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Linux virtual machine / host machine filesystem performance

Posted on 2009-07-02
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I've seen many benchmarks on filesystem performance, but what I'm wondering is how well a particular filesystem performs in a virtual machine when sitting on top of a different filesystem.  In particular I have several VMs running in QEMU/KVM formatted to ext4 running on top of a 5 disk RAID 5 formatted to xfs.  Performance is questionable but may be due to other factors such as memory allocation.  I'd like to know if this is even an issue, or if keeping host/guest filesystems the same has any performance benefit.
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Question by:GoTravel
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garycase earned 250 total points
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Keeping the filesystems the same doesn't matter.   What DOES make a difference is the virtualization of the disk ==> it takes resources to manage that virtualization, which will slow down the overall performance relative to what you would see with a native disk.

If your virtual host allows assigning a physical disk (or partition) to the VM, that would give you notably better performance.    Alternatively, you'll also get better performance if you set the hypervisor to permanently assign the entire allocated disk space rather than dynamically managing it.
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by:rindi
rindi earned 250 total points
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Another thing that will make a difference is your raid array. If you host many VM's on the same array there will be a lot of input / output going on in that array, particularly if all VM's are being used simultaneously. Therefore you should choose a raid form that is particularly fast. Raid 10 is very fast for both reads and writes, so that would be the form of raid I'd go for. performance will also get better the more disks are used for an array (this also applies for raid 5 read access).
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by:GoTravel
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Does mapping to a partition hurt the VMs portability?  I'd like to be able to virt-copy them to other machines or to new instances with little headache.
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