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  1. #16
    Join Date
    Aug 2000
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    2,749
    Xen is my choice :0)

  2. #17
    Quote Originally Posted by InsDel View Post
    Okay then - will anybody's opinion change if the OpenVZ has the common burst RAM of twice the 'guaranteed' amount?
    If OpenVZ has xMB "guaranteed" RAM, and burstable to 3*x MB RAM, it means that if the host is not heavily loaded, It is possible for you to use up to 3*x MB RAM in total.
    If Xen has xMB RAM, and 2*MB swap on disk, I will take Xen. Though part of it might be swap, Xen can always use all its specified memory, while you may not always be able to burst to the limit under OpenVZ.
    cherng-wht@cellfone.kwik.to
    Last edited by whzcjint; 12-17-2009 at 09:05 PM.

  3. #18
    Join Date
    Feb 2004
    Location
    Sacramento CA
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    3,200
    I never consider "burst ram" in my decision, never count on it and never expect to have it. That said I've chosen OpenVZ almost as often as Xen.

  4. #19
    Join Date
    Feb 2005
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    Australia
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    5,837
    Quote Originally Posted by whzcjint View Post
    If OpenVZ has xMB "guaranteed" RAM, and burstable to 3*x MB RAM, it means that if the host is not heavily loaded, It is possible for you to use up to 3*x MB RAM in total.
    No, it's not (possible for you to use up to 3*x MB RAM). This is the myth about burst RAM. The reality is:
    • You're constantly being monitored on two (actually more than two, but let's keep it simple) measures: One called "burst", the other called "guaranteed", both misnamed.
    • Memory actually used corresponds roughly to the guaranteed measurement.
    • Memory usage shown in top and free corresponds to the burst measurement.
    • The burst measurement is always higher than the guaranteed, sometimes considerably so, depending on the programs you're running. For the sake of argument, say it's double.
    • So if you ever want to be able to actually use as much memory as your guaranteed limit you need the burst limit to be double - and then you'll hit both limits at the same time.

    UBC is a very weird system, but none of this is a problem as long as the provider knows what they're doing. Unfortunately you can't always rely on this being the case.
    Chris

    "Some problems are so complex that you have to be highly intelligent and well informed just to be undecided about them." - Laurence J. Peter

  5. #20
    Join Date
    Oct 2009
    Posts
    138
    Xen. It offers true virtualization and better security.

  6. #21
    Join Date
    Oct 2009
    Location
    Florida
    Posts
    176
    Quote Originally Posted by foobic View Post
    • Memory actually used corresponds roughly to the guaranteed measurement.
    • Memory usage shown in top and free corresponds to the burst measurement.
    So if I run 'ps aux' on my box, the 'guaranteed' measurement corresponds to RSS, while the 'burst' measurement corresponds to VSZ?

  7. #22
    Join Date
    Feb 2005
    Location
    Australia
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    5,837
    Good question. I've only looked at top / free against /proc/user_beancounters. VZ machines used to show the memory figures for the entire node there, but more recent versions fake the figures from either privvmpages (UBC "burst") or SLM memory. I'll go and do some adding up on RSS / VSZ...
    Chris

    "Some problems are so complex that you have to be highly intelligent and well informed just to be undecided about them." - Laurence J. Peter

  8. #23
    Join Date
    Feb 2005
    Location
    Australia
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    Ok, for interest, on an almost idle OpenVZ box (512MB guaranteed, 1024MB burst):
    oomguarpages held: 8467 = 33MB usage against "guaranteed"
    privvmpages held: 54619 = 213MB usage against "burst"
    free -m shows: total 1024MB used 213MB

    So free -m display simply represents privvmpages held, as expected.

    And from ps aux, adding all processes usage:
    Total VSZ: 376772 = 368MB
    Total RSS: 66832 = 65MB

    No idea what that means, except that RSS and VSZ don't seem to correspond to OpenVZ's measures. Unless I have something wrong, which is entirely possible. Any OpenVZ experts want to comment on this?
    Chris

    "Some problems are so complex that you have to be highly intelligent and well informed just to be undecided about them." - Laurence J. Peter

  9. #24
    I think Xen is best compared to the competitive technologies when its comes to performance.
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  10. #25
    Join Date
    Oct 2009
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    Florida
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    176
    So, after some echo `ps aux | cut -c33-37 | sed "s/ //g" | sed 1d | tr '\n' '+' | sed "s/+$//"` | bc (forgive my horrible attempt at shell commands) on my 128MB guaranteed/256MB burst box while comparing it to my UBC, my RSS is consistently about 3MB above my oomguarpages (57MB/54MB).

    Also @keserhosting, I think the extra overhead involved in truer virtualization would slow down the VM slightly compared to OpenVZ (since it shares kernels/etc.).

  11. #26
    Join Date
    Jan 2006
    Location
    Sydney, Australia
    Posts
    251
    Quote Originally Posted by InsDel View Post
    So if I run 'ps aux' on my box, the 'guaranteed' measurement corresponds to RSS, while the 'burst' measurement corresponds to VSZ?
    Not necessarily. Memory accounting on Linux is actually quite a complex matter. OpenVZ's user bean counters tried to simplify it for each VE, but at the end it is not really doing an optimal job...

    VSZ is the virtual memory size of your process -- that includes the memory that has been allocated, process/thread stack, shared library etc. It's closely related to privvmpages (allocated pages) but the readings from `ps aux` is not that accurate due to shared libraries.

    RSS is the residential memory, i.e. the actual physical memory used by this process. It should be corresponding to the held amount in oomguarpages in a non-overselling/non-swapping scenario. I think if your process gets swapped out on the physical server, RSS will reduce, but oomguarpages will actually stay the same (which is when your physpages is different from oomguarpages).

    And although beancounters are tracking two metrics (privvmpages/guaranteed and oomguarpages/burstable), I think there is really pointless to track the GUARANTEED memory due to the ability to swap on the host node. The "guaranteed" amount there is only to guarantee that your processes will not be killed during an OOM event. It can go above the barrier amount as long as the server does not run out of memory (OOM). HOWEVER, in order to reach an OOM event on a Linux server, you usually have to exhaust all the physical memory AND all the swap -- which by then the whole server will be extremely slow anyway and your processes are about as good as dead...

    Therefore -- just worry about the burstable memory Remember that's the amount your processes "allocated" rather than "used". So for programs doing slab allocation can usually kill the whole system as they allocate a lot more than they use (i.e. Java). Multi-threaded apps also use a lot more memory on OpenVZ as each thread has default 8MB stack on Linux, which again is allocated but rarely used.

    Yeah. Pick Xen or KVM if you can.

  12. #27
    Join Date
    Mar 2009
    Posts
    519
    Everyone is choosing Xen Is there anyone out there who wants to use OpenVZ?
    Learning...

  13. #28
    Join Date
    Dec 2005
    Location
    Berkshire, UK
    Posts
    3,074
    Xen
    System Administrator (RHCE, LFCS, cPPC::Tech)
    PCSmart Group.

  14. #29
    Join Date
    Jul 2008
    Location
    New Zealand
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    1,207
    Xeeeeeeeeen!!!

    Although OpenVZ could be alright for a temp VPS (for testing or something) as it's cheap.

  15. #30
    Join Date
    Jul 2009
    Location
    New Zealand
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    It depends how you intend to you the VPS both system has their own advantages and disadvantages, but i assuming i can only keep one for my own use i'll prefer Xen VPS.
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