Some RAID5 questions

[tdx]

Hi everyone,

I'm about to buy a 975x mobo that has RAID 5, and I'm thinking of putting 4-5 hard drives on it. There are some things I'm not sure about, though:

- Can I create a RAID 5 array starting with the minimum (3) number of hard drives, and then add other hard drives to it later on? Or will I have to create a new RAID array and lose all my data everytime I want to expand it?

- If, God forbid, one of the drives should crash, how am i supposed to determine wich one it is in order to replace it and repair the array (they will all be inside the case, you see)?

- Finalkly, I know the hard drives have to be all the same size, but do they have to be the same brand/model? or it doesnt matter, as long as they're the same size?

Thanks in advance for the help, guys...

Cheers, TDX

 

[defakto]

- Can I create a RAID 5 array starting with the minimum (3) number of hard drives, and then add other hard drives to it later on? Or will I have to create a new RAID array and lose all my data everytime I want to expand it?

Depends on the controller. Look for a feature called something along the lines of online array expansion or such.

- If, God forbid, one of the drives should crash, how am i supposed to determine wich one it is in order to replace it and repair the array (they will all be inside the case, you see)?

The controller will probably tell you drive x on port y has failed would be my guess but I don't have experience with this.

- Finalkly, I know the hard drives have to be all the same size, but do they have to be the same brand/model? or it doesnt matter, as long as they're the same size?

You can mix brands, but should you? It's normally considered bad form creating an array, at least with most the people I've talked to about it, with different manufacturers in it. Differences in firmware, platter density, etc can all affect performance on the raid, one problem drive could cause alot of issues.

 

[tdx]

Thanks for the quick response. So, just to make sure I got it right:

- In order to get online array expansion capability, i have to go with a PCI RAID controller, right? The mobo I'll be buying is going to be the P5WDG2-WS by Asus, which has 2 onboard controllers (ntel ICH7R & Marvell), but I guess none of them are sophisticated enough to let me add volumes to my array without reformatting, right?

- As for the replacement of a defective hard drive, i understand that the controller would tell me which drive is bad, but how would i know which one it's talking about?

 

[defakto]

1.) most onboard controllers are crap and don't have alot of features, I haven't read the manual on it, but it probably wouldn't support it.

2.) you label everything to make it easy, every port on the card should have a number, just number the cables and the drives to match.

 

[dirtydr]

1.) most onboard controllers are crap and don't have alot of features

Indeed... do yourself a favor and get a standalone pci-e or pxi-x (since it appears that board has both) controller with a built in xor processor.

 

[UICompE02]

- Finalkly, I know the hard drives have to be all the same size,

It looks like nobody has covered this yet, but you don't have to have all drives in a RAID 5 array be the same size. However, the size of the array is dependent on the size of the smallest disk in the array - the rest of the bigger drives will just be unused.

The size of a RAID 5 array can be calculated as follows:
X*(N-1)

Where X is the size of the smallest disk in the array and N is the number of drives in the array.

 

[Tilde]

Raid5 Arrays generally can't be grown.

 

[Megalomaniac]

but you can add a hot spare, which kind of makes it into a poor mans raid 6

 

[defakto]

Raid5 Arrays generally can't be grown.

On cheap hardware no, but any decent hardware raid 5 card out there supports online capacity expansion.

 

[defakto]

It looks like nobody has covered this yet, but you don't have to have all drives in a RAID 5 array be the same size. However, the size of the array is dependent on the size of the smallest disk in the array - the rest of the bigger drives will just be unused.

The size of a RAID 5 array can be calculated as follows:
X*(N-1)

Where X is the size of the smallest disk in the array and N is the number of drives in the array.

If he wants to use the expansion capabilities he must add drives that are the same size or larger for it to work right, correct?


Sorry for the double post, spaced and did two for no good reason.

 

[The_Mage18]

If he wants to use the expansion capabilities he must add drives that are the same size or larger for it to work right, correct?


Sorry for the double post, spaced and did two for no good reason.

No, the drives must at least MATCH the size of the smallest drive in the current array. They can be bigger but you'll have waisted space.

The Highpoint RocketRaid 2220 I have has ORE. If I create an array with two 160GB drives and try to add a 250GB to it, the 250GB is chopped down by the card to 160GB so that it matches size-wise.

As already stated though, it's not a good idea to mix and match.

 

[whbonny]

I always thought that extra drive space would be wasted too, but then I found this:

http://www.adaptec.com/worldwide/product/proddetail.html?sess=no&language=English+US&prodkey=AAR-2810SA&cat=%2fTechnology%2fRAID+Controllers%2fSerial+ATA+RAID

for the lazy, the important part is:

Optimized Disk Utilization, exclusive to Adaptec, enables the Adaptec 2810SA to utilize the full available capacity of all attached drives, even if drive sizes are different. This avoids wasted drive capacity and provides advantages over competing cards that limit capacity usage to the smallest drive in an array.

and it goes on with:

Online Capacity Expansion enables IT managers to create multiple arrays across a single set of drives connected to the Adaptec 2810SA, so that specific arrays can be optimally tuned to meet the differing needs of multiple applications.

Seems that it's little 4-channel brother 2410SA does this also.
I wish these were arround when I got my promise sx6000. I'm rebuilding a 5 drive 1TB array right now in fact. Anybody want to buy it...Cheap. Just kidding (kind of).