vsanSparse – TechNote for Virtual SAN 6.0 Snapshots


Virtual SAN 6.0 introduces a new on-disk format that includes VirstoFS technology. This always-sparse filesystem provides the basis for a new snapshot format, also introduced with Virtual SAN 6.0, called vsanSparse. Through the use of the underlying sparseness of the filesystem and a new, in-memory metadata cache for lookups, vsanSparse offers greatly improved performance when compared to previous virtual machine snapshot implementations.

Introducing vsanSparse snapshots

As mentioned in the introduction, Virtual SAN 6.0 has a new on-disk (v2) format that facilitates the introduction of a new type of performance-based snapshot. The new vsanSparse format leverages the underlying sparseness of the new VirstoFS filesystem (v2) on-disk format and a new in-memory caching mechanism for tracking updates. This v2 format is an always-sparse file system (512-byte block size instead of 1MB block size on VMFS-L) and is only available with Virtual SAN 6.0.

NPMD data diagram

Figure 1. vsanSparse disk format

When a virtual machine snapshot is created on Virtual SAN 5.5, a vmfsSparse/redo log object is created (you can find out more about this format in appendix A of this paper). In Virtual SAN 6.0, when a virtual machine snapshot is created, vsanSparse “delta” objects get created.

Why is vsanSparse needed?

The new vsanSparse snapshot format provides Virtual SAN administrators with enterprise class snapshots and clones. The goal is to improve snapshot performance by continuing to use the existing redo logs mechanism but now utilizing an “inmemory” metadata cache and a more efficient sparse filesystem layout.

How does vsanSparse work?

When a vsanSparse snapshot is taken of a base disk, a child delta disk is created. The parent is now considered a point-in-time (PIT) copy. The running point of the virtual machine is now the delta.New writes by the virtual machine go to the delta but the base disk and other snapshots in the chain satisfy reads. To get current state of the disk, one can take the “parent” disk and redo all writes from “children” chain.
Thus children are referred to as “redo logs”. In this way, vsanSparse format is very similar to the earlier vmfsSparse format.


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