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RE: Question on Compression for Raw Image

From: Wang, Wei W
Subject: RE: Question on Compression for Raw Image
Date: Tue, 20 Oct 2020 14:22:43 +0000

On Tuesday, October 20, 2020 4:01 PM, Kevin Wolf wrote:
> Am 20.10.2020 um 03:31 hat Wang, Wei W geschrieben:
> > Hi,
> >
> > Does anyone know the reason why raw-format.c doesn't have
> compression
> > support (but qcow has the supported added)?  For example, raw image
> > backup with compression, "qemu-img convert -c -O raw origin.img
> > dist.img", doesn't work.
> A raw image is by definition a file that contains the exact same sequence of
> bytes as the guest sees, without any additional information or encoding. If
> you compress a raw file, the guest will see compressed data on its hard disk
> instead of the real data.

Ok, thanks. I'm thinking QEMU could do decompression of the compressed data in 
raw.img when guest reads data.

> Anything you could do to add transparent compression to it would mean that
> it's not a raw image any more, but a new image format.
Yes, decompression makes it transparent to the guest. Would you think it's good 
to reuse the raw image implementation, just add the compress/decompress option?


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