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  1. #1
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    Can KVM Based VPS oversell Memory ?

    thank you.I see someone test the kvm vps,the result is kvm vps can oversell the memory,but I'm not sure,if anyone can tell me the true
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  2. #2
    As of now no since KSM isn't merged into mainline yet but currently I have KSM on a few machines with kvm, cgroups and the works and it does everything quite nicely

  3. #3
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    Yes. KVM can overcommit CPU and RAM. I do not recommend doing it.

    Here is a document from RedHat that explains it:
    http://www.redhat.com/docs/en-US/Red..._with_KVM.html

  4. #4
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    Question. Why would you *want* to oversell memory, surely that is suicide for a VPS provider? I mean, isn't that asking for issues causing a whole node to fall over or just creating a management nightmare for yourself?
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  5. #5
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    I think he was asking from a customer standpoint. There are probably el-cheapo providers out there that oversell everything.

  6. #6
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    Because the cost of KVM ($0) can keep costs low, you really shouldn't be trying to oversell VPS with KVM.
    KVM uses very little overhead and therefore is an economical usage of the resources of the host node server. I think KVM has quite a bit of promise for the hosting and VPS business.
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  7. #7
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    Yes it can, so watch out for providers who claim KVM can't. They obviously don't know the technology well enough.

  8. #8
    KSM is currently being merged into mainline it allows identical shared pages to be mapped to a single page e.g 20 vm's running apache with 20MB in shared pages each without KSM it would be 20 x 20MB worth of ram used, with KSM it simply maps all 20 vm's 20MB worth of shared pages to one that is copy on write.

  9. #9
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    Oversubscribing VPS memory is not really the end of the world, in fact, it's probably the future of the VPS industry.. or rather, the future of the "shared" industry.

    It just doesn't make sense to have a 32GB server with only 32 1GB "chunks" on it, when the average usage on each one is 256MB. It's probably 100% safe to go ahead and put 64 "1GB" chunks on it, and now you can a. make twice as much money or b. charge half as much money... perhaps eventually bringing the benefits of VPS into the price range of shared!

    Not that we do that (we're Xen based), but I wouldn't say it's an INSANE strategy or anything!

  10. #10
    Quote Originally Posted by PCS-Chris View Post
    Yes it can, so watch out for providers who claim KVM can't. They obviously don't know the technology well enough.
    It's not that it can't, it's that it can't do it *reliably*. Go ahead and try to oversubscribe a KVM host and see what happens when a VM's memory starts swapping to disk.

    For the record, if I have 32GB of RAM in a box, I'd provision about 80% of it for VMs, leave the rest for the host (low memory conditions can cause problems). There is no need to oversubscribe, just asking for trouble.
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  11. #11
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    Quote Originally Posted by Toby H View Post
    Question. Why would you *want* to oversell memory, surely that is suicide for a VPS provider? I mean, isn't that asking for issues causing a whole node to fall over or just creating a management nightmare for yourself?
    Ditto, it seems like it would make more sense to not oversell at all because all its going to do is cause you more trouble in the long run.
    Especially for the people who try and oversell too much... there will still be problems if not more.



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  12. #12
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    There's a very nice feature with the memory of Linux: when a file is read, and as long as there's enough RAM for it, it stays cached in the RAM (well, AFAIR, it's at the device block level, but well, what counts here is the feature).

    If you oversell RAM, what happens is that you totally LOOSE the caching feature, and the server goes faster, with more I/O.

    So, YES, you can oversell the RAM. It's not the end of the world ... it's just that you will loose a lot in performances, and that your customers will NOT have what they paid for (so in fact, I highly believe you are cheating them).

    Now, there's all sorts of providers in the industry, with all sorts of policy/behavior. This RAM overselling thing is really showing who's who...

    Now, a small advice for customers. Just right after you purchase your VPS, right after the first boot, do an md5sum on a file that would fit on the available free RAM. Wait a day or 2. Try again. If you see that the VPS is reading again in the HDD, it means that your RAM has been used by another VM, and that you are not getting what you paid for...

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  13. #13
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    Quote Originally Posted by garrycc View Post

    For the record, if I have 32GB of RAM in a box, I'd provision about 80% of it for VMs, leave the rest for the host (low memory conditions can cause problems). There is no need to oversubscribe, just asking for trouble.
    I agree entirley, we always leave free capacity on our VPS nodes regardless of virtualization plaform. Overselling only leads to issues further down the line.

  14. #14
    For the record, if I have 32GB of RAM in a box, I'd provision about 80% of it for VMs, leave the rest for the host (low memory conditions can cause problems). There is no need to oversubscribe, just asking for trouble.
    Quote Originally Posted by PCS-Chris View Post
    I agree entirley, we always leave free capacity on our VPS nodes regardless of virtualization plaform. Overselling only leads to issues further down the line.
    You two are right, but.....

    If you have 32GB, and just use 80%, you are leaveing about 6.4 GB "free"..... Unless you are using the hardware node for something else than VPS selling, there is no point on leaving that much free, just leave about 2GB "free" for the HW node, that's enough (you may even go lower, but that would require some previous analysis). Also, make sure you reduce your kernel's swappiness.

    I personally like KVM, it is good, fast, cheap, and comes with the vanilla kernel. My advices:

    1. Don't oversell. It just doesn't worth it.
    2. Get *fast* disks (raid 1+0, maybe).
    3. Make sure you keep the HW node's OS correctly updated.
    4. If HW node have more that 3GB (which is likely), make sure you use 64 bits OS (that will improve memory access speed, and will let you sell 64 bits guests).

    I hope this helps,

    Ildefonso Camargo

  15. #15
    Quote Originally Posted by MIYU-HITORI View Post
    thank you.I see someone test the kvm vps,the result is kvm vps can oversell the memory,but I'm not sure,if anyone can tell me the true
    It is possible to oversubscribe under any virtualization technology, so its a matter of choosing a reputable service provider regardless of the underlying technology. If you have doubts you can simply ask the company in question, and I'm sure they'll tell you what their allocation policies are. KVM has some technical advantages over other solutions that will become more apparent as time goes on. It will be entertaining to watch the current Xen-based providers turn completely around on their positioning.

  16. #16
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    Quote Originally Posted by Toby H View Post
    Question. Why would you *want* to oversell memory, surely that is suicide for a VPS provider? I mean, isn't that asking for issues causing a whole node to fall over or just creating a management nightmare for yourself?
    Tell that to the thousands of OpenVZ providers which sell 512MB slices for $5 on their boxes...

    Regardless, yes, KVM allows memory overcommitment.

  17. #17
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    Quote Originally Posted by Toby H View Post
    Question. Why would you *want* to oversell memory, surely that is suicide for a VPS provider? I mean, isn't that asking for issues causing a whole node to fall over or just creating a management nightmare for yourself?
    The theory behind successfully overselling memory is the same as the theory behind successfully overselling anything -- only a minority of users will actually use all of the resources allotted to them.

    If managed correctly it can be a successful strategy. If.
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