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A virtual machine can freeze under load when you take quiesced snapshots or use custom quiescing scripts

Details

VMware products require file systems within a guest operating system to be quiesced prior to a snapshot operation for the purposes of backup and data integrity. VMware products which use quiesced snapshots include, but are not limited to, VMware Consolidated Backup and VMware Data Recovery.

Virtual machine generating heavy I/O workload may encounter issues when quiescing prior to a snapshot operation. These issues may be related to the component that does the quiescing or the custom quiescing scripts as described in the Virtual Machine Backup Guide.

Services which have been known to generate heavy I/O workload include, but are not limited to, Exchange, Active Directory, LDAP, and MS-SQL.

The quiescing operation is done by an optional VMware Tools component called the SYNC driver. As of ESX 3.5 Update 2, quiescing is also done by Microsoft's Volume Shadow Copy Service (VSS). VSS is provided by Microsoft in their operating systems as of Windows Server 2003 and Windows XP. Operating systems which do not have the Volume Shadow Copy Service make use of the SYNC driver for quiescing operations.

Solution

Quiescing with the SYNC Driver

A guest operating system may appear to be unresponsive when there is a conflict between the SYNC driver and services generating heavy I/O. If installed, the SYNC driver holds incoming I/O writes while it flushes all dirty data to a disk, thus making file systems consistent. Under heavy loads, the delay in I/O can become too long, which affects many time-sensitive applications, including the services which generate the heavy I/O (such as an Exchange Server). If writes issued by these services get delayed for too long, the service may stop and issues error messages.

To avoid this issue, disable the SYNC driver or stop the service generating heavy I/O before taking a snapshot.

Note: The sync driver is only required for legacy versions of Windows such as Windows XP and Windows 2000 which do not include the Microsoft VSS service. Updated versions of VMware Tools will automatically uninstall the SYNC driver.

Disabling the VCB SYNC Driver (LGTO_Sync)

Disabling the SYNC driver allows you to keep the heavy I/O services on-line, but results in snapshots being only crash-consistent.

To disable the VCB SYNC driver:

In Device Manager, click View > Show hidden devices.
Expand Non-Plug and Play Drivers.
Right-click Sync Driver and click Disable.
Click Yes twice to disable the device and restart the computer.

Stopping services generating heavy I/O

Use the following pre-freeze and post-thaw scripts to take the service generating heavy I/O offline for approximately 60 seconds and then restart it again after the snapshot is taken. This approach leaves the service inactive, but keeps the SYNC driver enabled while the snapshot is taken, ensuring application consistency. Using this method, you create the quiesced snapshot of guest operating system.

This example shuts down Exchange Services prior to a quiescing operation:

C:\Windows\pre-freeze-script.bat
@echo off
net stop MSExchangeSA /yes
C:\Windows\post-thaw-script.bat
@echo off
Net Start MsExchangeSA
Net Start MsExchangeIS
Net Start MsExchangeMTA

For additional information, see the Virtual Machine Backup Guide.

Quiescing with the Volume Shadow Copy Service

If you experience issues quiescing while using Microsoft's VSS, seeTroubleshooting Volume Shadow Copy quiesce related issues (1007696).

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