kernel has support for something called
ATA-RAID. It translates roughly to support for Promise and Highpoint
controllers. Besides that it has support for at number of "real"
RAID-controllers. Common to these are the fact that they're true open
source drives, that is, you can compile support into every new kernel you
make. And it's still compatible with the Windows drivers, so you can
install Windows on the same disc set.

Linux md RAID works on a per partition base rather than a per disk base.
That means, that if you choose to install two disks on a non-RAID
controller, you can have a windows partition on one of them (non-RAIDed of
course) and an md-RAIDed set of linux partitions. It's actually very
flexible and clever to use if you don't care about RAID'ing Windows (I
don't!). Some actually say, that linux md RAID is faster than software IDE
RAID as implemented by the Promise and Highpoint controllers.

Quote:>> - What is the real scoop on the benefits/risks of using RAID 0?

> Read some documentation about RAID. Google is your friend.

Actually it's pretty simple: If you use RAID 0 on a two disk set and one of
your disks crash (hardware failure) you lose your data on the crashed disk
and the data on the remaining disk are unreconstructable.
The benefit is increased througput: I have a linux software RAID setup at
home, and i get approx. 60MB/s from the RAID-partitions and 35MB/s from the
non-RAID-partitions. ("hdparm -t /dev/md0" vs. "hdparm -t /dev/hde") from
an onboard ATA-100 controller and two IBM GXP-60 40GB 7200RPM disks.

Quote:>> - Will hardware IDE RAID or Linux software RAID support RAID >0?

> Hardware raid-> read documentation of your device, software -> yes.

The more expensive the RAID controller is, the more levels you get :-)
Highpoint and Promise controllers support level 0 and 1 (striping and
mirroring).

Quote:>> (also, is it true that to go beyond RAID 0, you need more than two
>> disks?)

> Yes, minimum 3 disks.

Two disks will suffice for RAID 1 (mirroring).

Quote:>> Any pointers to good intros would also be helpful.

> As I said, google is your friend.

> Davide

I always start out searching the enormous knowledge base presented at
www.tldp.org (The Linux Documentation Project). The info is organised in
HOWTO's and mini-HOWTOS each about a very specific subject.

A bit of my experience: If you have two disks and want to RAID both linux
and windows using a "fake" RAID-controller (Highpoint and Promise) use
RedHat and the vendor-supplied drivers. If you don't care about RAID'ing
windows, use linux md RAID for the linux partitions and no RAID for
windows. If using a "real" RAID-controller use your distro and setup of
choise and hardware RAID it all.

In my Mandrake 9.0 setup at home I have three disks: One for Windows, and
two for linux. On one of the linux disks i have /boot (ext2) and swap
non-RAID'ed. On linux md RAID i have / (ext2), /usr (Reiserfs) and /home
(Reiserfs). The choise of ext2 on / was made due to compiled-in support on
the default Mandrake kernel. Reiserfs is only available as modules, which
requires / to be mounted. I have a Highpoint RAID-controller, but as you
may have figured out, I don't use its RAID-functionality but only it's
ATA-100 support... :-)  If you have a similar setup, you are welcome to
contact me.

Good luck!
Martin Fejrskov Pedersen

 
 
 

built-in IDE RAID vs. Linux-level RAID?

Post by Steve B » Tue, 15 Oct 2002 15:03:10



Quote:> <snip>
> > Hardware raid-> read documentation of your device, software -> yes.

> > > (also, is it true that to go beyond RAID 0, you need more than two
> > > disks?)

> > Yes, minimum 3 disks.
> Actually, for RAID 1, (mirroring) only 2 disks are required.
> You effectively only have half the total physical disk space, but you you
> have redundancy.

> > > Any pointers to good intros would also be helpful.

> > As I said, google is your friend.

For a good book, but probably more than you want to know try...
Building Storage Networks
by Marc Farley, Mark Farley

Main thing is to understand which RAID is which   0, 1, 5, 0 + 1, 1 + 0.
And the pro's and con's of one RAID level versus another.  Such as which is
better for writes, for reads.   How to estimate available disk space after
RAID overhead.
The dark side of RAID. Like what happens if you don't replace a dead drive
in a RAID 5 array soon enough?

Figure out the answer to those questions and you'll be RAID worthy.

Steve B.