feat: use copy-on-write qcow2 images for VM image

Previously, we made a copy. This is faster.

Signed-off-by: Lars Wirzenius <liw@liw.fi>
Sponsored-by: author

1 files changed, 29 insertions, 2 deletions

diff --git a/src/qemu.rs b/src/qemu.rs
index 22c0bb9..4f5afff 100644
--- a/src/qemu.rs
+++ b/src/qemu.rs
@@ -115,8 +115,8 @@ impl Qemu {
         let vars = tmp.path().join("vars.fd");
         let console_log = tmp.path().join("console.log");
 
-        debug!("copy image and vars");
-        copy(&self.image, &image).map_err(|e| QemuError::Copy(self.image.clone(), e))?;
+        debug!("create copy-on-write image and UEFI vars file");
+        create_cow_image(&self.image, &image)?;
         copy(OVMF_FD, &vars).map_err(|e| QemuError::Copy(OVMF_FD.into(), e))?;
 
         debug!("create run-ci drive from {}", self.run_ci.display());
@@ -288,6 +288,30 @@ impl Qemu {
     }
 }
 
+fn create_cow_image(backing_file: &Path, new_file: &Path) -> Result<(), QemuError> {
+    let output = Command::new("qemu-img")
+        .arg("create")
+        .arg("-b")
+        .arg(backing_file)
+        .args(["-F", "qcow2"])
+        .args(["-f", "qcow2"])
+        .arg(new_file)
+        .output()
+        .map_err(QemuError::Run)?;
+    if !output.status.success() {
+        let exit = output.status.code().unwrap_or(255);
+        let out = String::from_utf8_lossy(&output.stdout).to_string();
+        let err = String::from_utf8_lossy(&output.stderr).to_string();
+        error!(
+            "qemu-img failed: exit={}\nstdout: {:?}\nstderr: {:?}",
+            exit, out, err,
+        );
+        Err(QemuError::QemuImg(exit, err))
+    } else {
+        Ok(())
+    }
+}
+
 #[derive(Debug, Default)]
 struct QemuArgs {
     args: Vec<String>,
@@ -348,6 +372,9 @@ pub enum QemuError {
     #[error("QEMU failed")]
     Run(#[source] std::io::Error),
 
+    #[error("qemu-img failed: exit code {0}: stderr:\n{1}")]
+    QemuImg(i32, String),
+
     #[error("failed to run QEMU (kvm): exit code {0}, stderr:\n{1}\nconsole: {2}")]
     Kvm(i32, String, String),