RAID5 questions

[GTengineer]

I have read the sticky but I still have unanswered questions. I was thinking on adding one or two more Seagates 250GB (perps) to the two I currently have.

Question 1
So let's say I add another one (3 total) in RAID5, would that mean I have 375GB usable disk space? And if I add 2 more drives I would have 500GB usable disk space?

Question 2
Would RAID5 be close to RAID0 performance with full parity as safety in case one drive fails? Are there any benchmarks out there comparing RAID0 vs RAID5?

Question 3
Can the ICH9R south bridge handle a matrix array of Raid 0+1 and a separate RAID1 array?

Thanks! :beer:

[thideras]

Question 1: If you have 3 250gig drives, you would have 500gig (about 465gb).

Question 2: It will be slower with RAID 5 unless you have a dedicated card, even then with the card it will still be slower.

Question 3: You mean with 4 drives? It should, I did not tinker around with 4 drives when I had that SB. But I would assume you should be able to.

[GTengineer]

Wooo fast replies! cheers!

Question 1: If you have 3 250gig drives, you would have 500gig (about 465gb).

I don't understand how you can have a back up of your entire drive that way if you have 500GB usable with 3 drives. It seems like trying to back up 2/3 of the drives into 1.

I think I am confusing RAID5 with RAID10, but I just dont see how RAID5 would be able to always have a backup. Would RAID10 provide RAID0 performance with RAID1 parity?

Question 2: It will be slower with RAID 5 unless you have a dedicated card, even then with the card it will still be slower.

Wish I could find some benchmarks to determine if it is worth it :o

[th3]

I don't understand how you can have a back up of your entire drive that way if you have 500GB usable with 3 drives. It seems like trying to back up 2/3 of the drives into 1.
Raid5 doesnt back up anything, it creates parity information so the data can be recreated in the case of a single drive failure. The capacity of Raid 5 is SIzeOfDrive x NumOfDrives - SIzeOfDrive, so 5x 250GB disks in Raid 5 will be 250 x 5 - 250 = 1000GB.

[thideras]

Raid5 doesnt back up anything, it creates parity information so the data can be recreated in the case of a single drive failure. The capacity of Raid 5 is SIzeOfDrive x NumOfDrives - SIzeOfDrive, so 5x 250GB disks in Raid 5 will be 250 x 5 - 250 = 1000GB.Correct.

OP, I think you are mixing up RAID5 and RAID0+1 ;)

RAID5:

3 drives. Parity is split into 3 pieces and split across the drives. Incase of 1 drive failing, the RAID can continue to function, when a new drive is added (or the one comes back online) it will recreate the RAID.

Drive 1............Drive 2.............Drive 3
....A.....................B.................Parity
....C.................Parity.................D
.Parity..................E....................F



RAID 0+1:
4 drives. There are two sets of drives, both in RAID0, then the two RAID0 groups are put in a RAID1 for mirroring. In this case, you could have up to two drives fail (if two drives fail that are the same stripe (drive 1 and 3), the RAID fails.

Drive 1.............Drive 2
............Stripe A...........

Drive 3.............Drive 4
.....Copy of Stripe A....

[g0dM@n]

Correct.

OP, I think you are mixing up RAID5 and RAID0+1 ;)

RAID5:

3 drives. Parity is split into 3 pieces and split across the drives. Incase of 1 drive failing, the RAID can continue to function, when a new drive is added (or the one comes back online) it will recreate the RAID.

Drive 1............Drive 2.............Drive 3
....A.....................B.................Parity
....C.................Parity.................D
.Parity..................E....................F



RAID 0+1:
4 drives. There are two sets of drives, both in RAID0, then the two RAID0 groups are put in a RAID1 for mirroring. In this case, you could have up to two drives fail (if two drives fail that are the same stripe (drive 1 and 3), the RAID fails.

Drive 1.............Drive 2
............Stripe A...........

Drive 3.............Drive 4
.....Copy of Stripe A....

good explanation, my friend.

GTengineer, Raid5 is easiest to calculate by taking a single drive size and multiplying it by one less than the total amount of drives.

