[vbox-dev] writing to VHD/VDI is 50% slower of VMDK?

Huihong Luo huisinro at yahoo.com
Mon Oct 19 21:59:37 GMT 2009

yes, it's on windows, but it may perform similar on Linux too.
As Klaus pointed out in previous emails, it's more likely to do with the bitmap. Block size might be a factor too.
If you examine vhdWrite(), it does the following:
(1) read bitmap (1 sector)
(2) write block
(3) write footer
(4) write the bitmap
this is 4 disk i/o, and especially, (3) - > (4) repositions the disk head back, very expensive

--- On Mon, 10/19/09, Frank Mehnert <Frank.Mehnert at Sun.COM> wrote:

From: Frank Mehnert <Frank.Mehnert at Sun.COM>
Subject: Re: [vbox-dev] writing to VHD/VDI is 50% slower of VMDK?
To: vbox-dev at virtualbox.org
Date: Monday, October 19, 2009, 3:15 AM

On Tuesday 13 October 2009, Huihong Luo wrote:
> This seems to be always the case, writing a large amount of data to vdi/vhd
> is much much slower than writing to a vmdk disk. 
> I am using the the vboxddu.dll code, and clone physical disk to virtual
> disk. 
> for example, clone 250G takes about 2.50 hrs to  a vmdk file,  whereas it
> takes 4 hrs to a vdi file, under exact conditions, quite reproducible.
> There is only writes to disks, no reads. 

So this is a Windows host, correct? I can't imagine the reason for
such a slowdown as there are not many differences between the backends.
One differnece between .vmdk and .vdi is the block size. This needs
further investigation.

Kind regards,

Dr.-Ing. Frank Mehnert    Sun Microsystems, Inc.    www.sun.com

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