Virtual environment has made business systems easily created while the environment also needs to be continuously maintained. Backup and snapshot are often the ways that IT administrators use to protect VM data.
Snapshot is more convenient than backup. Users will get both the VM data and status saved. In fact, snapshot can also help backup VM.
There are generally 2 kinds of snapshot in VMware, standard and quiesced snapshots. Quiesced snapshot ensures that a snapshot disk represents a consistent state of the guest file systems. Quiesced guest file system will be suitable for backup so that's why backup supplier would like to create quiesced snapshot before backup.
You can create quiesced snapshot in ESXi client or vCenter. Just check Quiesce guest file system (requires VMware Tools) when taking snapshot to create quiesced snapshot.
What if you can't create quiesced snapshot or your backup software reports that quiescing VM failed? You will know the causes in the next section.
Why you failed to quiesce the virtual machine?
How Quiescing Works in VMware
When you take a quiesced snapshot, VMware Tools inside the guest pauses writes and flushes data from memory to disk before the snapshot is created, so the snapshot captures a consistent file system.
Windows guests: VMware Tools calls the Volume Shadow Copy Service (VSS). VSS writers for applications such as SQL Server or Exchange finish pending transactions, which makes the snapshot application-consistent.
Linux guests: VMware Tools (or open-vm-tools) freezes the file systems and can run custom pre-freeze and post-thaw scripts to prepare applications.
The guest can only hold writes for a short time. If flushing takes too long, or any part of this chain fails, the snapshot can’t be quiesced and you see errors like “failed to quiesce the virtual machine.”
Creating quiesced snapshot requires VMware Tools installed on the guest so that VMware Tools issue might be the cause.
I/O in the virtual machine and space on VM or datastore will also influence taking quiesced snapshot.
Some services like VMware Snapshot Provider and Volume Shadow Copy are influencing the process.
If your backup system report this issue, you should also check whether quiesced snapshot can be taken in ESXi or vCenter. On Linux guests, also check that the VMware Tools service (vmtoolsd) is running, and that no file system the guest can’t freeze, such as some network or FUSE mounts, is blocking the process.
How to fix quiescing virtual machine failed issue?
There are some solutions from the communities that users have used to solve the problem.
Before You Start: Find the Cause in the Logs
The exact error message tells you which solution to try first:
In vSphere: open the VM’s Monitor > Tasks and Events and read the full error of the failed snapshot task.
In the VM’s vmware.log (in the VM folder on the datastore): look for lines containing quiesce, VssSyncStart or ToolsBackup near the time of the failure.
On Windows guests: run vssadmin list writers in an elevated command prompt. Any writer that is not in the Stable state with No error can block quiescing. Restarting the related service (or the guest) usually clears it.
For example, “Cannot create a quiesced snapshot because the snapshot operation exceeded the time limit for holding off I/O” points to high I/O (Solution 3), while “‘VssSyncStart’ operation failed: IDispatch error #8449” points to a VSS problem inside the guest (Solutions 2 and 5).
Solution 1. Reboot the guest
Rebooting the guest machine is often the easiest way to fix most of the issues in IT environment. Some users reported that this problem appears randomly so you can simply restart the guest machine to see whether quiescing snapshot can be created later.
Solution 2. Update VMware Tools
VMware Tools is a must for quiescing the virtual machine so you should check whether it is on the guest machine or accidentally removed.
Sometimes the out-of-date VMware Tools will cause this problem. Update VMware Tools might solve the problem. You can also read the description of the update to see whether this is a proven issue.
Some users reported that this issue might be related to the installation order of VMware Tools and backup agent. VMware Tools needs to be installed before backup agent and when VMware Tools needs to be updated, you need to uninstall the backup agent first and reinstall it after VMware Tools update, so if you have installed a backup agent, you can remove both VMware Tools and backup agent and reinstall them in order.
If you are going to reinstall VMware Tools, you can try selecting Custom and uncheck VSS option this time. Note that without the VSS component, VMware Tools can only quiesce the file system and can no longer create application-consistent snapshots for applications such as SQL Server or Exchange.
