Storage Basics

Data store:-
Formatted logical container analogous to a file system on a logical volume. The datastore holds virtual machine files and can exist on different types of physical storage including SCSI, iSCSI, Fibre Channel SAN, or NFS. Data stores can be of the two types: VMFS-based or NFS-based.

Disk partition

Reserved part of hard disk that is set aside for specific purposes. In the context of ESX Server storage, disk partitions on various physical storage devices can be reserved and formatted as datastores.

Extent
In the ESX Server context, an extent is a hard disk partition on a physical storage device that can be dynamically added to an existing VMFS-based datastore. The datastore can stretch over multiple extents, yet appear as a single volume analogous to a spanned volume.

Failover path
Redundant physical path that the ESX Server system can use when communicating with its networked storage. The ESX Server system uses the failover path if any component responsible for transferring storage data fails. See Multipathing.

Fibre Channel (FC)
High-speed data transmitting technology that ESX Server systems use to transport SCSI traffic from virtual machines to storage devices on a SAN. The Fibre Channel Protocol (FCP) packages SCSI commands into Fibre Channel frames.

iSCSI (Internet SCSI)
Packages SCSI storage traffic into TCP so it can travel through IP networks instead of the specialized FC network. With iSCSI connection, your ESX Server system (initiator) communicates with a remote storage device (target) as it would do with a local hard disk.
 

LUN (logical unit number)
Address uniquely identifying each SCSI disk that an ESX Server system uses for storage. The terms hard disk and LUN are often used interchangeably.

Multipathing
Technique that lets you use more than one physical path, or an element on this path, responsible for transferring data between the ESX Server system and its remote storage. This redundant use of physical paths or elements, such as adapters, helps ensure uninterrupted traffic between the ESX Server system and storage devices.

NAS (network-attached storage)
 Specialized storage device that connects to a network and can provide file access services to ESX Server systems. ESX Server systems use the NFS protocol to communicate with NAS servers.

NFS (network file system)
File sharing protocol ESX Server supports to communicate with a NAS device.

Raw device (raw disk)
SCSI device used directly by a virtual machine. The device might be accessed through an RDM.

Raw device mapping (RDM)
Special mapping file in a VMFS volume that acts as a proxy for a raw device and maps SAN LUNs directly to a virtual machine. RDM can be also called metadata file.

Raw LUN
Logical disk located in a SAN.

Spanned volume
Dynamic volume that uses disk space on more than one physical disk, yet appears as a single logical volume.

Storage device
Physical disk or storage array that can either be internal or located outside of your system and connected to the system either directly or through an adapter.

VMFS (VMware File System)
High-performance cluster file system that provides storage virtualization optimized for virtual machines.

Volume
Logical storage unit, which can use disk space on one physical device, or its part, or span several physical devices. 

Overview

In the most configuration, a virtual machine uses a virtual hard disk to store its operating system, program files, and other data associated with its activities. A virtual disk is a large physical file that can be copied, moved, archived, and backed up as easy as any other file.

Virtual disk files reside on specially formatted volumes called datastores. A datastore can be deployed on the host machine’s internal direct-attached storage devices or on networked storage devices. A networked storage device represents an external shared storage device or array that is located outside of your system and is typically accessed over a network through an adapter

Datastores and File Systems

ESX Server virtual machines store their virtual disk files on specially formatted logical containers, or datastores, that can exist on different types of physical storage devices

Your datastore management process starts with storage space that your storage administrator preallocates for your ESX Server system on different storage devices. The storage space is presented to your ESX Server system as LUNs (logical unit numbers) or, in case of a network attached storage, as NFS volumes.

File System Formats

Datastores that you use can have the following file system formats:

·         VMFS – ESX Server deploys this type of file system on local SCSI disks, iSCSI LUNs, or Fibre Channel LUNs, creating one directory for each virtual machine. VMFS is a clustered file system that can be accessed simultaneously by multiple ESX Server systems.

·         As an alternative to using the VMFS-based datastore, your virtual machine can have direct access to raw devices using a mapping file (RDM) as a proxy

·         NFS – ESX Server can use a designated NFS volume located on an NFS server. ESX Server mounts the NFS volume creating one directory for each virtual machine. From the viewpoint of the user on a client computer, the mounted files are indistinguishable from local files.

Types of Storage

Datastores can reside on a variety of storage devices. You can deploy a datastore on your system’s direct-attached storage device or on a networked storage device.

ESX Server supports the following types of storage devices:

• Local – Stores files locally on an internal or external SCSI device.
• Fibre Channel – Stores files remotely on a Storage Area Network (SAN). Requires Fibre Channel adapters.
• iSCSI (hardware initiated) – Stores files on remote iSCSI storage devices. Files are accessed over TCP/IP network using hardware-based iSCSI HBAs (host bus adapters).
• iSCSI (software initiated) – Stores files on remote iSCSI storage devices. Files are accessed over TCP/IP network using software-based iSCSI code in the VMkernel. Requires a standard network adapter for network connectivity.
• Network file system (NFS) – Stores files on remote file servers. Files are accessed over TCP/IP network using the NFS protocol. Requires a standard network adapter for network connectivity

Note: We can’t store virtual machine on IDE or SATA drives.

Supported Storage Adapters

To access different types of storage, your ESX Server system needs different adapters that provide connectivity to the storage device or network. ESX Server supports PCI-based SCSI and iSCSI, RAID, Fibre Channel, and Ethernet adapters and accesses them directly through device drivers in the VMkernel.