May 29, 2015

vSphere Storage Terminologies - Local vs. Shared Storage

Local vs. shared storage

“An ESXi host can have one or more storage options actively configured, including the following:”
  • Local SAS/SATA/SCSI storage
  • Fibre Channel
  • Fibre Channel over Ethernet (FCoE)
  • iSCSI using software and hardware initiators
  • NAS (specifically, NFS)
  • InfiniBand
Many advanced vSphere features, vMotion, high availability (HA), distributed resource scheduler (DRS), fault tolerance (FT), and etc. required shared storage. Local storage has limited use in a vSphere environment.

With vSphere 5.0, VMware introduced vSphere Storage Appliance (VSA). VSA provides a way to take local storage and present it to ESXi hosts as a shared NFS mount. This is implemented through the installation of a virtual appliance called the vSphere Storage Appliance.

VSA enables provides failover capabilities for VMs, without requiring shared SAN storage.

There are some limitations however. It can be configured with only two or three hosts, there are strict rules around the hardware that can run the VSA, and on top of this, it is licensed as a separate product. While it does utilize the underused local storage of servers, the use case for the VSA simply is not valid for many organizations.

VSA Limitations:
  • can scale to two or three storage nodes
  • each VSA cluster can support up to eight disks
  • cannot add storage after cluster has been configured
vSphere 5.5, introduced two other features that allow the consumption of local storage: vSphere Flash Read Cache (vFRC) and VSAN.

vSphere Flash Read Cache takes flash-based storage and allows administrators to allocate portions of it as a read cache for VM read I/O.

VSAN extends the VSA concept and presents the local storage as a distributed datastore across many hosts. Unlike with VSA, VSAN does not require an appliance or reliance on NFS. The functionality is built into the ESXi hypervisor.

Fibre Channel (FC)

"Fibre Channel (FC) stores virtual machine files remotely on an FC storage area network (SAN).”

The network uses FC protocol to transport SCSI traffic from virtual machines to the FC SAN devices. Fibre Channel host bus adapters (HBAs) are used by the ESXi host to connect to the FC SAN. The datastores on FC storage use the VMFS format.

A comparison of the vSphere features supported on different types of storage:

Protocols Transfers Interface
Fibre Channel FC/SCSI Block access of data/LUN FC HBA
Fibre Channel over Ethernet FCoE/SCSI Block access of data/LUN Converged Network Adapter (hardware FCoE)
NIC with FCoE support (software FCoE)
iSCSI IP/SCSI Block access of data/LUN SCSI HBA or iSCSI-enabled NIC (hardware iSCSI)
Network adapter (software iSCSI)
NAS IP/NFS File (no direct LUN access) Network adapter

Shared storage allows multiple ESXi hosts access to the same storage. Some vSphere features which also require shared storage include:
  • DRS
  • DPM
  • Storage DRS
  • High Availability
  • Fault Tolerance

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