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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 run as expected. Subsequent ones, until the next reboot, will only consider
parts of the filesystem that changed since then. 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 Troubleshooting
^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^