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pmg4lktan · Jul 7, 2003, 02:01 AM

Hi there ...

just a simple and straight-forward question about ATA RAID here ...

currently, i have an almost full 120 GB WD1200JB and i'm planning to get another identical one to build ATA RAID ...

the question is ... must i format my almost full 120 GB to do it (build the RAID)? Thanks in advance!

BrettOZ · Jul 7, 2003, 05:36 AM

The short answer : Yes

I know you didn't want to hear that

OreoCookie · Jul 7, 2003, 05:46 AM

Or you could mirror the contents of your old drive onto you new one

But I am sure you have made regular backups

pmg4lktan · Jul 7, 2003, 06:13 AM

Originally posted by BrettOZ:
The short answer : Yes

I know you didn't want to hear that

yeah ... really don't want to hear that ... anyway ... at least i am aware of this ...

well ... actually i am getting two 120 GB ... and one will fit in to my G4 with the old one to build RAID ... so i can probably build the RAID with the two new drive then copy my stuffs in old drive to it ...

i have no experience playing RAID ... will the manual of the ATA RAID card cover this? Oreo ... i am going to do stripping instead of mirroring ... will i see significant performance increase?

Thanks in advance for any further explanation ...

scottiB · Jul 7, 2003, 07:57 AM

i have no experience playing RAID ... will the manual of the ATA RAID card cover this? Oreo ... i am going to do stripping instead of mirroring ... will i see significant performance increase?

The manual should, or the RAID card's website will.

I have 2x120 striped with a SIIG card. It is faster, but I primarily wanted a large volume. At the time 200GB drives were much more expensive than now.

CIA · Jul 7, 2003, 10:37 AM

I strapped two WD120GB's together in a striped RAID. (The Special Edition Ones)
Preformance was great. I turned my 240GB, actually, like 224GB after formatting, into my system drive.
It ran like a champ for 8 months, now I'm getting ready to downgrade back to just the two seperate drives. I've had some problems recently, and I don't have the time to deal with the drives failing every now and then. I've never had a HARD crash and lost everything, but it feels like I'm rolling the dice with my sytem everytime I have to restart. The performance gains were nice tho...

pmg4lktan · Jul 7, 2003, 10:49 AM

hmm ... that means striping hard disks shorten their lives?

should i reconsider then? haven't place my order for the RAID card nor the HDs yet ... Thanks ...

Catfish_Man · Jul 7, 2003, 11:14 AM

Originally posted by pmg4lktan:
hmm ... that means striping hard disks shorten their lives?

should i reconsider then? haven't place my order for the RAID card nor the HDs yet ... Thanks ...

Well, it essentially doubles your failure rate, since if one drive goes out, you lose everything. The individual drives should last just as long though.

Spliffdaddy · Jul 7, 2003, 11:43 AM

If you have one hard drive and it fails, you lose your data.

If you have two hard drives in a striped array and one fails, you lose your data.

Doesn't appear that a single drive offers much comfort.

Worrying about hard drive failure is a pointless endeavour. If your data was that important it should exist in more than one place.

Most hard drives will outlast their usefulness, anyway. I've been worrying about the 2.1GB drives in the printserver at work since about 1996. They still function perfectly.

My point is, striping a pair of drives does double the chance of data loss - but the chance was slim to begin with. Sorta like buying 2 lottery tickets, you double your chances but, overall, your odds of winning don't improve much.

Stripe those drives and double your read/write speeds. The risk is really no worse than having a single drive.

OreoCookie · Jul 7, 2003, 12:59 PM

Originally posted by pmg4lktan:
yeah ... really don't want to hear that ... anyway ... at least i am aware of this ...

well ... actually i am getting two 120 GB ... and one will fit in to my G4 with the old one to build RAID ... so i can probably build the RAID with the two new drive then copy my stuffs in old drive to it ...

i have no experience playing RAID ... will the manual of the ATA RAID card cover this? Oreo ... i am going to do stripping instead of mirroring ... will i see significant performance increase?

Thanks in advance for any further explanation ...

First of all, harddrives don't strip

ATA cards that do RAID in hardware are slower than a software RAID (at least on pcs). You will see a performance gain when striping your drive and a (miniscule) performance hit when you mirror a hd (when you write data).

Despite the fact that no hd has physically given up on me yet, there are plenty of hds that did (friends, university, etc.). But the problem doesn't stop at physical problems: if your data is corrupted by software, the problem multiplies, too.

In any case, I wouldn't do a RAID (except for mirroring) unless
1. I need the extra contiguous space or
2. I need the performance for data that's not worth worrying about (i. e. you got backups) -- say for video rendering and stuff).

If I were you, I would do some smart partitioning. Get the new hd and separate the essential data from the `usual' one.

CIA · Jul 7, 2003, 05:14 PM

Yes, the important data that I have is backed up, but my RAID is my system and my application drive. So if I have to do a complete re-format, and re-install the system (a long process in any respect). I then have to re-install all my apps, and drivers, etc....
I used to say drives rarely fail, but I have had a Maxtor 60GB die on me. Maxtor replaced it for free, but it was still a pain in the ass.

tooki · Jul 8, 2003, 03:07 PM

Originally posted by Spliffdaddy:
If you have one hard drive and it fails, you lose your data.

