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Thread: Xen query

  1. #1
    Join Date
    Jan 2006
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    663

    Xen query

    Hi

    Is it normal for a xen based cpanel vps with no domains hosted to keep spiking on %wa and load keeps creaping up



    top - 11:22:16 up 1 day, 18:08, 1 user, load average: 0.16, 0.18, 0.10
    Tasks: 96 total, 2 running, 93 sleeping, 0 stopped, 1 zombie
    Cpu(s): 0.0%us, 0.0%sy, 0.0%ni, 92.9%id, 8.9%wa, 0.0%hi, 0.0%si, 0.2%st
    Mem: 1048704k total, 1005760k used, 42944k free, 177116k buffers
    Swap: 1048568k total, 0k used, 1048568k free, 581744k cached
    PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
    1 root 15 0 10352 704 592 S 0.0 0.1 0:00.05 init
    2 root RT 0 0 0 0 S 0.0 0.0 0:00.06 migration/0
    3 root 34 19 0 0 0 S 0.0 0.0 0:00.01 ksoftirqd/0
    Cheers

    Keith
    VPS & Dedi with -
    UKServers (8/10) Clouvider (7/10) 12 month, Rapidswitch (1/10) 15Years+, Hivelocity (10/10) 7 Years, Clustered net (10+/10) 17Years+

  2. #2
    Join Date
    Dec 2005
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    For an empty VPS no thats not normal. That shows the processes in the background running are causing some iowait before you have even got sites on there.

    The host machine is probably under-specification and/or under a lot of load.

    Our empty Xen VPS stay at 0.00 as an empty server should, occasionally going to 0.01 / 0.02 if something runs in the background

  3. #3
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    Higest io wait hit 29%, cpu was hovering 0.22 ~, with no sites or traffic
    Just cancelled, cant afford to move 50+ sites to find out it folds over under one website

    Loosing faith in vps's now :-(
    VPS & Dedi with -
    UKServers (8/10) Clouvider (7/10) 12 month, Rapidswitch (1/10) 15Years+, Hivelocity (10/10) 7 Years, Clustered net (10+/10) 17Years+

  4. #4
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    Did you have spamassassin enabled?

  5. #5
    Hi,

    That has nothing to do with VPS may be try some other Hosting companies.

    Anderson
    Astraxis - Offshore IT Outsourcing Services.
    Server Management | Web Design and Development
    Astraxis.com

  6. #6
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    Quote Originally Posted by keith007 View Post
    Higest io wait hit 29%, cpu was hovering 0.22 ~, with no sites or traffic
    Just cancelled, cant afford to move 50+ sites to find out it folds over under one website

    Loosing faith in vps's now :-(
    Understandable, but you must know (based on your sig ) that all VPSs are not created equal. Was this a cheap VPS where you might expect the provider to be economising on hardware, or a more expensive one?

    I've had a similar experience with a small Xen VPS (not a particularly cheap one) and as a result I tend to prefer VZ. I wonder if disk i/o is harder to manage under Xen because your neighbours can thrash the swap if they overload their VPSs. Anyone...?
    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

  7. #7
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    Quote Originally Posted by foobic View Post
    Understandable, but you must know (based on your sig ) that all VPSs are not created equal. Was this a cheap VPS where you might expect the provider to be economising on hardware, or a more expensive one?

    I've had a similar experience with a small Xen VPS (not a particularly cheap one) and as a result I tend to prefer VZ. I wonder if disk i/o is harder to manage under Xen because your neighbours can thrash the swap if they overload their VPSs. Anyone...?
    You are right foobic, if the others trash swap it'll degrade the performance on your end.


    That's why I, like you, prefer VZ.
    Jacob Wall

  8. #8
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    the Xen server was from a reputable company in the UK, not cheap either
    unfortunately this didnt work out and now moved to another host
    I cancelled for numerous reasons including the high loads on a unloaded server
    VPS & Dedi with -
    UKServers (8/10) Clouvider (7/10) 12 month, Rapidswitch (1/10) 15Years+, Hivelocity (10/10) 7 Years, Clustered net (10+/10) 17Years+

  9. #9
    People give Xen a better reputation because it "can't be oversold" but there are many things to a VPS server. If you load up a server with 1 1TB hard drive and a dual core cpu running 8GB of RAM, then your going to have issues on both Xen and VZ.

  10. #10
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    I wonder if disk i/o is harder to manage under Xen because your neighbours can thrash the swap if they overload their VPSs. Anyone...?
    Having used both Xen and OpenVZ I can shed a bit of light on this.

    I/O is more difficult to manage in a Xen environment, but at the end of the day all you need to do is be proactive and keep an eye on it. Ever since rolling out 4-8 disk RAID-10 setups accross our nodes, we have not seen any I/O related issues as a result of VPS swapping out for over a year.

    On some of our earlier systems with RAID-1, a single VPS heavily swapping out would start to cause issues. Not immediatley I/O issues inside other VPS, but you start noticing things like packet loss appearing on a node.

    With what I would call hardware in "proportion" and careful monitoring, it really isn't an issue.

