From 62bf5d754e4edf3a5a5a2827cc9119c21abc23c9 Mon Sep 17 00:00:00 2001 From: Christoph Heiss Date: Thu, 23 Feb 2023 15:18:05 +0100 Subject: [PATCH] qm: Add section explaining fs-freeze/-thaw QGA option Signed-off-by: Christoph Heiss [ T: resolve trivial merge conflict due to context additions ] Signed-off-by: Thomas Lamprecht --- qm.adoc | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/qm.adoc b/qm.adoc index 3ba2a3c..1a5ad3b 100644 --- a/qm.adoc +++ b/qm.adoc @@ -1069,6 +1069,28 @@ know about the change in the underlying storage, only the first guest-trim will run as expected. Subsequent ones, until the next reboot, will only consider parts of the filesystem that changed since then. +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 ^^^^^^^^^^^^^^^