Re: General use of zstd instead of zlib compression
On Mon, Mar 17, 2025 at 09:31:55 +0000, Daniel P. Berrangé wrote:
On Fri, Mar 14, 2025 at 03:34:46PM +0100, Peter Krempa wrote:
> On Fri, Mar 14, 2025 at 14:18:03 -0000, Michael Niehren wrote:
> > one more ...
> >
> > what would happen, if i create the qcow2 image with zstd compression enabled
and
> > use the '--reuse-external' flag ?
> > Would qemu write compressed data in it or not ?
>
> It would not be compressed, as I've explained earlier (On technical
> lists it's preferred if context from what you're replying to is not
> trimmed so that it can be reffred back to) actual compression of blocks
> needs to be requested explicitly. The qcow2 header setting merely states
> which algorithm should be used if blocks are compressed.
Also, AFAIK, qemu will not write compressed data during "normal" usage
by a running VM. The compression only gets applied during full image
So there is the 'compress' filter in qemu although based on the only bit
of "documentation" it has (commit message) QCOW2 supports compressed
writes only for previously unallocated clusters so in fact it is used
only for backups IIUC.