RAID 0 & RAID 1

[Hunston]

Hi,

I have just decided to use 2 x 30 Gb Deskstar 75GXP drives in a Raid 0 array. Every time I mention this to someone, I usually get the reply:

"Ah, but RAID 0 offers no fault tolerance"

I have this thrown back at me more than once now - are they just stating a fact, or is RAID 0 really that unreliable that its a good idea to mirror the drives? I won't be running a server or anything majorly important, but I don't want to risk losing data either!

[Stan]

Hi

RAID0 = stripe
- RAID 0 implements a striped disk array, the data is broken down into blocks and each block is written to a separate disk drive
- I/O performance is greatly improved by spreading the I/O load across many channels and drives
- Not a "True" RAID because it is NOT fault-tolerant
- The failure of just one drive will result in all data in an array being lost
- Should never be used in mission critical environments

RAID1 = mirror
- 100% redundancy of data means no rebuild is necessary in case of a disk failure, just a copy to the replacement disk
- Twice the Read transaction rate of single disks, same Write transaction rate as single disks

Of course, I am talking about hardware RAID

RAID0 is less reliable than RAID1. In RAID0, if you loose one HD, all your data is lost...
I am using RAID0 as well (two IBM 36LZX) and I don't care if I loose one HD as all my data is on my server (RAID1 with weekly backups)... http://www.sysopt.com/forum/wink.gif

You say that you do not want to risk losing data. For me, that a good reason NOT to implement RAID0.

Stan


[This message has been edited by Stan (edited 04-18-2001).]

[daverme]

"Why are you considering RAID?"

That is the question you must answer in order to make a decision. As Stan pointed out, RAID-0 gets you high throughput (but only for SUSTAINED operations such as video capture) while RAID-1 gets you fault tolerence. You CAN combine the two but then you need four disks instead of just two.

Since many mobo's today have RAID built in, my choice would be to put the OS and programs (stuff that can be replaced) on a RAID-0 array and my data on a RAID-1 array ... IF I were going to do it at all.

Another advantage to having a RAID controller built in is that they often provide higher BURST rates than would be available with the standard IDE controller. This is the case with the Abit BX133-RAID that I bought for my wife. Being based on the BX chipset, the standard IDE controller only supports ATA-33 drives while the HighPoint RAID controller supports ATA-100 drives. Thus, we get the stability (rock solid) of the BX chipset AND ATA-100 speed as well.

Thaz my two cents worth ... Dave

BTW, To my knowledge, RAID-0 is NOT inherantly unreliable. HOWEVER ... since it requires two disks instead of one, there are two potential points of failure. Thus, there is a higher PROBABILITY of a failure, but just how much higher depends upon the quality of the disks, how well cooled they are, how much use they get, what KIND of use (how much arm movement), etc.

[This message has been edited by daverme (edited 04-18-2001).]

[Andy_L]

Dave, you forgot the most important reason for any home raid system - Geek factor/bragging rights LOL. (well thats my reason anyway)

[Stan]

Yep, Andy_L is **** right...
My PC is never fast enough... http://www.sysopt.com/forum/wink.gif

[AuraEdge]

My reason too http://www.sysopt.com/forum/smile.gif
A single drive has no fault tolerance either, so its no better than that.

Actually its slightly worse, because if one fails, all the data is kaput. If you had them both seperate, then if one fails, you only loose the data on that disk. However, you have the same disk of a single disk failing.
So if you have a RAID0 of 2 30GB drives vs 2x 30GB single drives, you get double the speed, but you give up the chance that if one disk fails, you loose 100% of your data on those disks (In a configuration of 2 singles, you have a 50% chance of loosing one disk's data and 50% chance of loosing the other's data). Thats really no better than the RAID0's fault tolerance, since its a random half of your data you lose.
Say you had a 60GB single drive - Then you would have the same speed as two single drives, but you have half the chance of loosing your data as the 2x single drives or the RAID0, but double the risk of the single drives, and the same risk as the RAID0 (if that drive goes, 100% of the data goes).

If you run a RAID1, If a drive fails, you lose NONE of your data, but you give up half your disk space for that fault tolerance.

Its a whole bunch of mumbo jumbo about risk and probability, but if you understand all of it, you're more apt to pick the best configuration for what you want.

[jl123]

Here's a new idea! Why don't you just backup what's on your RAID0. http://www.sysopt.com/forum/wink.gif Duh why go through the trouble of setting up a server with RAID1 and yet back up the RAID1 server? Just back up whatever is on the system with RAID0. Get you a nice tape drive. http://www.sysopt.com/forum/smile.gif Sounds more logical to me.

~Joel

[AuraEdge]

I said that didnt I?
Let me reread it for grammatical problems, cuz thats what I meant.

I dont know if you read that, but I did make those two points
I said you have half the data if you have 2x single drives and a drive fails, but in RAID0, its all gone.

The other points here -
Actually its slightly worse, because if one fails, all the data is kaput. If you had them both seperate, then if one fails, you only loose the data on that disk. However, you have the same disk of a single disk failing.
I see the typo bug bit me on the last sentence.. I meant the same Risk, not disk. I also stated that a single 60GB drive has half the chance of loosing the data as either 2 single drives or RAID0 configurations.

[daverme]

Andy, Stan, U R both absolutely correct !!! I thought of that but just couldn't think of a clever way to say it at the time.

Aura, I hate to argue but I think U R off base on two accounts: (1) With a RAID-0 array, there is NO WAY to recover half the data on a failing drive; if one fails, you lose ALL of it! (2) Having two drives will more than double the probability of a failure. If you don't trust me, ask a statistician.

[Stan]

jl123

My server at home has a RAID1 config and yes, I back it up every week.
2 reasons:
- if one disk fails, the server is still online. There is no downtime and I just have to replace the dead disk. If it was a RAID0 PC, I'll have to re-install everything
- I back up the data only. It is just in case there is a fire or someome steals my PC's (and the backup tape is NOT in the same building http://www.sysopt.com/forum/wink.gif)

Stan

[This message has been edited by Stan (edited 04-19-2001).]

[shady]

what about RAID0+1?? Isn't it stripping and mirroring? Both in one.....

[Stan]

True, but you need a minimum of 4 disks.

Stan