Re: qemu-img create with preallocation=full and compression_type=zlib

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On 30 Nov 2021, at 15:27, Daniel P. Berrangé <berrange@xxxxxxxxxx> wrote:
> On Tue, Nov 30, 2021 at 02:51:22PM +0100, Richard Taubo wrote:
>> Hi!
>> When running this command on Fedora 35 I get:
>> [#] qemu-img create -o preallocation=full -f qcow2 /var/vm_mail/mytest.qcow2 10GB
>> => Formatting '/var/vm_mail//mytest.qcow2', fmt=qcow2 . . . preallocation=full compression_type=zlib . . . ."
>> Question:
>> When using 'preallocation=full’ I was surprised that the resulting qcow2 image includes the 'compression_type=zlib’ part.
>> Why/when is this compression used when the qcow2 image is fully preallocated?
> It doesn't mean the image is currently compressed.
> Rather it just records the desired compression algorithm for this image,
> if any future writes were to be told to use compression.

Okay, good to know.

Thanks a lot! :-)

Best regards
Richard Taubo

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