This article provides an insight on :
Symptoms:
When backing up a virtual machine, the following symptoms are observed.
VMware vSphere ESXi 6.x
VMware vSphere ESXi 7.x
VMware vSphere ESXi 8.x
Actions or Features | Supported | Unsupported | Notes |
Snapshots of VMs with dependent disks | × | Virtual backup solutions leverage snapshots through the vStorage APIs; for example, VMware Data Recovery, vSphere Data Protection. These are also not supported. | |
Snapshots of VMs with independent-persistent disks | √ | Supported in vSphere 5.1 update2 and later versions. The snapshot task will be completed successfully but will skip the independent disks from the snapshot operation. Powered on virtual machines with independent disks cannot support memory snapshots. |
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Changed Block Tracking (CBT) | × |
Multi-writer disks are virtual disks configured to allow simultaneous read/write access by multiple virtual machines (VMs). This configuration is commonly used in scenarios like clustering, distributed databases, and applications requiring shared storage. Multi-writer disks are ideal for environments where data consistency and performance are critical across multiple VMs.
VMFS is a clustered file system that disables (by default) multiple virtual machines from opening and writing to the same virtual disk (.vmdk file). This prevents more than one virtual machine from inadvertently accessing the same .vmdk file.
The multi-writer option allows VMFS-backed disks to be shared by multiple virtual machines. This option is used to support VMware fault tolerance, which allows a primary virtual machine and a standby virtual machine to simultaneously access a .vmdk file.
Dependent Mode Multi-Writer Disks:
Snapshots are not supported for dependent mode disks with the multi-writer flag. Dependent disks are typically included in snapshots, but multi-writer configurations disable this functionality due to the inability to handle redo logs (delta files) in shared-disk environments. This restriction prevents data corruption caused by conflicting writes.
Independent Mode Multi-Writer Disks:
When a snapshot is created, VMware temporarily suspends I/O operations to the disk to establish a consistent state. A redo log (delta file) is created to track changes, while the base disk becomes read-only. For multi-writer disks, this mechanism introduces the following challenges:
These inherent limitations of the snapshot mechanism make it unsuitable for multi-writer disks.
Multi-writer disks larger than 2TB do not support the VMFSSPARSE snapshot format. This is because VMFSSPARSE has a size limitation that does not extend to disks larger than 2TB. For disks larger than 2TB, snapshots automatically use the SESPARSE format instead of VMFSSPARSE. However, enabling multi-writer mode on a disk disables snapshot functionality entirely, regardless of whether SESPARSE could theoretically be used.
Here’s a detailed breakdown:
Snapshot-Based Backups
Agent-Based Backups