qemu: Configure caching properly

For the scratch device, we can use unsafe caching as it's thrown
away immediately after the machine shuts down. For the root disk,
we can use unsafe caching only if --ephemeral is enabled.

diff --git a/mkosi/qemu.py b/mkosi/qemu.py
index 86b673f5d2d78b30206266cdfa6a4ac713045c18..6702835399cd5d1e917afce2257d995144690212 100644 (file)
--- a/mkosi/qemu.py
+++ b/mkosi/qemu.py
@@ -36,6 +36,7 @@ from mkosi.config import (
     format_bytes,
     systemd_tool_version,
     want_selinux_relabel,
+    yes_no,
 )
 from mkosi.log import ARG_DEBUG, die
 from mkosi.mounts import finalize_source_mounts
@@ -1035,8 +1036,9 @@ def run_qemu(args: Args, config: Config) -> None:
 
         if want_scratch(config):
             scratch = stack.enter_context(generate_scratch_fs(config))
+            cache = "cache.writeback=on,cache.direct=on,cache.no-flush=yes"
             cmdline += [
-                "-drive", f"if=none,id=scratch,file={scratch},format=raw,discard=on",
+                "-drive", f"if=none,id=scratch,file={scratch},format=raw,discard=on,{cache}",
                 "-device", "scsi-hd,drive=scratch",
             ]
             kcl += [f"systemd.mount-extra=LABEL=scratch:/var/tmp:{config.distribution.filesystem()}"]
@@ -1051,7 +1053,8 @@ def run_qemu(args: Args, config: Config) -> None:
             cmdline += ["-initrd", config.output_dir_or_cwd() / config.output_split_initrd]
 
         if config.output_format in (OutputFormat.disk, OutputFormat.esp):
-            cmdline += ["-drive", f"if=none,id=mkosi,file={fname},format=raw,discard=on",
+            cache = f"cache.writeback=on,cache.direct=on,cache.no-flush={yes_no(config.ephemeral)}"
+            cmdline += ["-drive", f"if=none,id=mkosi,file={fname},format=raw,discard=on,{cache}",
                         "-device", f"scsi-{'cd' if config.qemu_cdrom else 'hd'},drive=mkosi,bootindex=1"]
 
         if (