Introduction to Storage

Fibre Channel SAN arrays, iSCSI SAN arrays, and NAS arrays are widely used storage technologies supported by VMware vSphere to meet different data center storage needs. The storage arrays are connected to and shared between groups of servers through storage area networks. This arrangement allows aggregation of the storage resources and provides more flexibility in provisioning them to virtual machines.

vSphere Data Center Physical Topology

How Virtual Machines Access Storage

A virtual disk hides the physical storage layer from the virtual machine’s operating system. Regardless of the type of storage device that your host uses, the virtual disk always appears to the virtual machine as a mounted SCSI device. As a result, you can run operating systems that are not certified for specific storage equipment, such as SAN, in the virtual machine.

When a virtual machine communicates with its virtual disk stored on a datastore, it issues SCSI commands. Because datastores can exist on various types of physical storage, these commands are encapsulated into other forms, depending on the protocol that the ESXi host uses to connect to a storage device.

Virtual Machines Accessing Different Types of Storage depicts five virtual machines that use different types of storage to illustrate the differences between each type.

Virtual Machines Accessing Different Types of Storage

You can use vCLI commands to manage the virtual machine file system and storage devices.

■	VMFS. Use vmkfstools to create, modify, and manage VMFS virtual disks and raw device mappings. See Managing the Virtual Machine File System with vmkfstools for an introduction and the vSphere Storage documentation for a detailed reference.

■	Datastores. Several commands allow you to manage datastores and are useful for multiple protocols.

■	LUNs. Use esxcli storage core or vicfg-scsidevs commands to display available LUNs and mappings for each VMFS volume to its corresponding partition. See Examining LUNs.

■	Path management. Use esxcli storage core or vicfg-mpath commands to list information about Fibre Channel or iSCSI LUNs and to change a path’s state. See Managing Paths. Use the ESXCLI command to view and modify path policies. See Managing Path Policies.

■	Rescan. Use esxcli storage core or vicfg-rescan adapter rescan to perform a rescan operation each time you reconfigure your storage setup. See Scanning Storage Adapters.

■	Storage devices. Several commands manage only specific storage devices.

■	NFS storage. Use esxcli storage nfs or vicfg-nas to manage NAS storage devices. See Managing NFS/NAS Datastores.

■	iSCSI storage. Use esxcli iscsi or vicfg-iscsi to manage both hardware and software iSCSI. See Managing iSCSI Storage.

■	Software-defined storage. vSphere supports several types of software-defined storage.

■	Virtual SAN storage. Use commands in the esxcli vsan namespace to manage Virtual SAN. See Monitoring and Managing Virtual SAN Storage.

■	Virtual Flash storage. Use commands in the esxcli storage vflash namespace to manage VMware vSphere Flash Read Cache.

■	Virtual volumes. Virtual volumes offer a different layer of abstraction than datastores. As a result, finer-grained management is possible.. Use commands in the esxcli storage vvol namespace.

Datastores

ESXi hosts use storage space on a variety of physical storage systems, including internal and external devices and networked storage. A host can discover storage devices to which it has access and format them as datastores. Each datastore is a special logical container, analogous to a file system on a logical volume, where the host places virtual disk files and other virtual machine files. Datastores hide specifics of each storage product and provide a uniform model for storing virtual machine files.

Depending on the type of storage you use, datastores can be backed by the following file system formats:

■	Virtual Machine File System (VMFS). High-performance file system optimized for storing virtual machines. Your host can deploy a VMFS datastore on any SCSI-based local or networked storage device, including Fibre Channel and iSCSI SAN equipment.

As an alternative to using the VMFS datastore, your virtual machine can have direct access to raw devices and use a mapping file (RDM) as a proxy. See Managing the Virtual Machine File System with vmkfstools.

■	Network File System (NFS). File system on a NAS storage device. ESXi supports NFS version 3 over TCP/IP. The host can access a designated NFS volume located on an NFS server, mount the volume, and use it for any storage needs.

Storage Device Naming

Each storage device, or LUN, is identified by several device identifier names.

Device Identifiers

Depending on the type of storage, the ESXi host uses different algorithms and conventions to generate an identifier for each storage device.

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SCSI INQUIRY identifiers. The host uses the SCSI INQUIRY command to query a storage device and uses the resulting data, in particular the Page 83 information, to generate a unique identifier. SCSI INQUIRY device identifiers are unique across all hosts, persistent, and have one of the following formats:

■	naa.<number>

■	t10.<number>

■	eui.<number>

These formats follow the T10 committee standards. See the SCSI-3 documentation on the T10 committe Web site for information on Page 83.

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Path-based identifier. If the device does not provide the information on Page 83 of the T10 committee SCSI-3 documentation, the host generates an mpx.<path> name, where <path> represents the first path to the device, for example, mpx.vmhba1:C0:T1:L3. This identifier can be used in the same way as the SCSI inquiry identifiers.

The mpx. identifier is created for local devices on the assumption that their path names are unique. However, this identifier is neither unique nor persistent and could change after every boot.

Typically, the path to the device has the following format:

vmhba<adapter>:C<channel>:T<target>:L<LUN>

■	vmbh<adapter> is the name of the storage adapter. The name refers to the physical adapter on the host, not the SCSI controller used by the virtual machines.

■	C<channel> is the storage channel number. Software iSCSI adapters and dependent hardware adapters use the channel number to show multiple paths to the same target.

■	T<target> is the target number. Target numbering is determined by the host and might change if the mappings of targets that are visible to the host change. Targets that are shared by different hosts might not have the same target number.

■	L<LUN> is the LUN number that shows the position of the LUN within the target. The number is provided by the storage system. If a target has only one LUN, the LUN number is always zero (0).

Legacy Identifiers

In addition to the SCSI INQUIRY or mpx identifiers, ESXi generates an alternative legacy name, called VML name, for each device. Use the device UID instead.