[pve-devel] [PATCH v2 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>
---
Changes v1 -> v2:
 * Incorporate suggestions made by Thomas

 qm.adoc | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/qm.adoc b/qm.adoc
index 8a49283..93ec29d 100644
--- a/qm.adoc
+++ b/qm.adoc
@@ -1061,6 +1061,28 @@ 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 on Backup
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+By default, guest filesystems are synced via the 'fs-freeze' QEMU Guest Agent
+Command when a backup is performed, to provide consistency.
+
+On Windows guests, some applications might handle consistent backups themselves
+by hooking into the Windows VSS (Volume Shadow Copy Service) layer, a
+'fs-freeze' then might interfere with that. For example, it has been observed
+that calling 'fs-freeze' with some SQL Servers triggers VSS to call the SQL
+Writer VSS module in a mode that breaks the SQL Server backup chain for
+differential backups.
+
+For such setups you can configure {pve} to not issue a freeze-and-thaw cycle on
+backup by setting the `freeze-fs-on-backup` QGA option to `0`. This can also be
+done via the GUI with the 'Freeze/thaw guest filesystems on backup for
+consistency' option.
+
+NOTE: Disabling this option can potentially lead to backups with inconsistent
+filesystems and should therefore only be disabled if you know what you are
+doing.
+
 Troubleshooting
 ^^^^^^^^^^^^^^^

--
2.39.1