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RAID 5 Help

Posted on 2004-10-11
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I want to buy a server that has a RAID 5 configuration. I just want to make sure I understand it fully.

If I get 5 drives that will be a Raid 5 correct? Do I need a 6th drive as a hot spare? I guess that only matters how quick I want to be back up right? If I have that hot spare, then it will instantly start rebuilding the data onto that drive when another drive fails...correct?

Also, if I get 5 30gb drives...does that mean i will have 150gb of disk space, or does it mean I will only have 30gb, and it just splits the info up on each drive?

I need about 150 gb of usable space....with my RAID5 configuration.

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Question by:caminator
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Luc Franken earned 25 total points
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Hi caminator,

>>If I get 5 drives that will be a Raid 5 correct?
Nope, you can create a RAID-5 array out of it, by default those are just 5 disks.

>>Do I need a 6th drive as a hot spare?
Depends on if you like this extra disk of protection

>>I guess that only matters how quick I want to be back up right?
That has little or nothing to do with this :o)

>>If I have that hot spare, then it will instantly start rebuilding the data onto that drive when another drive fails...correct?
Yes, correct, that's the idea of a hot spare. It'll replace the faulty one, so you can safely replace that one and still have redundancy in the time the rebuild takes.

>>if I get 5 30gb drives...does that mean i will have 150gb of disk space, or does it mean I will only have 30gb, and it just splits the info up on each drive?
It'd mean you have 120GB of disk space, exactly the amount of one disk is used for parity information.

>>I need about 150 gb of usable space
Why not go for three 73GB SCSI disks then? It'll be much cheaper.

It seems like you don't understand RAID at this moment, please take a little time to check this and the following pages:
http://www.raid.com/04_00.html
For RAID-5 => http://www.raid.com/04_01_05.html

Greetings,

LucF
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by:tonyteri
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Hey there,

The backups time will depend on how much storage size you are backing up.

Raid 5 uses 1/3 of total disk space available for parity, so if you need 150GB useable, then purchase the server with 225GB (sorry if my math is off).  Anyway, if you purchase a good server system; i.e Dell, IBM, and not one from a cheap reseller, the raid config utility will allow you to keep a hot spare in their aside from the spoken storage, which will autiomatically sych up when a drive fails.

TT
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by:caminator
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Are HP's as good as Dell's r IBM's?  I want to purchase one from a known vendor.  It seems like to most lkely candiates would be Dell IBM, and HP.  Correct?
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by:Luc Franken
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tonyteri,
That's only when you use three disks, but you can use a lot more in a RAID-5 array. Three is just the minimum. So your calculations are not off :o)

caminator,
The brand of the server doesn't really matter, all you mentioned are made with qualitly equipment and you shouldn't have too many problems with them. I personally have build my own with only intel equipment, stable as can be. I also have a IBM at home with a nice Adaptec RAID controller which also suits me perfectly (I know it's a biased oppinion, but I just love Adaptec hardware Raid controllers)

LucF
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by:caminator
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Looks like RAID 5 is def my best option.  I thought u needed 5 drives for that...for some reason.  If i go with like 3 ...i could get 3 75 gb drives, and a hot spare that is 75 also.  Does the OD just install on one of these drives also?  When looking at dell and HP website, it makes me choose the same 4 drives.  If the OS goes on a seperate one, i would thingk u could get like a 10 gig'er or something....but it doesnt look like dell or hp offer that option.  Does that seem weird to anyone else?
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by:Luc Franken
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One little question... I'm dutch so this might be pretty easy for you, but what's OD?

For a RAID-5 array, you'll need at least 3 disks, so if you want a hot-spare you'll need four. You can partition the array the way you like, so if you want to have a C: and a D: you can do that on just one array. You can also decide to go for multiple arrays, this is usefull for things like databases as a RAID 10 is much faster in write performance, but it'll need more disks to implant (a minimum of four)

One thing you need to remember is that one drive can have more partitions, but in case of a raid array several drives can also make one partition. It all depends on what you want. The OS doesn't have to go on a seperate one, it'll be on the same array but on a different partition.
If you really want to seperate the OS from the data you'll need two arrays. The most common configuration (for a small company) is a mirrored array (RAID-1) of two disks for the OS and then a RAID-5 array for the data (minimum of three disks again). Both arrays can share the same hot-spare.

