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  1. #1

    Trying to understand cloud


    If a server has 8 Gigs RAM, and a customer buys a VM with 4, it'll still cost the same amount to power it up, right?
    So how is this cost effective?

    Or is there a way that say 6 Gigs could be assigned with 4 from this machine, and 2 from ANOTHER???!!! Is this how it's done? HOW, please? (I find this incredible).


    Thanks.

  2. #2
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    This isn't necessarily a cloud thing - the term cloud applies to a range of applications.

    A server can run multiple VMs. You won't be able to run 2 x 4 gig VMs on an 8 gig machine because the VM itself will consume some memory. The VM will request the memory from the OS when it starts and then uses that for all allocations provided to the software it hosts. Essentially the virtual machine acts as though it is a computer and provides registers, ram and buses to the software.

    There is software called parallel virtualization that allows software running on multiple machines to synchronize as though it is run on one but I don' think any VMs are hosted that way.

  3. #3
    Linux Engineer docbop's Avatar
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    To put it simply... The VM(s) get their resources (RAM, CPU, disk, etc) from the host machine, they don't get resources from other machines.

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  5. #4
    To be clear, I was talking about a VM running on a hypervisor, not an OS. Is the "4 gigs from here, 2 from there" valid in that case?

  6. #5
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    No. A hypervisor doesn't change things - it just manages multiple VMs. It is conceivable that memory could be managed across a network, and that is what parallel virtualization does, but I don't imagine a virtual machine would be fast enough with network latency as a limiting factor. It is competing with real machines after all.

  7. #6
    Linux Engineer docbop's Avatar
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    Quote Originally Posted by gregm View Post
    No. A hypervisor doesn't change things - it just manages multiple VMs. It is conceivable that memory could be managed across a network, and that is what parallel virtualization does, but I don't imagine a virtual machine would be fast enough with network latency as a limiting factor. It is competing with real machines after all.
    The latency would be a killer, that's why even computer clusters the cluster nodes use resources of its local machine and return results.

    Remote memory always brings in latency. I remember the SGI modular computer with memory bricks, CPU bricks, etc bricks, there was latency and everything was local the the backpane. Any time a computer has to do a context switch there is latency.

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