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10.2.5. Creating a Guest Virtual Machine Disk Backup

libvirt can communicate with qemu-ga to assure that snapshots of guest virtual machine file systems are consistent internally and ready for use on an as needed basis. Improvements in Red Hat Enterprise Linux 6 have been made to make sure that both file and application level synchronization (flushing) is done. Guest system administrators can write and install application-specific freeze/thaw hook scripts. Before freezing the filesystems, the qemu-ga invokes the main hook script (included in the qemu-ga package). The freezing process temporarily deactivates all guest virtual machine applications.
Just before filesystems are frozen, the following actions occur:
  • File system applications / databases flush working buffers to the virtual disk and stop accepting client connections
  • Applications bring their data files into a consistent state
  • Main hook script returns
  • qemu-ga freezes the filesystems and management stack takes a snapshot
  • Snapshot is confirmed
  • Filesystem function resumes
Thawing happens in reverse order.
Use the snapshot-create-as command to create a snapshot of the guest disk. See Section 14.15.2.2, “Creating a snapshot for the current domain” for more details on this command.

Note

An application-specific hook script might need various SELinux permissions in order to run correctly, as is done when the script needs to connect to a socket in order to talk to a database. In general, local SELinux policies should be developed and installed for such purposes. Accessing file system nodes should work out of the box, after issuing the restorecon -FvvR command listed in Table 10.1, “QEMU guest agent package contents” in the table row labeled /etc/qemu-ga/fsfreeze-hook.d/.
The qemu-guest-agent binary RPM includes the following files:

Table 10.1. QEMU guest agent package contents

File nameDescription
/etc/rc.d/init.d/qemu-gaService control script (start/stop) for the QEMU guest agent.
/etc/sysconfig/qemu-gaConfiguration file for the QEMU guest agent, as it is read by the /etc/rc.d/init.d/qemu-ga control script. The settings are documented in the file with shell script comments.
/usr/bin/qemu-gaQEMU guest agent binary file.
/usr/libexec/qemu-ga/Root directory for hook scripts.
/usr/libexec/qemu-ga/fsfreeze-hookMain hook script. No modifications are needed here.
/usr/libexec/qemu-ga/fsfreeze-hook.d/Directory for individual, application-specific hook scripts. The guest system administrator should copy hook scripts manually into this directory, ensure proper file mode bits for them, and then run restorecon -FvvR on this directory.
/usr/share/qemu-kvm/qemu-ga/Directory with sample scripts (for example purposes only). The scripts contained here are not executed.
The main hook script, /usr/libexec/qemu-ga/fsfreeze-hook logs its own messages, as well as the application-specific script's standard output and error messages, in the following log file: /var/log/qemu-ga/fsfreeze-hook.log. For more information, refer to the qemu-guest-agent wiki page at wiki.qemu.org or libvirt.org.