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Posted on 2012-09-14
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I have a question on how a virtual machine file is stored and how oracle is stored when installing on the virtual machine.


If i have a dell machine with 8 physical disks configured as raid10 (array has 8 disks)

When sys admin creates a Virtual machine (VMware ESX) a file is created for that machine.

I assume the file is located on one of the 8 disks.

If i install oracle to the virtual machine does not that mean all the files and databases will be created within that VM file which means on one disk which is not good for database performance. I  would like to install system software on one disk, databases on second disk, and backup areas and archive file on 3rd disk..


When you nornally create a virtual machine do you specify different disks for the VM file?
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Question by:sam15
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Andrew Hancock (VMware vExpert / EE MVE^2) earned 1000 total points
ID: 38399176
the virtual disk (vmdk), which is created by the VMware Hypervisor, will be stored in the datastore, which will be stripped over your 8 physical disks configured as RAID 10.

the vmdk is not located on a single physical disk, VMware ESX does not detect 8 p[hysical disks, it's presented a RAID 10 datastore, which is a single "block" or RAID 10 storage.

it's correct, that when you install Oracle, all the files and database will be in that single file (vmdk). But...

the benefit is the virtual disk is stripped over eight disks which will increase performance.

But you can still if you like create three seperate disks, which will all be stored on the same datastore, but I do not think there will be any performance gain, because all three disks are on the same RAID 10 datastore, compared to a single virtual disk, with all the same data.
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by:sam15
ID: 38399320
I think you mean striped over 4 disks since the other 4 are simply mirror disks.

So you are saying the the virtual datastore is really stored on the 4 physical disks.
Is this always automatic when you install a Virtual machine meaning it will always look for all the physical disks on the machine and create a virtual datastore on all disks?

It sounds when i install oracle, i dont really have an option other than selecting the virtual datastore for all directoires and files.
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by:slightwv (䄆 Netminder)
slightwv (䄆 Netminder) earned 1000 total points
ID: 38399395
I think I mentioned this in your previous related questions:  I feel you are over-thinking this.

Think of it in layers:
Bottom layer:  Physical drives
On top of that you bind physical disks together to create logical devices.

You can then create one or more file systems on one or more logical devices.

Now you have Virtual machines on top of that.

So do you have 4 disks bound as one large device and mirrored or 8 disks bound together as raid 10?

However you have them bound:  When you create a file on disk, it must obey the filesystem it is on.  If that filesystem is on all 8 stripped/mirrored disks:  any file is also on those same disks.

The VM's VHD is just a file on disk.  It obeys the same rules.  If you create two VHDs on the same host filesystem, it will be on the exact same disks as any other file on the host file system so no real performance gains.

Now where you might benefit later is if you add additional disks to the host OS and create a different raid binding.  Then you can move the VMs VHD to the other raid binding and Oracle inside the VM can take advantage of the extra disks and there isn't anything to change inside the VM itself.
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Author Comment

by:sam15
ID: 38399431
I need to check with sys admin on the internal structure.
What do you mean by "bound"? is it a physical link.

So you can have 8 filesystems (one on each disk) or one fielssytem for all 8 disks?

Do you know of a good link with pictures that shows all of this. a picture is worth a thousand words.
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it's best to have RAID10, with all disks in a single RAID 10.
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by:slightwv (䄆 Netminder)
ID: 38399697
>>What do you mean by "bound"? is it a physical link.

More or less it is a physical link.  When you create a 'group' of physical disks for a RAID setup they are said to be 'bound' together.  This makes a logical device of more than one physical device.

http://en.wikipedia.org/wiki/Logical_device

Here is a decent description that hopefully will help you see the physical to logical device mapping:
http://en.wikipedia.org/wiki/Logical_Volume_Manager_(Linux)



>>So you can have 8 filesystems (one on each disk) or one fielssytem for all 8 disks?

Sort of.  A file system is just how the OS allocates space on a device:
http://en.wikipedia.org/wiki/File_system
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by:slightwv (䄆 Netminder)
ID: 38399716
>>it's best to have RAID10, with all disks in a single RAID 10.

All things being equal, this is probably true.  However things are rarely ever equal.

The main one being administrative staff.  The 'safest' bet is RAID-5 if the systems using the disks can handle the write performance hit.  Since the disks in question will mainly handle smaller Oracle databases where raw performance isn't a main consideration RAID-5 would probably be fine here and a lot fewer headaches when a disk failure happens.
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