New to RAID and not sure about HD config

[hoffmak]

Hi. I am about to move from a Dell server without RAID to a Dell PowerEdge 6600 with RAID. It will be an Oracle server, currently requires about 5 hard drives.
I am totally new to RAID, and while I understand its theory, I am having a hard time planning out how many drives I need in the reality of things. I would like to run it at RAID 5.
Any suggestions from any seasoned professionals out there? I have never run on a RAID system before, but I would like it for its data redundancy and recoverability.
Thanks

 

[MJewell]

If you haven't bought the server yet, consider going with the 4x2 backplane, 4 drives per channel * 2 channels, and purchase 4 drives instead of 5, this will leave you with room for expansion later if necessary.

As for space, for Raid 5, the way to figure space is:
numdrives-1 * capacity of smallest drive = Useable space

ie.
Bay 1 Bay 2
4-1 * 18g = 54g Reserved for Later use
4-1 * 36g = 108g
4-1 * 73g = 219g

Then you can add additional space later if you outgrow it.

I would recomend going with either 36's or 73's.

If you are going to need more then this to begin with, then buy the 1x8 backplane and fill all 8, because the formula works the same...
Bay 1
8-1 * 18g = 126g
8-1 * 36g = 252g
8-1 * 73g = 511g

If your going to require more then 511G, then you could also purchase the optional drive bay that will hold 4 more drives, but take up the area below the CDRom....

Mike

 

[johnny99]

MJewell is right, but I would do it a little different.
The raid cntl. should have 2 channels so go for the dual backplane regardless of how many drives you will use.

Using 2 channels will be better for performance and is doesn't make any difference in how you use your system anyway.

Remember tat replacing drives later cost a lot of time and money. (that's why in our installation we only have the C: drive on the servers)

I don't know how many drives I would use because it depends a lot on your needs, but Most often I would go for raid 5 + a hot-spare.
The number of drives doesn't make the big difference. You can use anything from 3 to the max number in the server for raid 5.
If this server is for high performance remember that the more drives you have in your raid 5 the better the performance.

But tell a little about the needs you have. If the database is 5 GB or you need to be able to swap the disksystem to a different servers can make a big difference in how to configure the server.

If you database if relative small (eg less then 20-30 GB) you could also look at raid 1. In realy bad situations it's a lot more easy to recover a server that is running raid 1 then raid 5. But again more info on your needs will help to give you a better advice.

If you have the time try fooling around with the raid system before you start to make the "real" server.
Disksystems now a days are fairly stable so understanding how it works and knowing what to do in a given situation can be a little of a problem.
We have 35 Dell servers and they are almost 2 years old. In this period we have had one real diskfailure (the raid cntl had to be replaces)
Now a days you have to train so you can handle recovery of raid. In the "old times" it was part of daily life.

Always plan for recovery (not for backup)

/johnny

 

[MJewell]

qand it doesn't hurt to make sure that your tapes will do a full restore if you ever need to, have a spare machine around and do a restore everyonce in a while if possible...

Mike

 

[OUCATS]

FYI...
When planning for available drive space, remember that RAID 5 uses approx 30% of drive space as overhead (space not available for data). I almost made that mistake once.

A couple of RAID links:

http://www.acnc.com/04_01_00.html

http://www.webopedia.com/TERM/R/RAID.html

 

[SteveTheGeek]

phelpsm,
30% of drive space, in addition to the N-1 rule where essentially one drive's worth of space is devoted toward the RAID? I've never heard of 30% overhead, I'd assume that it's simply the fourth drive usage on a four-drive array.
-Steve

 

[cdjmh]

Consider your space requirements carefully. It is not easy to expand a RAID 5 array after it has been initially configured. A complete backup/restore and reinitialize are required. It's easier to plan for the maximum hard drive space in the initial installation.
cdjmh

 

[NickFerrar]

Why do people keeping posting this? IT IS PERFECTLY POSSIBLE TO EXTEND A RAID 5 VOLUME AFTER IT'S BEEN CREATED.

I've done it on four production servers in the last year - the only hassle is you have to create a volume set within Windows to logically 'attach' the new diskspace to the exisiting logical volume (assuming you don't want the new disk space to be a new volume). On Windows 2000 you can probably even avoid this if you configured the volume as a dynamic disk although I haven't tested this.

 

[OUCATS]

SteveTheGeek,
Sorry for my late response. RAID 5 stripes accross however many drives you have in your array. So because one disk has striped part of it's data onto another drive, that part of the drive is used up (unavailable). That's why I have heard that 30% is a good estimate to use. The way RAID 5 works is that if you loose a drive, the data on that drive is striped on other drives and can be restored from those drives (why you lose the 30%)