How to change zstd level for qcow2 image?

Issue

There is an option compression_type=zstd to enable zstd compression for qcow2 according to wiki.

But it always uses zstd level 3, how could I do to compress it with level 19? This image is read-only and I just want the max compression level.

The command I use now is:

qemu-img convert -p -f qcow2 -O qcow2 -c -o compression_type=zstd,preallocation=off win10.qcow2 win10-zstd.qcow2

Other info:

Linux fedora 5.17.5-300.fc36.x86_64
qemu-img.x86_64 2:6.2.0-9.fc36  

Solution

There are not a command line option, so we need to modify the source code. 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. 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. Try to convert a file:

./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

Also posted on my blog.

Answered By - kkocdko