Re: BAD disk I/O performance

Zoltan Frombach <zoltan@xxxxxxxxxxxx> · Sun, 04 May 2014 14:38:01 +0200



    I also experienced really bad disk I/O performance with qcow2 images
    (under CentOS 6.4 hosts.)

    When I converted the disk image to a raw logical volume (created
    with lvm2) I get almost bare-metal disk I/O performance.

    
    Also note mentioning: check if your disk partitions are properly
    aligned and begin at 4k block boundaries. I use parted for this. For
    more info see
http://rainbow.chard.org/2013/01/30/how-to-align-partitions-for-best-performance-using-parted/
    or google it.

    
    There are more performance tuning options, e.g. you can set
    vm.swappiness = 0 on the host's Linux kernel. You can also try
    different kernel scheduling options, etc. These gave me only minor
    performace gains. The most important part was getting away from
    qcow2 and using properly aligned disk partitions.

    
    Zoltan

    
    On 5/4/2014 12:58 PM, Luca Gervasi
      wrote:

    
      Hello,
        

        i'm trying to convert my physical web servers to a virtual
          guest. What i'm experiencing is a poor disk i/o, compared to
          the physical counterpart (having strace telling me that each
          write takes approximately 100 times the time needed on
          physical).
        

        Tested hardware is pretty good (HP Proliant 360p Gen8 with
          2xSAS 15k rpm 48 Gb Ram).
        

        The hypervisor part is a minimal Centos 6.5 with libvirt.
        The guest is configured using: VirtIO as disk bus, qcow2
          storage format (thick allocation), cache mode: none (needed
          for for live migration - this could be changed if is the
          bottleneck), IO mode: default.
        

        Is someone willing to give me some adivices? :)
        

        Thanks
        
          
        Luca
      
      
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