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Some info on Instance Snapshots causing VM to pause on KVM #571

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Sep 24, 2025
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Some info on Instance Snapshots causing VM to pause on KVM #571

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c9aae60
vm pause documentation
abh1sar Sep 24, 2025
4b74f72
Update typo in source/adminguide/virtual_machines.rst
abh1sar Sep 24, 2025
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21 changes: 21 additions & 0 deletions source/adminguide/virtual_machines.rst
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Expand Up @@ -1265,6 +1265,8 @@ When the snapshotting is complete, the Instance is thawed.

You can use this functionality on Instances with raw block storages (E.g. Ceph/SolidFire/Linstor).

.. _Disk-only-File-based-Storage-Instance-Snapshots-on-KVM:

Disk-only File-based Storage Instance Snapshot on KVM
-----------------------------------------------------

Expand Down Expand Up @@ -1298,6 +1300,25 @@ Limitations on Instance Snapshots
managed by CloudStack. Any Snapshots that you make directly on the
hypervisor will not be tracked in CloudStack.

Pause During Live Instance Snapshots on KVM
-------------------------------------------

When creating **Instance Snapshots with Memory**, CloudStack uses Libvirt’s
*domain snapshot* API to create an Internal Snapshot that includes Memory.
The guest’s memory state is written directly into the root volume’s QCOW2 file.
This causes the instance to pause for the duration of the memory dump. The pause
time is typically much longer than with VMware snapshots, but this is a limitation
with Internal Snapshots in Libvirt.

**Instance Snapshots without Memory** has seen significant improvements since Cloudstack 4.21 with the
:ref:`Disk-only-File-based-Storage-Instance-Snapshots-on-KVM` feature for NFS and local storage.
Pre 4.21, the Instance would be frozen for the entire duration of the snapshot create operation.
Since 4.21, the Instance is only frozen during the checkpointing operation, which is significantly less.

Users looking for the Instance Snapshot feature in KVM are recommended to use the
:ref:`Disk-only-File-based-Storage-Instance-Snapshots-on-KVM` feature, if the pause duration is a concern.
App consistent snapshots can be created by using the ``quiescevm`` parameter with pre and post-freeze hooks.
The Instance should have Qemu Guest Agent installed for this to work.

Configuring Instance Snapshots
------------------------------
Expand Down