Let's talk about RAID

[DoOLiE11]

if your thinkin of raid 0+1

y not look at raid 5?

 

[AMDPwred]

Oh the choices! So what would RAID 5 be and what are the contraints for it?

 

[DoOLiE11]

well definaly dont take my word for this cuz i'm not an expert of raid (i just started to do some research on it)

but raid 5 requires at least 3 drives

and i think basically waht happneds is that when data is written it is given a parity bit

and if u ever lose on of the three drives u can rebuilkd what was on from the other two drives..

haha that probalby doestn make any sense

 

[SinNisTeR]

you dont need raid 5..

With RAID-5, data is spread across multiple drives. An additional drive contains "parity" information -- data which is created via an algorithm which uses the contents of the other drives as input. If the data on any individual drive is lost, the data on the remaining drives can be used to create the lost data by using the inverse of the original algorithm.

pic or raid 5

most onboard raid controllers (i have the epox 8k7a+ with the HighPoint 370 controller) will do raid 0, raid 1, raid 0 + 1.

 

[AMDPwred]

I think I'll stick with RAID 0+1. My motherboard doesn't have onboard RAID though. So I'm guessing I'll need a PCI controller. What's a good one that supports RAID 0+1?

 

[SinNisTeR]

i use a HP controller and dont know too much about the Promise controllers.. but i think this is a good one.. LINKY

 

[AMDPwred]

So I'll need two more hard drives and this card. I should be good to go with those three items, huh?

 

[Budman]

all i can say is this, If you want speed go with SCSI, if you want large storage go with IDE.

Even the best ide raid 0 cant touch a fast single SCSI drive but costs too much for large storage space.

So my question is, what do you want? speed or space?


the best setup would be SCSI raid 0+1 but that would cost a lot.

 

[AMDPwred]

Space and safety.

 

[Budman]

Space & safety then just go buy a 100gig 7200 HD & you're all set.

 

[AMDPwred]

They are so expensive though. I really like the idea of having a backup copy of my hard drive also. But I can't just do RAID 1 because I need more space now (down to about 3GB from 20GB). A second drive would be perfect.

 

[Budman]

Any Raid choice you chose you will have to wipe you current drive to Create the Array,so if you can lose anything on your current drive then it's better to just buy 1 more drive & use it.

setting up an array you have to wipe both hd's.

 

[AMDPwred]

Welp, screw RAID. I guess I'll be buying a second 20GB drive soon. Thanks for your help everyone!

 

[thomsbrain]

Slower boot times with RAID? Whatever... Built a machine for a guy recently with a RAID 0 setup. That thing booted wicked fast into W2K. With multiple network adapters, tons of peripherals, software etc.

Incedentally, 0+1 requires 4 drives, not 3 like someone said. RAID 3 only needs 3 drives. (Hence the 3).

 

[kjmcdonald]

It's not RAID 3 because it uses 3 drives.

There actually originally were RAID 2 and 4 standards also. But they didn't make much Sense.

RAID 3 and 5 are very similiar. They both stripe data across all drives (like RAID 0) and they
both keep parity data (which 0 doesn't hence it's lower reliability) RAID 3 always keeps the
parity data on the same disk, and RAID 5 rotates the parity date round-robin between all
the disks. RAID 5 it has turned out has better performance than 3 most under most circumstances
because the parity disk (which needs to be updated when ever the data changes) becomes
a bottleneck very easily. I think 2 and 4 were variations on other but with syncronized spindles
or something.

RAID basically boils down to a trade off between:

RAID 0 Striping:
2 Drive Min.
No wasted space
Better performance - performance improvement varies based on stripe size and access patterns.
N x more likely to have a failure (N is # of drives)

RAID 1 Mirroring:
2 Drive Min.
50% less 'usable' space
No performance improvement - Writes Can be worse, and Reads can be faster depending on implementation.
Enough Redundancy to survive a single drive failure.

RAID 5 Striping with Parity
3 Drive Min.
1 drive's worth of 'unusable' space.
Can have significant performance penalty compared to RAID 0 if not done in Hardware. Even in HW will probably
be slower than 0. With HW writes should be faster then RAID 1 but reads may not be as fast as good RAID 1.
Like RAID 0 stripe width and access patterns will affect (or can be used to tune) performance. (You want the stripe
width to closely match the average size of a read or write)
This also can withstand a single drive failure.

RAID 0+1 or 1+0 Stiped mirrors and Mirrored stripes ( I don't know which is which though)
4 Drive Min.
50% 'unusable' space
This is the most expensive of all RAID It attempts to match the performance of 0 with the
redundancy of 1 without the parity penalty of 5.
One of these stripes across mirrored pairs of drives. The other Mirrors across 2 sets of
Striped Drives. The one which stripes across mirrored pairs of drives can actually withstand
losing half of the drives in array as long as it doesn't lose both drives in any one pair.
The other (Mirroring between 2 sets of striped disks) can really only withstand one failure.
(Well it can withstand other failures but only if they are in the same stripe set as the first.)
There are performance differences between these two, but I have to admit I don't know which
is better at what or why.

