Change ZSTD Level in QCOW2

Currently it's QEMU 8.2.0

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:

  224 |  */
+     | ZSTD_CCtx_setParameter(cctx, ZSTD_c_strategy, ZSTD_btultra2);
+     | ZSTD_CCtx_setParameter(cctx, ZSTD_c_compressionLevel, ZSTD_maxCLevel());
+     | ZSTD_CCtx_setParameter(cctx, ZSTD_c_enableLongDistanceMatching, 1);
  225 | zstd_ret = ZSTD_compressStream2(cctx, &output, &input, ZSTD_e_end);

Compiling by yourself or use my prebuilt binary (built on GitHub Actions).

Now we found qemu-img at ./build. 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 parameters from docs.