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Just curiousity - RAID arrays and partitions

Posted on 2006-03-21
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I'm just curious what others are doing or thinking in these terms.

Let's say you have a server with (3) 36GB SCSI drives and a RAID controller card.

Let's say you set it to a simple RAID 5 array.

Now comes the question and thought:

Once this is set, regardless of the logical drives and partitions I create, it's still going to access the entire array for read/writes, correct?

I mean, a D: drive for data isn't going to do much good if it's still on the same RAID array, right?  Other than maybe saneness on my part from a management aspect.

I guess I always found it funny that MS and others would recommend things like the Pagefile on separate disks, Exchange logs on separate disks, etc.  That to me would mean that you would need multiple array controller cards, and multiple arrays on each card, with quite a few physical disks, just to actually get separate disk I/O to happen.


Again, I'm not needing an answer to a problem...just curiousity as to whether my theory is right, and what others do.  I'll split the points based on decent comments.
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Question by:TheCleaner
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Netman66 earned 800 total points
ID: 16247776
There would be no performance gain with a pagefile on a different volume on the same array, no.

For I/O performance gains you need to think "spindles".  Physically different drives will perform better as reads and writes can occur simultaneously across them.

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by:sam_cogan
sam_cogan earned 600 total points
ID: 16247912
The only benifit of partionioning your raid array is for organistion, you will gain no performance improvement.
Microsofts suggestion to move page file and logs to another drive will help, but as you sya you would need a seperate array. A number of raid controllers allow you to run 2 or more different arrays from 1 controller, so seperating your drives this way would allow you to do as MS suggests and would give you a performance increase. Obviously having multiple controllers and multiple drives is the best option.
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by:TheCleaner
ID: 16250171
Yeah, that's what I figured...I just was curious what others do.  Do you actually bother with more than 1 or 2 partitions/volumes if they are on the same array?
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by:Netman66
ID: 16252409
Not normally, no.  Two for sure, perhaps three depending on how big your drives are or how many you can load into the server - but normally two on one RAID 5 array.  Keep in mind that many controllers come with more than one port on them.  Depending on how you do this, you can connect an external storage enclosure for more options.

We normally put 5 drives in our servers.  Disk 0 and 1 are mirrored for the OS, the other 3 are RAID5 and can be one or two logical drives depending on how big they are.

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by:TheCleaner
ID: 16252489
Understood, so on your RAID5 with 2 logical drives it is simply for simplicity/management sake, not for disk I/O performance....
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by:Netman66
ID: 16253516
Yes, exactly.  Speed comes with separation.  If you have room for (say) 10 drives, you can create 2 arrays of 5 drives each.  In this instance you improve performance since each array is addressed independant of the other.  Throughput of the controller itself (even though it's the same controller for both arrays) won't be the bottleneck as the I/O from both arrays simply could not saturate the channels.  It does present a single point of failure though.

Picture this;

Your server is one large RAID5, single volume array.  You hose the server installation somehow.  Your data is ...... hopefully on tape!

If you partition 2 logical volumes, then the data goes on (let's call it) D:\.  Hose away!  You can still reformat C: and reinstall without losing the second partition.  The same applies to your normal desktops as well.

Also, if you want to install something like RIS, you require a second partition to store the images on - this isn't optional for this service.  

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by:csk_73
csk_73 earned 600 total points
ID: 16256816
The only thing that can improve if you have a separate partition for the pagefile on the same array is fragmentation.

If you only have the pagefile in the partition then it'll have no framentation

Only a suposition

Cesc
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by:TheCleaner
ID: 16257648
Thanks guys...that'll work for me...thanks for the confirmations and insights.
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