[pve-devel] [PATCH docs 3/3] qm: Add section explaining fs-freeze/thaw QGA option

Christoph Heiss c.heiss at proxmox.com
Wed Feb 1 13:59:18 CET 2023

Signed-off-by: Christoph Heiss <c.heiss at proxmox.com>
 qm.adoc | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/qm.adoc b/qm.adoc
index 8a49283..f029ceb 100644
--- a/qm.adoc
+++ b/qm.adoc
@@ -1061,6 +1061,26 @@ operations that have the potential to write out zeros to the storage:
 On a thin provisioned storage, this can help to free up unused space.
+Filesystem freeze/thaw
+By default, VM filesystems are synced via the 'fs-freeze' command when a backup
+is performed, to provide consistency. On Windows guests, this calls into the VSS
+(volume shadow copy) subsystem.
+There might be cases where this needs to be disabled, e.g. when running
+applications which hook into the Windows VSS subsystem themselves. This has
+been observed to be necessary when running SQL Server, which otherwise might
+break its own differential backup mode.
+This behavior can be turned off using the 'Freeze/thaw filesystems on backup
+for consistency' option. With this option disabled, {pve} will skip the
+'fs-freeze' and 'fs-thaw' commands when running backup jobs.
+NOTE: Disabling this option can potentially lead to backups with inconsistent
+filesystems and should therefore only be changed if you know what you are

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