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Re: [libvirt] [PATCH 2/2] Adjust block size used by dd to speed QEMU domain save operations.



On Thu, Jun 03, 2010 at 11:57:33PM -0400, Laine Stump wrote:
> See https://bugzilla.redhat.com/show_bug.cgi?id=599091
> 
> Saving a paused 512MB domain took 3m47s with the old block size of 512
> bytes. Changing the block size to 1024*1024 decreased the time to 56
> seconds. (Doubling again to 2048*1024 yielded 0 improvement; lowering
> to 512k increased the save time to 1m10s, about 20%)

Surely we should have gone for 1024*1024  in this case ?

NB, since our XML header gets rounded up to a multiple of the block
size, smaller is better, because we're filling the disk with zeros
here :-)

> ---
>  src/qemu/qemu_monitor.h |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/src/qemu/qemu_monitor.h b/src/qemu/qemu_monitor.h
> index 1870b22..fd39b1c 100644
> --- a/src/qemu/qemu_monitor.h
> +++ b/src/qemu/qemu_monitor.h
> @@ -261,7 +261,7 @@ int qemuMonitorMigrateToCommand(qemuMonitorPtr mon,
>                                  unsigned int background,
>                                  const char * const *argv);
>  
> -# define QEMU_MONITOR_MIGRATE_TO_FILE_BS 512llu
> +# define QEMU_MONITOR_MIGRATE_TO_FILE_BS (1024llu * 2048)
>  
>  int qemuMonitorMigrateToFile(qemuMonitorPtr mon,
>                               unsigned int background,
> -- 


Daniel
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