[Gluster-users] IO performance cut down when VM on Gluster

[Joe Julian]

On 01/13/2013 04:14 AM, glusterzhxue wrote:
> Hi all,
> We placed Virtual Machine Imame(based on kvm) on gluster file system, 
> but IO performance of the VM is only half of the bandwidth.
> If we mount it on a physical machine using the same volume as the 
> above VM, physical host reaches full bandwidth. We performed it many 
> times, but each had the same result.
What you're seeing is the difference between bandwidth and latency. When 
you're writing a big file to a VM filesystem, you're not performing the 
same operations as writing a file to a GlusterFS mount thus you're able 
to measure bandwidth. The filesystem within the VM is doing things like 
journaling, inode operations, etc. that you don't have to do when 
writing to the client requiring a lot more I/O operations per second, 
thus amplifying the latency present in both your network and the context 
switching through FUSE.

You have two options:
1. Mount the GlusterFS volume from within the VM and host the data 
you're operating on there. This avoids all the additional overhead of 
managing a filesystem on top of FUSE.
2. Try the 3.4 qa release and native GlusterFS support in the latest 
qemu-kvm.

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