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RAID 5 using two physical disks?

Is it possible to create a RAID5 array of three equally sized partitions that are physically located on two disks?

The objective is fault tolerant storage without hurting performance too much

The server has 1x250GB and 1x500GB HDD's

If RAID5 is not possible, would you recommend using mirroring?
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george82
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george82
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9 Solutions
 
John HurstBusiness Consultant (Owner)Commented:
I don't know if you can, but it would totally defeat the purpose of Raid 5. A simple Raid failure could trash the system with only two drives.  If you want two disks only, use Mirroring. ... T
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NotLogicalCommented:
RAID 5 requires a minimum of 3 disks: otherwise it is just mirroring... If you mirror your drives, half of your 500GB will go unused, as the smallest disk's area is used as the mirror size. RAID is always used on physical devices, not partitions - otherwise a failure could (and most likely would) render your array unusable. That's not to say that you cannot put several partitions and RAID them across different disks!

Mirroring is definitely better than no backup at all - RAID5 would be best. I am generalizing on your application, since there are some performance hits on RAID5 as compared to other RAID types.
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Lee W, MVPTechnology and Business Process AdvisorCommented:
I do not recommend RAID 5 anymore because it has been too flaky for me using various controllers and drives.  It SHOULD be possible to create a 3 disk RAID 5 using the 250 GB drive and the 500 GB drives.  It is NOT POSSIBLE to create a two disk RAID because that would NOT be redundant.  Consider this: to be redundant, RAID 5 uses 1 drive worth of space for parity.  IF you had 2 drives, one partitioned in two simulating a 3rd drive, then if that one PHYSICAL drive failed (that had the "simulated" drive), ALLL DATA WOULD BE LOST.  Total space would 3x250 (the smallest drive size).  It is POSSIBLE that a hardware RAID controller would not allow it, but all the hardware RAID controllers I've used DO allow it.  SOFTWARE RAID will definitely do it, but that is NOT RECOMMENDED for performance reasons.

I would strongly recommend you use the mirroring option. OR, drives are dirt cheap - go buy another couple of drives.
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aleghartCommented:
RAID5 will hurt your performance, unless you spend a lot of money on a hardware RAID controller, like a 3WARE, which is rated far faster than the individual drives attached.

Since you are slicing and dicing old drives, I assume this is not in your budget.

For cheap RAID without taking on a performance hit, the best you can do is RAID1 mirror.  You will lose 250GB on the 500GB drive, but you will not see too much of a performance hit on reads or writes.

You can't build a RAID5 with only two physical drives.  The RAID controller will not let you.  As far as Windows Dynamic Disk...haven't tried it, but the OS know the difference between a physical drive and a logical partition.  Hopefully it will give a warning message or refuse to build the array in that manner.

A 3x250GB array nets you ~500GB.  You should just buy another 500GB drive and build a mirror.  I know it doesn't fit a "zero" budget...but how much is your data worth?

http://www.newegg.com/Product/Product.aspx?Item=N82E16822148136

$70 for a 500GB SATA drive.
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SysExpertCommented:
Not possille as already mentioned.

Time to buy 2 more drives and to do it properly.


I hope this helps !
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jdietrichCommented:
As everyone said, you need minimum 3 drives for Raid V and should have a 4th (spare).  That said, here's a few things to think about:

As mentioned, if you mirror the two you will have 250 of usable mirrored raid.
If you end up buying 2-3 more 500's to do a raid V, I would recommend taking the 4 and doing Raid 10 (striped for performance, mirrored for fault tolerance).  Best of both worlds.

If you are set on Raid V, a few things to consider:  Reads are not bad on raid V, but due to the overhead of parity (writes set parity on all drives for recovery) your writes will be slow.  Now that doesn't matter if you are loading data during non peak times, such as via batch at night, and during the day the data is only read from.  But if this is a database or OLTP (on line transaction processing) system where there is solid reads and writes raid V may not be the best for you.  BUT...if your heart is set on it, at least get a 3 channel controller, so that you can put one drive on each channel, that will improve performance for you.

JD
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Handy HolderSaggar makers bottom knockerCommented:
Veritas volume manager used to allow you to create a software RAID 5 array as described in the question but I think they've patched it to disable that now. It's really poor performance because of the long seeks and only protects against flaws on the platters, not head or spindle failure.
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george82Author Commented:
thanks for all responses, I will use RAID1 for now, with 1x250GB and 1x500GB at half capacity.. and later on I can add two more drives and create a RAID1+0 array.

In a RAID1+0, do you suggest having another drive setup as a hot spare or is it considered an overkill since everything is already mirrored?

I hope the Proliant ml150 g5 built-in Intel ICH9R controller supports RAID1+0
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aleghartCommented:
Hot spare is good if your controller supports it.  Cheap insurance.
If you don't have it, RAID 1+0 tolerance should buy you enough time to swap out the bad drive(s).
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Handy HolderSaggar makers bottom knockerCommented:
ML150 isn't a proper Proliant, just HP pinching a good name to sell poo to compete with Dell on price. Might as well get a generic white box.
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george82Author Commented:
andyalder what would you suggest instead at this entry-level price point?
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george82Author Commented:
I have a last question -- when the new RAID1 Array is set-up, with 2 HDD's, do I lose all data on the First HDD? Right now the first HDD (the one I want mirrored) already has ~10GB of data

If we lose the data can I just use Ghost to take a full image of the first disk and then restore it after the Array is setup?

Thank you
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aleghartCommented:
With a RAID1, you should be able to specify the source disk, which will then be mirrored to the other.

But, to avoid the size mismatch problem, it is sometimes easier to backup the data (or simply copy it) then restore to the finalized array.  If your source drive is 72,000.1 MB and the target for the mirror is 72,000.0 MB, you'll have problems.  The array is supposed to be sized down to the smallest drive.
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george82Author Commented:
thanks for clarifying aleghart

I have a 250GB drive split in three partitions with ~10GB of data (new windows server 2003), and a brand new 500GB drive with no data

All i want is have the 250GB mirrored to the 500GB, inevitably i will lose the 250gb of the second drive

Do i need to clone the current 250GB to a ghost image and then restore this image back to the array after the BIOS RAID setup?

Thank you
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aleghartCommented:
I would clone it as a backup anyway.

But, the 250GB source should have no problem making a mirror on a 500GB drive.  You have plenty of extra space!
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