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  1. #1
    Join Date
    Sep 2011
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    USA
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    Best way to use SSD in KVM host?

    Hello,

    I'm new to hosting, but have been a Unix/Linux admin since '98. This is a great forum.

    I couldn't find this in search - does anyone know what the best use of an SSD drive in a KVM host would be? The spinning disks are RAID1 SATA. The server is Quad w/ HT and 8 GB ram, most of my guests run LAMP with Wordpres/Drupal type sites and a few other random things. I haven't used KVM much so I haven't seen where the biggest server bottleneck will be, and I would prefer not to disrupt any services once it's being used.

    I was thinking either to give the guests an OS disk on the SSD or using it for swap. The Redhat document seemed to suggest using it for swap, but I thought I wanted to avoid any swapping of the host.

    Can anyone share experience with this? Thanks.

  2. #2
    Join Date
    Jul 2010
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    ~/
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    1,261
    First of all... Welcome

    For what its worth:

    It very much depends on the way the clients use there servers, sure avoiding swapping all together would be a dream but in reality almost impossible to avoid which is why the recommendations are to use it for swap.

    If you mean for the HOST OS Swap then, sure you could use it for that but the cost would outweigh the benefit of just giving the Host OS enough RAM.

    Another option would be to allocate a small chunk of the SSD to each guest OS that they can use for databases as they are once of the IO hogs and the IO is *almost* always the biggest bottleneck and in my experience customer disk IO bottlenecks are caused by them buying the package with the least amount of ram they think they can get away with which inevitably caused swapping and then IO load so your back to the first suggestion of using it for swap.
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  3. #3
    Join Date
    Jul 2007
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    Ashburn, VA
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    Quote Originally Posted by backtogeek View Post
    Another option would be to allocate a small chunk of the SSD to each guest OS that they can use for databases
    I'd do this.

    Just to make sure: you do have the SSD drive in a RAID1 array, yes?
    Preetam Jinka

    Isomerous - High performance web services for business and individuals.
    Bitcable Colocation, KVMs, cPanel hosting, Oracle expertise, and more.

  4. #4
    Join Date
    Mar 2010
    Location
    Germany
    Posts
    656
    Quote Originally Posted by FractalGrid View Post
    I was thinking either to give the guests an OS disk on the SSD or using it for swap. The Redhat document seemed to suggest using it for swap, but I thought I wanted to avoid any swapping of the host.
    The RedHat virtualization guide has a few bright areas, and many that are ... not so ... bright? It could be called the "dev with a laptop" problem. They write about what they know and can try. The problem is they don't do anything else than spinning up a few virtual OS for testing software. This is utterly irrelevant when you're supposed to actually run servers.

    Ideas:
    - You can slice the SSD into many swap spaces for the virtual machines
    - if you use ZFS you could use it as mixed ZIL + L2ARC device. L2ARC is simply cache, ZIL speeds up the FS log. Advantage here is that ZFS has really smart code that will automagically detach an SSD that has too many errors coming up.

    - if you wanna go an uncommon route, use it as a "FlashCache"* device. Flash Cache has a mode called "write-around-read" which is perfect as a poor mans tiered storage. Writes go directly to disk avoiding wear on the SSD, but commonly read stuff is cached to the SSD.

    From your introduction I guess you can handle building a kernel module )


    (This is a project by FaceBook which they are using in production for quite some time. So less bleeding edge than ZFS on Linux in a way, although one could argue whats more robust: Linux port of modern unix-grade filesystem or a facebook patch to linux)
    Last edited by wartungsfenster; 11-22-2011 at 09:05 AM. Reason: one billion typos and missing words to be added.
    Check out my SSD guides for Samsung, HGST (Hitachi Global Storage) and Intel!

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