Solution 3. Reduce the amount of ongoing I/O to the virtual machine
You might have seen the alert that Cannot create a quiesced snapshot because the snapshot operation exceeded the time limit for holding off I/O in the frozen virtual machine.
It means that VM I/O is too high and data transfer can't be completed so don't make VM too busy when taking quiesced snapshot.
Solution 4. Release space on VM or datastore
If the space on VM is used up, you can't create quiesced snapshot. This issue will also occur if the VM has free space but datastore doesn't.
VMware document says that you cannot quiesce virtual machines that have large capacity disks but haven't define what is large capacity.
Therefore, check the space on the VM and datastore. Make sure there is enough storage for quiesced snapshot.
Solution 5. Stop "VMware Snapshot Provider" service
Users reported that they fixed this issue by disabling “VMware Snapshot Provider” service on the guest.
Press Windows Key + R to open the Run dialog > type services.msc and press Enter > locate VMware Snapshot Provider > right-click it and select Properties > set Startup type to Disabled > click Stop > click OK.
Stopping the service alone only lasts until the next reboot, so disable it as well. Once it’s disabled, VMware Tools uses the Windows built-in VSS provider instead. Don’t disable the Volume Shadow Copy service itself, because quiescing on Windows depends on it.
If none of these solutions works and you need a backup right away, take the snapshot without quiescing. A crash-consistent snapshot captures the disks as they are, like pulling the power plug. Most file systems and databases can recover from that state, but you should fix the quiescing issue before relying on such backups for critical applications.
Quiesced snapshot is supported by Vinchin Backup & Recovery
Vinchin is VMware official partner and Vinchin Backup & Recovery is the professional VMware vSphere backup solution which has helped thousands of companies protect virtual environments.
You don't need to install backup agent on every guest but just add the host into Vinchin backup server and then you can make backup for any guest on the host agentlessly.
Vinchin Backup & Recovery provides many backup strategies to let you backup VM in every way you wish. By enabling Quiesced Snapshot, you can keep file systems or applications on the VM in a consistent state via application-awareness processing before snapshot is taken.
You can easily create a VMware backup job in the intuitive web console.
1. Select the VMware VM
2. Select the backup storage
3. Select the backup strategies
4. Submit the job
60-day full-featured free trial is available now. You can experience all the features of Vinchin Backup & Recovery including Instant Recovery (recover VM of any size from its backup in 15 seconds), Cross-Platform Recovery (if you have other platforms like Hyper-V, Proxmox, XenServer, Red Hat Virtualization, etc., you can migrate VM between different platforms), Data Compression and Deduplication (saving 60% backup storage), etc.
Failed to quiesce the virtual machine FAQs
Q1: How do I check if VMware Tools can quiesce the VM?
A: Ensure VMware Tools is installed, running, and up to date inside the guest to enable quiescence support.
Q2: Can corrupted VM configuration cause quiesce failures?
A: Yes. Invalid entries or phantom disks in the VMX file can block quiescing and may require cleaning or recreating the VM.
Q3: What role does Windows VSS play in quiescing errors?
A: Faulty or hung VSS writers inside the guest OS can prevent filesystem quiescence, so check vssadmin list writers for errors.
Q4: Does a quiesced snapshot guarantee application consistency?
A: Not always. Quiescing always makes the file system consistent, but application consistency on Windows requires VSS writers for the application, and on Linux it requires pre-freeze scripts. Also, application-consistent quiescing on Windows needs the VM setting disk.EnableUUID = TRUE and is not supported for VMs with dynamic disks.
Q5: Why is the “Quiesce guest file system” option greyed out?
A: The option is only available when Include virtual machine’s memory is unchecked and VMware Tools is installed and running in the guest. Uncheck the memory option, or install or start VMware Tools, and the quiesce option becomes available.
Sum Up
Quiesced snapshot can ensure that a snapshot disk represents a consistent state of the guest file systems and make a suitable environment for backup.
If you failed to quiesce the virtual machine or received related alert from your backup system, you can read the possible reasons in this post and use the 5 solutions to fix the issue.
Vinchin Backup & Recovery also supports Quiesced Snapshot during VM backup. You can try it for free.
