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On 03/06/2013 02:56 PM, Samuel Winchenbach wrote:
> On Tue, Mar 5, 2013 at 12:11 PM, Pádraig Brady <P at draigbrady.com <mailto:P at draigbrady.com>> wrote:
>     Just did some testing here, writing in a VM backed by a local file system, using:
>       dd if=/dev/zero of=file bs=1M count=1k conv=notrunc,fdatasync oflag=append
> 
>     I didn't see a degredation after a while, but did see
>     quite different performance depending on the formats used:
> 
>     disk performance outside VM = 120MB/s
>     raw in $instance_dir/ = 105MB/s
>     qcow copy with preallocation=metadata in $instance_dir/ = 100MB/s
>     qcow CoW with fallocate full size in $instance_dir/ = 55MB/s
>       Note perf a bit more stable than without fallocate
>       I didn't test with full host disk where improvements would be more noticeable
>     qcow CoW in $instance_dir/ = 52MB/s
>     qcow CoW in $instance_dir/ backed by qcow with preallocation=metadata in base = 52MB/s
> 
> Wow thanks for testing that.   I don't think I can preallocate on an NFS mount.  I keep getting an "Operation not supported" error.

Right, fallocate isn't supported on NFS currently and won't make much perf difference anyway as noted above.
Using raw is probably your best bet currently.

thanks,
Pádraig.