Application-Consistent backups are unsupported in Linux virtual machine's

Products

• VMware vSphere ESXi

Issue/Introduction

An explanation of quiescing in Linux virtual machine's relating to snapshots/backups. While only crash-consistent backups are supported, application-consistent backups are possible through the use of customer pre-freeze and post-thaw scripts.

Cause

Refer to this chart for supported quiescing mechanism for various Guest Operating Systems:<?xml:namespace prefix = o />

 

Driver Type and Quiescing Mechanisms Used According to Guest Operating Systems
Guest Operating System	Driver Type Used	Quiescing Type Used
Windows XP 32-bit Windows 2000 32-bit	Sync Driver	File-system consistent quiescing
Windows Vista 32- or 64-bit Windows 7 32- or 64-bit	VMware VSS component	File-system consistent quiescing
Windows 2003 32- or 64-bit	VMware VSS component	Application-consistent quiescing
Windows 2008 32- or 64-bit Windows Server 2008 R2	VMware VSS component	Application-consistent quiescing. For application-consistent quiescing to be available, several conditions must be met: ■ Virtual machine must be running on ESXi 4.1 or later. ■ The UUID attribute must be enabled. It is enabled by default for virtual machines created on 4.1 or later. For details about enabling this attribute see Enable Windows 2008 Virtual Machine Application Consistent Quiescing.   ■ The virtual machine must use SCSI disks only and have as many free SCSI slots as the number of disks. Application-consistent quiescing is not supported for virtual machines with IDE disks. ■ The virtual machine must not use dynamic disks.
Windows Server 2012	VMware VSS component	Same as above.
Other guest operating system	Not applicable	Crash-consistent quiescing

Resolution

There are three methods which are attempted sequentially if a quiescing snapshot is requested of a Linux Virtual Machine:

 

1. Using FIFREEZE/FITHAW ioctls build into the kernel

 

* this option is only available in kernels newer than 2.6.32 on Linux VMs running vmtools 5.x or higher (vSphere 5).

 

 

2. Using a VMware kernel module (vmsync)

 

* this option is available in kernels 2.6.6 -> 2.6.29 on Linux VMs running vmtools 4.x or higher (vSphere 4) and is experimental

 

**Message seen when installing tools in Linux***

 

[EXPERIMENTAL] The VMware FileSystem Sync Driver (vmsync) is a new feature that

creates backups of virtual machines. Please refer to the VMware Knowledge Base

for more details on this capability. Do you wish to enable this feature?

 

 

3. Using the Linux sync command
 

Each of the above methods also allows you define custom scripts which are called by vmtools before and after the quiesce.

 

An example of creating your own freeze/thaw scripts is below:

 

Ex: (http://www.virten.net/2012/05/filesystem-consistent-linux-backups-with-vmware/)

 

/usr/sbin/pre-freeze-script

----------------------------------

fsfreeze -f /

 

/usr/sbin/post-thaw-script

---------------------------------

fsfreeze -u /

 

**Note: mount point being used in above scripts needs to be mounted with the "noatime" option for the above to work

 

Additional Information

http://pubs.vmware.com/vsphere-51/index.jsp?topic=%2Fcom.vmware.vddk.pg.doc%2FvddkBkupVadp.9.6.html&resultof=%22VDDK%22%20%22vddk%22%20%22Sync%22%20%22sync%22%20%22Driver%22%20%22driver%22

 

http://pubs.vmware.com/vsphere-50/index.jsp?topic=%2Fcom.vmware.datarecovery.admin.doc_20%2FGUID-6F339449-8A9F-48C0-BE70-91A2654A79D2.html

 

 

 

Impact/Risks:
Using the default behavior should result in a snapshot/backup which would represent the state of the virtual machine, if it had been shutdown gracefully at that point in time. Unfortunately, this does not include application level graceful shutdown i,e flushing DB transaction logs, therefore restoring to such a backup would require application crash recovery operations.

 

Through the definition of custom pre-freeze and post-thaw scripts, interaction with running applications is possible allowing for application-consistent backups.