June 10, 2015

VSAN - Virtual SAN


VMware Virtual SAN abstracts and pools server-side flash and disk into shared pools of capacity with policy-based management and application-centric data services.

Virtual SAN pools server-attached hard disk drives and flash (SSDs, and PCI-e) drives to create a distributed shared datastore that abstracts the storage hardware and provides a Software-Defined Storage (SDS) tier for virtual machines.

VSAN leverages local storage from a number of ESXi hosts in a cluster and creates a distributed shared datastore. This shared datastore can be used for VM placement and core vSphere technologies such as vMotion, DRS, VMware Site Recovery Manager, etc.

“VSAN leverages the power of any solid state drives (SSDs) on the hosts in the cluster for read caching and write buffering to improve performance."

In a hybrid architecture VSAN has both Flash (SSDs and PCI-e) and hard disk drive (HDD) devices. The Flash devices are utilized as a read cache and the HDDs are pooled to ceate a distributed shared datastore.

For applications that require high performance, Virtual SAN 6.0 can be deployed in an All-Flash storage architecture in which flash-based devices are intelligently utilized only as a write cache while other flash-based devices provide high endurance for data persistence.
  • Introduced with vSphere 5.5
  • Creates Virtual SAN shared datastore
  • Embedded in the hypervisor kernel
  • Hybrid or all-Flash architecture
  • Flash provides caching
  • Managed via the vSphere Web Client
  • Per-VM policies
  • Minimum of three hosts per VSAN cluster
  • Maximum of eight hosts per VSAN cluster
  • Requires at least one VMkernel port per host
  • One or more SSDs per host
  • One or more HDDs per host
  • Uses a proprietary protocol of IP over a VMKernel port to communicate between the nodes
  • 1 Gbps network between hosts (10 Gbps recommended)
VSAN uses the SSD as a read/write cache, the capacity of the SSD is not actually added to the overall usable space of the VSAN datastore.

Caching: When blocks are written to the underlying datastore, they are first written to the SSDs, and later the data can be relocated to the HDDs.

VSAN uses "RAIN, or Reliable Array of Independent Nodes" instead of RAID. With RAIN, all the disks across all the hosts are combined into a large pool of storage. This storage is accessible across all the hosts in the cluster. RAIN enables HA across disk or host failures.

VSA (vSphere Storage Appliance) runs in a VM while VMware’s Virtual SAN (VSAN) is integrated into vSphere hypervisor (ESXi).

vSphere Storage Appliance VMware Virtual SAN
Description Low cost, simple shared storage for small deployments Scale-out distributed storage designed for virtualized/cloud environments
Form Factor Virtual Appliance Built into vSphere kernel
Scalability 2 to 3 vSphere servers
Does not scale beyond 3 hosts
Minimum 3 node deployment
Scale out to 8 nodes
Performance No SSD requirement SSD Caching requirement

"VSAN employs algorithms to help protect against data loss, such as ensuring that the data exists on multiple participating VSAN nodes at the same time."

VSAN is conceptually similar to the VSA, however VSAN provides better scaling, does not require NFS mounts, is embedded into the hypervisor, thus eliminating the complexity of deploying and managing a virtual appliance.


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