[PATCH qemu-server 2/5] snapshot: document how consistency between multiple disks is achieved

[Fiona Ebner <f.ebner@proxmox.com>]

Document how the savevm-start and savevm-end QMP handlers affect the
run-state of the VM and the block layer and how consistency between
multiple disks is ensured.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---
 src/PVE/QemuConfig.pm | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/PVE/QemuConfig.pm b/src/PVE/QemuConfig.pm
index 26f0fda2..cadcd9e9 100644
--- a/src/PVE/QemuConfig.pm
+++ b/src/PVE/QemuConfig.pm
@@ -321,6 +321,13 @@ sub __snapshot_create_vol_snapshots_hook {
     if ($running) {
         my $storecfg = PVE::Storage::config();
 
+        # NOTE savevm-start is called even if no state is saved to ensure consistency between
+        # multiple disks. The savevm-start QMP handler does a vm_stop() which drains and flushes all
+        # block devices via QEMU's block layer. If the state is saved, vm_stop() is called when
+        # saving the state converges (right before the status from query-savevm becomes
+        # 'completed'). The savevm-end QMP handler resumes the VM if it was running (and not
+        # paused/suspended/etc.) before.
+
         if ($hook eq "before") {
             if ($snap->{vmstate}) {
                 my $path = PVE::Storage::path($storecfg, $snap->{vmstate});
-- 
2.47.3