On Tue, Aug 29, 2023 at 05:49:24PM -0000, Daniel Alley wrote: > > The background to this is I've spent far too long trying to optimize > > the conversion of qcow2 files to raw files. Most existing qcow2 files > > that you can find online are zlib compressed, including the qcow2 > > images provided by Fedora. Each cluster in the file is separately > > compressed as a zlib stream, and qemu uses zlib library functions to > > decompress them. When downloading and decompressing these files, I > > measured 40%+ of the total CPU time is doing zlib decompression. > > > > [You don't need to tell me how great Zstd is, qcow2 supports this for > > compression also, but it is not widely used by existing content.] > > Independent from the decision to use zlib-ng as a distro-wide zlib > replacement, which is a good idea regardless. > > Are there reasons why Fedora's qcow2 images cannot switch to Zstd > compression? Zstd support appears to have been added by QEMU 5.1 in > August 2020, and both EL8 and EL9 appear to have newer versions QEMU > available (therefore, they ought to be able to support those > images). TBH I think the most probable reason is that people don't know about it and it is not obvious that you have to enable it. To generate a zlib-compressed qcow2 file, you simply add the -c option, easy. To use zstd compression you have to use this mouthful: qemu-img convert -f raw disk.img -O qcow2 disk.qcow2 -c -o compression_type=zstd The qemu-img man page doesn't even mention it. I think all recent qcow2-based tools should be fine with zstd, but I didn't check them all (RHEL 7 is still quite popular so that platform would no longer be compatible). > Sure, it doesn't help for existing content, but it would help for > future content. And I'm pretty sure zstd remains faster than > zlib-ng despite the speed improvements in the latter. Yup. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-builder quickly builds VMs from scratch http://libguestfs.org/virt-builder.1.html _______________________________________________ devel mailing list -- devel@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to devel-leave@xxxxxxxxxxxxxxxxxxxxxxx Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/devel@xxxxxxxxxxxxxxxxxxxxxxx Do not reply to spam, report it: https://pagure.io/fedora-infrastructure/new_issue
