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qm: Add section explaining fs-freeze/-thaw QGA option

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Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>
 [ T: resolve trivial merge conflict due to context additions ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
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Christoph Heiss 2023-02-23 15:18:05 +01:00 committed by Thomas Lamprecht
parent 5b105f3efd
commit 62bf5d754e
1 changed files with 22 additions and 0 deletions
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@ -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
^^^^^^^^^^^^^^^