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

[Huihong Luo]

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
 
Huihong

--- 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,

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

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