If you have two hard drives in a striped array and one fails, you lose your data.

Doesn't appear that a single drive offers much comfort.

Worrying about hard drive failure is a pointless endeavour. If your data was that important it should exist in more than one place.

Most hard drives will outlast their usefulness, anyway. I've been worrying about the 2.1GB drives in the printserver at work since about 1996. They still function perfectly.

My point is, striping a pair of drives does double the chance of data loss - but the chance was slim to begin with. Sorta like buying 2 lottery tickets, you double your chances but, overall, your odds of winning don't improve much.

Stripe those drives and double your read/write speeds. The risk is really no worse than having a single drive.

It is much worse than a single drive.

The thing is, let's say that drives have a one in 10 chance of failing within a given period of time (i'm making up the failure rate, but the principle applies).

So if you have one drive, you have a 1/10 chance it'll fail and hose all your data.

If you have two drives (unstriped), each drive has a 1/10 chance of death, but if it dies, it'll only take one drive's worth of data -- the rest will be intact. So your chance of losing all your data is reduced to 1/20 (5%).

If you stripe the drives, you have doubled the chance that ALL the data gets lost -- 2/10, or 1/5 (20%) chance of total data loss.

As you add drives to a striped array, it gets worse:

3 drives: 3/10 (30%) chance of total data loss.

4 drives: 40% chance of total data loss.

5 drives: 50% chance of total data loss.

and so on.

I really, really don't recommend striping drives unless your time is not valuable, because you'll need to spend time being completely diligent about backups, and will eventually spend lots of time reinstalling stuff.

All of the above refers to a striped array without parity (RAID 0).

Now, if you use a striped array with parity (RAID 3, 4 or 5; not supported by cheap ATA RAID cards), your data is safe, but you are always sacrificing one drive's worth of capacity for the redundancy that keeps your data safe. (And it's normal to have one more drive on top of this -- the "hot spare" that kicks in to automatically and seamlessly replace a failed drive in the array.)

tooki

spam · Jul 8, 2003, 04:51 PM

Has anybody done enough benchmarking of the MacOS X software RAID to be confident that it really does improve performance? In the little testing I've done, I felt like any possible improvment in disk speed was negated by the increased overall sluggishness of the machine. I attributed that to the extra CPU needed to calculate where to put the stripes. (I tried it on an older, 450 mhz G4.)

I'm also pretty uncomfortable with striping, since I've had several hard drives fail in the past couple of years. I have an old Mac IIci, with an Apple 40 meg (not gig) hard drive that works fine. In fact, I don't think I ever had an SCSI drive in a Mac fail. But recent IDE drives seem pretty flakey.

CatOne · Jul 9, 2003, 10:21 PM

Originally posted by tooki:
It is much worse than a single drive.

The thing is, let's say that drives have a one in 10 chance of failing within a given period of time (i'm making up the failure rate, but the principle applies).

So if you have one drive, you have a 1/10 chance it'll fail and hose all your data.

If you have two drives (unstriped), each drive has a 1/10 chance of death, but if it dies, it'll only take one drive's worth of data -- the rest will be intact. So your chance of losing all your data is reduced to 1/20 (5%).

If you stripe the drives, you have doubled the chance that ALL the data gets lost -- 2/10, or 1/5 (20%) chance of total data loss.

As you add drives to a striped array, it gets worse:

3 drives: 3/10 (30%) chance of total data loss.

4 drives: 40% chance of total data loss.

5 drives: 50% chance of total data loss.

and so on.

I really, really don't recommend striping drives unless your time is not valuable, because you'll need to spend time being completely diligent about backups, and will eventually spend lots of time reinstalling stuff.

All of the above refers to a striped array without parity (RAID 0).

...

tooki

Tooki,

You need to work on your statistics. If you took it in school, you forgot most of it or you got a bad grade

You're saying if you have 10 drives striped you have a 100% chance of data loss. Wrong

Each drive is independent and as such has a 10% chance of failure (by your figures... of course really it's like .01% or something in a year, but that's another issue). So the chances of 1 drive failing in 2 is actually... er... crap I can't recall actually. But you can't just add the numbers. That's bad math. It's an exponential function, something like 1-(0.9^n) where n is the number of drives.

so 1-(.9^2) or 1 - .81 = 19% for 2 drives.

1-(.9^3) = .271 = 27% for 3 drives.

Etc.

kkant · Jul 10, 2003, 07:55 AM

Statistics remedial, folks.

Since HD failures are independent events that we want to tie together for a RAID, we have to look at the inverse, that is the "success rate" rather than the failure rate.

So. Each drive has a 10% failure rate. That is, each drive has a 90% success rate. For a two drive RAID, the success rate is calculated as the probability that both drives will independently succeed, that is

0.90 * 0.90 = 0.81

so the failure rate is 19%. For ten drives, the success rate is

0.90^10 = 0.35

so the failure rate is 65%.

tooki · Jul 10, 2003, 12:32 PM

Hehe OK. But at least I got my point across -- RAIDs without redundancy data are just asking for trouble.

CatOne: I haven't taken stats yet.

And yes, I did mention that I pulled that failure rate number out of my behind! Just illustrating the principle, you know!

tooki