Change in vdsm[master]: freeze: Freeze guest even when creating memory snapshot

nsoffer at redhat.com nsoffer at redhat.com
Wed Jul 8 11:55:55 UTC 2015


Nir Soffer has posted comments on this change.

Change subject: freeze: Freeze guest even when creating memory snapshot
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Patch Set 3:

(1 comment)

https://gerrit.ovirt.org/#/c/43299/3//COMMIT_MSG
Commit Message:

Line 10: probably done because qemu is pausing the vm for creating memory
Line 11: snapshot.
Line 12: 
Line 13: However, this is not consistent with snapshots of external disks such as
Line 14: network disks, where we always freeze the vm before taking the snapshot.
> why does it need to be consistent? AFAIK we freeze non-memory snapshots for
I think we have 2 reason for freezing:

- Keep disk state consistent with each other - all io is frozen in same time
  on all disks.

- Keep application state consistent - freezing will run application specific
  hooks (e.g. /etc/qemu-ga/fsfreeze-hooks.d/myapp-fsfreeze-hook). This 
  gives an application chance to prepare for freezing before filesystems
  are freezed. Also if an application is writing directly to device (e.g. database)
  filesystem freezing does not effect the application, but freezing hooks
  may be used to prepare for snapshot.

Depending on pause during memory snapshot does not achive the second
goal. And indeed libvirt provide the option to freeze when you do a memory
snapshot.

See https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Virtualization_Deployment_and_Administration_Guide/sect-QEMU_Guest_Agent-Using_the_QEMU_guest_virtual_machine_agent_protocol_CLI.html#sect-Using_the_QEMU_guest_virtual_machine_agent_protocol_CLI-Creating_a_guest_virtual_machine_disk_backup
Line 15: Also, it is probably safer to freeze even when creating memory snapshot,
Line 16: giving applications on the guest chance to pause in consistent state.
Line 17: 
Line 18: This patch removes the check for memory snapshot, and freeze the guest


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Gerrit-MessageType: comment
Gerrit-Change-Id: I8aa7ac0dea8690ca33df8067f84734d788da8bf8
Gerrit-PatchSet: 3
Gerrit-Project: vdsm
Gerrit-Branch: master
Gerrit-Owner: Nir Soffer <nsoffer at redhat.com>
Gerrit-Reviewer: Adam Litke <alitke at redhat.com>
Gerrit-Reviewer: Allon Mureinik <amureini at redhat.com>
Gerrit-Reviewer: Dan Kenigsberg <danken at redhat.com>
Gerrit-Reviewer: Federico Simoncelli <fsimonce at redhat.com>
Gerrit-Reviewer: Francesco Romani <fromani at redhat.com>
Gerrit-Reviewer: Jenkins CI
Gerrit-Reviewer: Michal Skrivanek <michal.skrivanek at redhat.com>
Gerrit-Reviewer: Michal Skrivanek <mskrivan at redhat.com>
Gerrit-Reviewer: Nir Soffer <nsoffer at redhat.com>
Gerrit-Reviewer: automation at ovirt.org
Gerrit-HasComments: Yes


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