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

[Christoph Heiss <c.heiss@proxmox.com>]

Signed-off-by: Christoph Heiss <c.heiss@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
+doing.
+
 Troubleshooting
 ^^^^^^^^^^^^^^^
 
-- 
2.39.1