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Actualization in RHEL Change allocated memory and cpu

Posted on 2010-01-10
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My RHEL server specification is Quad core CPU 1.6 and 2GB memory. Basically I don't know how many virtual machine will be installed on this machine. If I allocated example 512 fro memory, can I change that in future after installing the virtual machine? In with your experience if I have 4 virtual machine how much shall I give to each machine?

thanks
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Question by:rawandnet
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Expert Comment

by:ryder0707
ID: 26279957
Yes you change amount of ram in future if you have enough physical ram, you can run multiple VMs with 2GB but the performance is not going to be great as each VM will need to do swapping due to isufficient physical ram
If your host RHEL dont really use lot of physical ram, probably you could run 3-4 VMs for 512MB each, but this also depends on what application you intend to run on each VM, over committing of ram is possible, if you need to run more VMs but you need to change the setting to "Swap all VMs memory" and move all VMs to dedicated disk drive NOT partition or the performance is going to be very bad for the host as well
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Author Comment

by:rawandnet
ID: 26280736
How about CPU, at the moment the system is Quad 1.6Ghz "4 CPU",  what number shall I put for Virtual CPU for each virtal system.

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by:ryder0707
ID: 26280760
1vCPU is normally enuf, but you can change the vCPU later but depends on guest OS, some OS will have problem if you changed the CPU count, you need to stop VM before modify ram or cpu
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Author Comment

by:rawandnet
ID: 26280825
normaly how many cpu do you allocate for one vertual OS, if you have quad cpu?  if each Virtual has been alocated 4 cpu, do they share the cpu between them?  and if I give one for one OS, it will use only one cpu?
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Expert Comment

by:ryder0707
ID: 26280850
best practice is 1vcpu is fine, you normally need to add additional cpu if the application can take advantage of SMP
otherwise no point
yes cpu resources are shared, so its better to use them wisely, over allocate cpu to a vm might impact overall performance
if you give only 1vcpu, to the guest OS is like one physical cpu, but its actually using a core of your physical cpu
for example, if you have dual core cpu, you cant assign quad core to a vm, you can assign but you will get error
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Expert Comment

by:ryder0707
ID: 26280870
let my finish my last sentence above
for example, if you have dual core cpu, you cant assign quad core to a vm, you can assign but you will get error if try to start the vm
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Author Comment

by:rawandnet
ID: 26280905
what do you mean by this "if you give only 1vcpu, to the guest OS is like one physical cpu, but its actually using a core of your physical cpu"? what do you mean by core of physical cpu?
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by:ryder0707
ID: 26280909
dual core cpu = 2 physical cores
quad core cpu = 4 physical cores
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by:ryder0707
ID: 26280937
just to make it clear, eg if you have dual core cpu, you can have 5 VMs with 1vCPU becoz it is actually sharing the same cpu but vmware is smart enough to allocate the core from the physical cpu, get it?
Not to say dual core cpu can only have 2 VMs, but dual core cpu can give 1 or 2 vcpu to each VM and at the same time allow more VMs to run at the same time
In enterprise product like esx/esxi you can actually assign vcpu to specific physical core, this is called cpu affinity
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Author Comment

by:rawandnet
ID: 26281040
Do you recommend VMware or Virtual Machine manager that comes with RHEL?

and with Red hat there is ParaVirtualizaed and Fully virtualized, which one do you recommend, I know paravirtualized has better performance, but as I read, it require more maintenance.

and how much hard disk space shall I alocate for each VM, don't you think it would be best to give minimum (example 5GB) and mound one big driver for other web and other applications to be shared between VM.  Thanks
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by:ryder0707
ID: 26281078
For me full virtualzation its perfectly fine to use plus it is easier to manage virtual as you said it yourself
I agree that paravirtual offers better performance but it really depends what is the VM for, in my comp we have 32 esx hosts and all VMs running full virtual, they are other ways to improve performance and VM performance not only reply on virtualization technique, you need to consider, cpu, ram, network & storage performance as well
I have no experience with rhel vmm so i have no clue about it
But i can tell you that vmware products for virtualization is matured & reliable if used properly
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Author Comment

by:rawandnet
ID: 26281089
you said you have 32 VMs on one system, what is the specification of that machine it must be very powerfull?

How much hard disk space shall I alocate for each VM, don't you think it would be best to give minimum (example 5GB) and mound one big driver for other web and other applications to be shared between VM?
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LVL 24

Expert Comment

by:ryder0707
ID: 26281135
actually 32 hosts and there are hundreds ov VMs :)
each host is actually a blade server, each blade has dual quad core with 32GB ram
model is HP Proliant BL460c G1, all host connected to fiber channel storage
you can set it to 20GB for future growth, with think disk virtual disk size will only grow as you put data into it so should be fine
if you use thick disk or preallocated disk, the virtual disk is created max to the size you specified, whether you have data or not, you already loose 20GB
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Expert Comment

by:ryder0707
ID: 26281137
but thick disk is supposedly to give better performance, if performance is something you need to consider you can use thick disk
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Expert Comment

by:ryder0707
ID: 26281154
sorry typo error, its actually thin disk not think disk :)
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Author Comment

by:rawandnet
ID: 26281552
thanks for all your support.
I have other question.  I have installed Linux VM, but when I insert CD or USB, it doesn't mount on that VM.  and when I try mount /dev/sda1 ...., I get does not exist.  does that mean virtual machine doesn't mount drivers?

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Expert Comment

by:ryder0707
ID: 26281584
You need to edit the VM properties to use the host disc drive
For USB devices you need to assign USB controller to the VM, then you can actually select which USB device to connect to the VM, but your host will loose the connection when it is given to the VM
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Author Comment

by:rawandnet
ID: 26282741
in a normal situation, when a system fail, the hard drive can be attached to another computer or mount the failed system and restore files.  but with vm there is only one img file, if the system couldn't boot to the img file how can I restore necessary files when img is only one file?
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Accepted Solution

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ryder0707 earned 2000 total points
ID: 26284141
You can use vmware disk mount tool to load the image as another drive in Windows
Refer to http://www.vmware.com/pdf/VMwareDiskMount.pdf
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Author Comment

by:rawandnet
ID: 26302250
thank you for your information
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