Change ZSTD Level in QCOW2

Currently it's QEMU 7.0.0

The zstd level of qcow2 created by qemu is always 3. For a read-only disk image, I just want the best compression ratio.

There are not a command line option, so we need to modify the source code(1). On ./block/qcow2-threads.c:

  206 |     return -EIO;
  207 | }
+ 208 | ZSTD_CCtx_setParameter(cctx, ZSTD_c_strategy, ZSTD_btultra2);
  209 | /*

Compiling it following offical tutorial (or prebuild binary). Here some tips:

# (Fedora) Install dependencies
dnf install bzip2 ninja-build libzstd-devel pixman-devel
# Enable ZSTD, disable useless features for shorter compile time
./configure --enable-zstd --disable-tcg --disable-kvm

Now we found qemu-img at ./build/softemu. Have a try?

./qemu-img convert -p -f qcow2 -O qcow2 -c -o compression_type=zstd win10.qcow2 win10-2.qcow2

We got it:

[kkocdko@fedora tmpfs]$ ls -l
total 3411264
-rw-r--r--. 1 kkocdko kkocdko 1651638272 Jun 12 01:57  win10-2.qcow2
-rw-r--r--. 1 kkocdko kkocdko 1841496064 May 25 18:21  win10.qcow2

(1) You can find more zstd strategy from docs.