From: Maximiliano Sandoval <m.sandoval@proxmox.com>
To: pve-devel@lists.proxmox.com
Subject: [pve-devel] [RFC PATCH docs] qm: document when fs-freeze is issued for VMs
Date: Thu, 14 Aug 2025 16:45:53 +0200 [thread overview]
Message-ID: <20250814144602.423927-1-m.sandoval@proxmox.com> (raw)
Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
---
I might be missing some condition so I am sending this to gather feedback. I
will grep the code with more gusto on monday to see if I am missing anything.
For the moment this is only about VMs and we are missing information for
containers.
qm.adoc | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
diff --git a/qm.adoc b/qm.adoc
index 55633fb..a97d5cc 100644
--- a/qm.adoc
+++ b/qm.adoc
@@ -1502,11 +1502,20 @@ run as expected. Subsequent ones, until the next reboot, will only consider
parts of the filesystem that changed since then.
[[qm_qga_fsfreeze]]
-Filesystem Freeze & Thaw on Backup
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Filesystem Freeze & Thaw
+^^^^^^^^^^^^^^^^^^^^^^^^
-By default, guest filesystems are synced via the 'fs-freeze' QEMU Guest Agent
-Command when a backup is performed, to provide consistency.
+By default, if the QEMU Guest Agent is enabled in the guest's config and if the
+agent is available inside of the guest, then the virtual machine's filesystems
+are synced via the 'fs-freeze' QEMU Guest Agent Command when certain operations
+are performed. This is done to provide data consistency.
+
+A 'fs-freeze' will be issued for any of the following operations on a VM:
+
+* Performing a backups in snapshot mode
+* Performing a clone of a VM while it is running
+* Replicating a VM while it is running
+* Taking a snapshots without RAM of a running VM
On Windows guests, some applications might handle consistent backups themselves
by hooking into the Windows VSS (Volume Shadow Copy Service) layer, a
--
2.47.2
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