May 27, 2015

vSphere Storage Terminologies - Virtual Disk Modes

Virtual Disk Modes

ESXi supports three virtual disk modes: Independent persistent, Independent nonpersistent, and Dependent.

An independent disk does not participate in virtual machine snapshots. That is, the disk state will be independent of the snapshot state; creating, consolidating, or reverting to snapshots will have no effect on the disk.

Independent persistent

In this mode changes are persistently written to the disk, providing the best performance.

Independent nonpersistent

In this mode disk writes are appended to a redo log.

The redo log is erased when you power off the virtual machine or revert to a snapshot, causing any changes made to the disk to be discarded.

When a virtual machine reads from an independent nonpersistent mode disk, ESXi first checks the redo log (by looking at a directory of disk blocks contained in the redo log) and, if the relevant blocks are listed, reads that information. Otherwise, the read goes to the base disk for the virtual machine.

Because of these redo logs, which track the changes in a virtual machine’s file system and allow you to commit changes or revert to a prior point in time, performance might not be as high as independent persistent mode disks.

Dependent

In this mode disk writes are appended to a redo log that persists between power cycles.

Thus, like the independent nonpersistent mode disks described above, dependent mode disk performance might not be as high as independent persistent mode disks.

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