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Re: [Qemu-discuss] qemu-img convert

From: Jakob Bohm
Subject: Re: [Qemu-discuss] qemu-img convert
Date: Tue, 01 Jul 2014 18:23:32 +0200
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On 7/1/2014 9:20 AM, Upendra Sahu wrote:

When I am using to convert VMDK to VHD (format vpc),

1.Why its creating new file with new format, can I change original file
itself to new format, if its creating new file it means its copying all
the block of disk image and also changing the metadata, headers?

Because if anything goes wrong, it would be a disaster to be left with
a file that is not valid in any format whatsoever. Also converting between arbitrary file formats in place is non-trivial because one format may want two pieces of data in the opposite order of what the
other format wants.

2.When I converting 8GB vmdk to vhd (format vpc) I see converted image
size as around 4GB, how its changing the size.

Many of the formats handled by qemu-img include special efficient
methods of storing sectors that contain nothing but zeroes.  This is
what VMWare refers to as "thin" disks.

3.In conversion process, does qemu-img removes free sectors and
defragment the virtual disk? Can I disable that and just create new
metadata for output format image to reduce the conversion time?

No, it doesn't touch the file system inside the virtual disk, but it
does copy all the sectors and regenerate the tables that say which
sector is stored where in the file.

There are other speed problems inside the qemu tools, which are most
annoying to those of us who use them every day, but that is a different

Thanks & Regards,



Jakob Bohm, CIO, Partner, WiseMo A/S.  http://www.wisemo.com
Transformervej 29, 2730 Herlev, Denmark.  Direct +45 31 13 16 10
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