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4.7. QEMU Guest Agent Overview

In Red Hat Enterprise Linux 6.4, the QEMU Guest Agent (QEMU GA) provided protection against the corruption of Linux guest virtual machines. Before issuing a snapshot request or creating a backup copy of the disk, the management stack (libvirt) sent a guest-fsfreeze-freeze QMP command to the QEMU GA via the virtio-serial port. This command caused the guest agent to freeze all of the guest virtual machine's filesystems, via the FIFREEZE ioctl() kernel function. This ioctl() function is implemented by the Linux kernel in the guest virtual machine. The function flushes the filesystem cache in the guest virtual machine's kernel, brings the filesystem into a consistent state, and denies all userspace threads write access to the filesystem.
Only after the QEMU GA reported success, libvirt would proceed with the snapshot. At its completion, libvirt sends the guest-fsfreeze-thaw QMP command to the QEMU GA over the virtio-serial port. This command tells the QEMU GA to issue a FITHAW ioctl(), which unblocks the userspace threads that were previously denied write access, and resumes normal processing. This process did not ensure that application-level data was in a consistent state when the virtual disk snapshot was taken. This was evident in cases where the fsck utility found no problems on filesystems restored from snapshots, and yet applications were not able to resume processing from the point where the snapshot was taken and userspace processes may not have written their internal buffers to files on the disk.
Red Hat Enterprise Linux 6.5 ensures that both file and application-level synchronization (flushing) are done. Guest system administrators can write and install application-specific freezing and thawing hook scripts. Before freezing the filesystems, the QEMU GA invokes the main hook script (included in the QEMU GA package). The main hook script in turn calls individual application-specific scripts, prepared by the guest system administrators, that temporarily deactivate all guest virtual machine applications. All of these actions occur when the mode is changed to "freeze".
Just before filesystems are frozen, the guest system administrator's scripts cause the databases and other file system applications to flush their working buffers to the virtual disk and to stop accepting further client connections. The applications then bring their data files into a consistent state where resumption of processing, with the reactivated (or a freshly started) instance of the application (after restoring the virtual disk from backup) is possible. When all scripts are done making their respective applications inactive, and the main hook script returns, QEMU GA proceeds to freeze filesystems, and the management stack takes the snapshot. Once all this is done, and it is confirmed that the snapshot is taken, the file system will resume to serve write requests. This process is called thawing.
Thawing is freezing in reverse order. Instructed by libvirt, QEMU GA thaws the guest virtual machine's filesystems. It then invokes individual hook scripts (via the main hook script) to resume or restart applications that had been inactivated during the freeze process.