    On the other hand, while I do love the amount of control you get as an admin of OpenVZ based systems, the lack of full isolation does make a difference. If a single VPS loads up or goes crazy on CPU or network you have sluggishness/issues on all VPS on that machine.

    The only thing that really ever causes issues on a Xen node is i/o issues as you say, as a result of swapping out. That said ever since rolling out our nodes with higher-end storage arrays, I've not had a single issue.

    Personally I like both technologies.

    OpenVZ is great for light applications, LAMP setups, Trixbox PBX etc. Real-world performance in general use on a good quality and well managed machine IS better than Xen Paravirtualization.

    However, Xen is great for more hardcore setups, requiring more CPU, heavier access to disk as it's just that more robust, and you dont have to worry about hitting any of the UBC limits, as you literally just have the hardware limits as you would in a physical box.

    As a consumer, I will always favour Xen.

    We have some customers absolutly thrashing their Xen VPS with 100tick Gameservers and the like, while other users on the node wouldn't even notice. With OpenVZ that would be a different story.

    Both have their + and - points.

    OpenVZ can be oversold/overcommit, Xen can be oversold/overcomitt and KVM can also be oversold/overcomitt. It might not necesarrily be easy or practical to do, but regardless of what many providers say, they are simply lying to you if they say otherwise. Thats my personal view on it anyway.

    P.S @OP. Since we are a UK VPS provider I'm assuming thats not one of ours you are having issues with. If so please PM me and I will get straight onto it. Monitoring is looking good at the moment through accross our cluster.
    Last edited by uksysadmin; 11-12-2009 at 05:36 PM.

  11. #11
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    You might want to contact your VPS host and have them look at the physical host your VPS is on. Someone(s) may be threshing the disk.

    If it's open source Xen, they need to look at the rate of VBD_OO, VBD_RD, & VBD_WR off xentop. Sort out the growth rate of any offending VPS. iostat off dom0 would give some ideas as well.

    I've seen some weird dom0 system configs in past causing %st (off top) to fluctuate as well. In term it causes all domU/VPS to get this sporadic "pauses".


    Since you stated this is a decent reputable VPS host, have you tried to get them to migrate your VPS to a different server?
    SysAdmin.xyz
    Having severs with customer data on it without proper monitoring is like having one night stand without using protections - eventually, there will be an 'oh s**t!' moment.

  12. #12
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    I agree with the above sentiments and I am definitely in the Xen camp. A properly managed Xen server should have no problems with I/O load. Since Xen memory caps are hard most hosts certainly will want to give users some swap space to allow a little bit of wiggle room. However multiple nodes on a single host consistently sitting in swap will at some point just begin swapping pages in and out and hammer I/O. Well managed VPS hosts won't let this become an issue and will minimize the effect that this can have by using RAID 10 arrays, imposing maximum swap size restrictions, etc.

  13. #13
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    I'm not sure about this, but couldn't you assign swap space off a separate SAN/RAID array/etc, so that way swapping would only affect others' swap, and not normal disk?

  14. #14
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    Just a comment as a client who has used four different OpenVz providers and another five Xen providers.

    - All four OpenVZ servers exhibited I/O wait issues at times. For example (on a nearly empty and idle server) at 2:00 PM I/O wait climbs to 25%, load to over 1, and the VPS is nearly paralyzed for 15 minutes. A fairly common occurrence.

    - The five Xen servers I've used have never exhibited this behaviour. Ever.

    Maybe it's a case of five good Xen providers and four crappy OpenVz ones. Heck of a coincidence though

  15. #15
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    Quote Originally Posted by quantumphysics View Post
    I'm not sure about this, but couldn't you assign swap space off a separate SAN/RAID array/etc, so that way swapping would only affect others' swap, and not normal disk?
    This isn't a bad idea but it would take a lot more work and extra hardware. To separate i/o you would either need a separate network card for the SAN or a separate RAID card for the raid array swap. Otherwise the swap usage would still affect other users.


  16. #16
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    Well, you can only control the swap partition you provide to the VPS's. Any swap files created will reside on the main VPS data drives and generally it is the VPS's that create extra swapfiles that are doing a large amount of swapping.

    As far as the person that said to limit the max swap size, so long as users have free disk space they can create swapfiles so there is no way to limit swap space usage.
    Matt Ayres - togglebox.com
    Linux and Windows Cloud Virtual Datacenters powered by Onapp / Xen
    Instant Setup, Instant Scalability, Full Lifecycle Hosting Solutions

    www.togglebox.com

  17. #17
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    Quote Originally Posted by sleddog View Post
    Maybe it's a case of five good Xen providers and four crappy OpenVz ones. Heck of a coincidence though
    Just out of curiosity: how did the prices compare between the good and the crappy?

    My own experiences for comparison: 6 Virtuozzo VPSs from 3 or 4 providers (none particularly cheap by current standards), 2 Xen from 2 providers (one cheap, one relatively expensive) and one cheap OpenVZ. The only one I've not been happy with was the more expensive Xen, which seemed generally underpowered and frequently suffered these i/o spikes. Different people, different experiences - such is life.
    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

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