-Kyle

 

[kjmcdonald]

Ok now that I typed that long winded RAID explanation I have some PC RAID questions for all of you.
You see all my RAID knowledge comes from Large 300+ drive UNIX arrays.

So I've seen posts about how all the 'low-end' HotPoint and Promise RAID controllers are supposedly
'Software RAID' but this to me doesn't make any sense.

1. Real 'Software RAID' doesn't require any HW at all so what are the Chips on the MB's for then??

2. Real 'Software RAID' doesn't work until the software is loaded up and running - so How can you
boot from one of these RAID 0 setups? From what I've seen It seems that the RAID chips make the
RAID Array (0,1,0+1,etc.) look like a single drive as far as the BIOS or OS is concerned - Even a
'stupid' OS like DOS it seems would see it as just one big drive *well* before any RAID software could
be loaded.

3. With the exception of RAID 5, RAID doesn't really require a whole lot of HW at all anyway - and none
of these chips do RAID 5 so what exactly do people think is missing?

4. If it's some sort of FCODE that gets DL'd and run in the RAID chip (not in the CPU) that makes
people call this 'Software RAID' then I've got news for you... according to that definition ALL RAID
is Software RAID. Most of the large SCSI RAID Arrays are just boards with dedicated CPUS and
software in Firmware. Heck, one of my companies most successful RAID arrays (that we still sell)
runs on dual (for redundancy not horsepower) 486's!!

Any one want to explain this to me or correct any misconceptions I have??

-Kyle

 

[Demonicon]

<< Slower boot times with RAID? Whatever... Built a machine for a guy recently with a RAID 0 setup. That thing booted wicked fast into W2K. With multiple network adapters, tons of peripherals, software etc. >>



OK, I guarantee you will have slower boot times on RAID or SCSI(yes, even SCSI). Just because it booted "wicked fast" means nothing.

 

[Quad]

lol wicked fast
haha

i'm gonna start using that one

 

[Doomguy]

RAID is a waste of money unless you do video capturing or work with huge files. Seek time is the most important factor in hard drive performance for the average user. When you load windows, does all the data come from one big file or a lot of smaller ones? A lot of smaller ones which means seek time matters much more than transfe rate.

 

[clarkmo]

Don't forget game loading times. They are decreased with raid 0. You'll never need burnproof with raid 0 array.
About software raid. I think it's called software because, although you have a controller chip on the mobo, it's your cpu which processes the striping. Cpu usage increases with a raid array. This differs from a raid card with a processor on board. I think Adaptec has an ide raid adapter with a processor on board. There is an interesting article on Anandtech with benchmarks and a better explanation.

 

[kjmcdonald]

<< About software raid. I think it's called software because, although you have a controller chip on the mobo, it's your cpu
which processes the striping. Cpu usage increases with a raid array. This differs from a raid card with a processor on
board. I think Adaptec has an ide raid adapter with a processor on board. There is an interesting article on Anandtech
with benchmarks and a better explanation. >>



I'm not sure I follow the logic there.

Striping (in RAID 0) doesn't require alot of CPU. It should be a simple modulus formula to figure out where and which
drive a block of data should be written, then you just feed the data to the drive just like a regular. An separate chip
like these RAID chips ought to easily be able to translate the block the OS wants to write on the large RAID device
into a (drive, block) pair and forward the data on to the drive without software. RAID1 needs even less computation,
it just needs to send everything to both the slave and master. RAID 5 on the other hand does quite a bit of computation,
along with additional reads and writes.

Also if it were true that your CPU needs to run this special software to read from the RAID device, then how can you
possibly boot of the device when no software is loaded yet? This is the thing I can't understand If it needs software
then how can you boot?

As for the higher CPU utilization.... That could be explained from the fact that the higher performance of the drives
fills up buffers quicker , generating more interupts etc. Even with DMA, the CPU does have to play traffic cop a
little bit. As for the Adaptec card that has a CPU, I'll bet you'll find it also offers a RAID 5 mode which does require
significant horsepower if you want to do it quickly. Also any reduction in CPU usage by that board in RAID 0
mode come most likely from the RAM (I guessing you're talking about one of the boards that has SIMM slots on it?)
it has on board that serve to buffer the data transfers.

-Kyle

 

[JDJ]

FYI, here's a decent link on raids

raid info link

Dont know if it helps, much of the info has been discussed in the thread. Also be aware that some (maybe most) raid set ups will not allow you to boot from your raid.