LucF
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by:caminator
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The OD was supposed to be OS (as in windows).  Sorry for the typo..and the added confusion.  Thanks for all you information!!!
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by:Luc Franken
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:) sorry about misunderstanding...
Would you like some more information, or do you have any more questions?

LucF
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by:caminator
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Only other question I got is, what kind of server would you get?  This will be the PDC, DNS, file, and print server.  I am looking into Dell, and HP.  I'm not sure what makes one better than the other, but HP seems to be a little bit more expensive.  

Do you have a preference, and why?  Thanks
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by:Luc Franken
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As I said at http:#12278482 I build my own now. My IBM is an old leftover from my work, now hosting my websites. (it's a IBM Netfinity 3000 so very old, but still running without any problems)
For what I've heard, sorry I don't have much experience with this, HP has a better support then Dell has, but I'm sure some one else will think otherwise. Both use very stable and high quality parts, so if you know your way around a server both should suit you great, otherwise HP would be my preference.

LucF
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by:Brainded
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I have had nothing but good luck with Dell.  And good performance too.

Make sure you understand the distinction between hardware and software RAID.  hardware is (obviously) controlled by the RAID controller hardware.  Software is run by the OS, and as such will not support hot-swap and is slower because there is more CPU overhead.  For a production server you  definitely want a hardware solution.

You have to think differently when working with RAID arrays.  There may be 3+ physical drives in there, but the OS only sees the number of containers (or volumes) created on the array.  For instance, if you take your example above (3 75GB drives) the OS would see a single 150GB container.  That can be partitioned up the same way you could a single standard drive.

You should also understand the different levels of RAID.  The most commonly used are RAID 0, 1 and 5.  RAID 0 (striping) takes data and stripes it across first one drive, then the next, and so on back to the first.  All stripes are the same size and are distributed equally across the drives.  Note there is NO fault tolerance here, since if one drive fails you lose the whole array.  But there is a performance increase, as data can be read from multiple drives at the same time.  Minimum 2 physical drives required.  0 overhead.

RAID 1 (mirroring) duplicates data between drives.  This is done real-time.  Data is written-to and read-from both drives at the same time.  Theoretically, both drives should have exactly the same sector layout.  2 identical size drives (only) required.  2 identical size RAID volumes can be mirrored this way as well.  50% overhead.

RAID 3 (stiping with parity) writes data accross disks similar to the way RAID 0 does, but one disk is a dedicated parity volume.  Very similar to RAID 5.  Fault tolerant in that the array can continue to function if one drive fails.  Minimum 3 identical drives.  One drive's worth of overhead.

RAID 5 (stiping with parity) writes data the same way as RAID 3, but the parity stripes are distributed evenly across all volumes.  Otherwise identical to RAID 3.

Sometimes RAID levels are combined (ex, 0+1, 5+1, etc.) to improve performance or fault tolerance, but bear in mind that this increases hardware overhead as well.

For more info, check out http://www.prepressure.com/techno/raid.htm
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by:caminator
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Brainded,  Dell over HP then?  It seems HP are higher than Dell in the ratings for the servers.  Is this true?  That is what I have found.  I don't understand the difference between the two.  They are probably just putting the same hardware in each system....don't ya think?
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by:Brainded
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I'm not necessarily saying Dell over HP, just that I have had great success with Dell equipment.  I have had good luck with IBM in the past as well.  Bad experiences with Compaq, but that's as close as I've come to HP.  That was before they merged.  Most of the reputable major vendors will have their critics and supporters, so just do your research and don't forget to listen to the comments on their support as well.  My bad experiences with Compaq were purely support-related.
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