I am planning on doing FIVE 320gb perpendiculars in a Raid5 in the future, so I'd be left with 4x320gb = ~1200gb

Or I'm thinking of doing FOUR 320gb perps in raid5 for ~950gb total and keeping the 5th drive as a spare for if/when one drive fails... this way i can rebuild the array instantly. :)

I say this b/c I have 2 perps in my file-server, and 3 perps in my main rig... i may combine them all someday and do something with these super-fast 250gb 410as on my main rig.

[GTengineer]

Thanks guys I think I've got it now, yes I was confusing RAID0+1 with RAID5. My next dilemma is to decide between RAID0+1 and RAID5 :D

[bing]

My next dilemma is to decide between RAID0+1 and RAID5 :D

That is not a difficult, once you had those drives in hand, it doesn't take too long to test out "BOTH" configurations at Raid 10 or Raid 5 to see which one fits you the best !

Just remember, when on Raid 5, the best strip size = 64K !

.

[thideras]

Just remember, when on Raid 5, the best strip size = 64K !Why is that? Is it something to do with the way it handles RAID?

[bing]

Why is that? Is it something to do with the way it handles RAID?

Dunno, its just the way it is, you can dig up inside that Matrix sticky though, if I'm not mistaken, there is OcF-er that tested different strip sizes, and tortured his rig with high load benchmark IOMeter and came with conclusion that 64K for RAID 5 (not Raid 0) is the best.

[GTengineer]

That is not a difficult, once you had those drives in hand, it doesn't take too long to test out "BOTH" configurations at Raid 10 or Raid 5 to see which one fits you the best !

Just remember, when on Raid 5, the best strip size = 64K !

.

cool bing, I actually ended up ordering only one. Newegg had a promo code $65 w/ free shipping but it was only one per customer. I will get another next time around since I am not that desperate for it ;)

BTW bing, I got the RAID1 and RAID0 arrays to mount on Linux. So now my backups go straight into RAID1. Awesome!

[Jon]

Why is that? Is it something to do with the way it handles RAID?

64K is just a good 'middling' setting for standard use. There is no 'best stripe size' for any RAID level - it's dependent upon many contributing factors such as the number of disks, access times, latency, file sizes, number of files, preferences in positional or transfer performance, IO operations, access type/frequency, etc.

If for benchmarking only, then maybe there is a 'best', but it would still be dependent upon what you're benching for and the app used. I won't argue individual benchmark results since I don't use my arrays for that.

If you want a hypothetical 'rule of thumb', then I'd say use smaller stripe sizes for fewer very large files, bigger stripe sizes for many small files, and something in the middle for a mixture. That is where the 64K stripe would come into play...not too low or high.

[bing]

Jon,

Those theory on strip sizes can not be applied easily in this type of "fake" in mobo raid controller. :)

Remember, they were designed not as a dedicated raid controller replacement.

Regarding the strip size, it is all depends on their limitation at their internal SB chip, also the SB to NB bandwidth, plus the raid driver latency and it's design.

So you won't have the luxury of choosing different strip size as the normal Raid theory & practice.

Even though it has a choices of strip sizes, all of those came with penalties because it involves SB, NB, CPU and RAM, we just choose the one with minimal impact, not like the dedicated raid controller that has a dedicated CPU,RAM and dedicated lane/bus just to pass the " final result" or cooked ready to serve data to/from the raid drives.

[GTengineer]

well RAID5 sounded good till I read this (http://www.miracleas.com/BAARF/RAID5_versus_RAID10.txt)

NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO RAID5!

So I bit the bullet and ordered a fourth drive, I am pretty sure I will end up running RAID10. But I may do some benchmarking of RAID0 vs RAID5 vs RAID10 to help other noobs like me.

Just to make sure I have this right. With 4x250GB drives in RAID10 I would have 500GB of usable space and the performance of RAID0 with mirroring?

[bing]

well RAID5 sounded good till I read this (http://www.miracleas.com/BAARF/RAID5_versus_RAID10.txt)

So I bit the bullet and ordered a fourth drive, I am pretty sure I will end up running RAID10. But I may do some benchmarking of RAID0 vs RAID5 vs RAID10 to help other noobs like me.

I have a "feeling" that after those juggling and intensive testings with different Raid types, you will fall into Raid 5 ! LOL :D

Yes, that is the best way to find what is the best for you !