Storage Virtualization
When we talk about storage virtualization, most often we are referring to a "Storage Area Network," or "SAN."
SANs were originally introduced to the datacenter to combat that portion of Murphy's Law that says that
when your servers all have their own local storage, excess storage capacity will never be in the server that
actually needs it. By consolidating storage capacity into a central repository, it can then be made available
to whichever server needs it. In fact, most SANs today support something called "thin-provisioning," which allows
you to, in effect, over-subscribe the available SAN physical storage. For example, you could tell ten different
servers that they each had 2 Terabytes of storage available to them, when in fact your SAN only had a total of
2 Terabytes of physical space. The SAN would then dynamically allocate to each server only as much storage as
it actually needed. This efficient allocation of storage space on demand
saves you money, because you only need
to buy the storage you actually need and expand as needed as time goes on.
Physical servers are typically connected to the SAN by means of a "Host Bus Adapter," or "HBA." The server operating
system view the HBA much the way it views, say, a SCSI controller card. But instead of connecting to local hard
disks, the HBA connects to one or more SAN "nodes," typically through some kind of switching fabric. The most
common implementations are Fibre Channel, which, as you might expect, uses fiber optic cable for its connections,
and iSCSI, which is "SCSI over IP," and uses ordinary gigabit Ethernet cables and switches. Fibre Channel is
somewhat faster than iSCSI, but iSCSI has the advantage of not introducing a different transmission and
switching technology to the data center.
SAN technology can also help with another increasingly common problem in the data center: often the night just
isn't long enough anymore to back up all the data that needs to be backed up! But most SANs support some kind
of "snapshot" technology that allows you to capture an image of a particular storage volume at a given moment in
time, then go on using the storage volume while the snapshot is backed up.
SANs are a crucial part of any serious server or desktop virtualization project. Placing the guest operating
system files on a SAN rather than on a virtual host server's local storage allows them to be accessed by
multiple virtual hosts. This enables advanced features such as live motion, high availability, and intelligent
placement of new virtual guests on the servers that have the most available resources.
Moose Logic partners with multiple SAN vendors, including
DataCore, Dell EqualLogic, and LeftHand Networks. (LeftHand is now
a part of the Hewlett-Packard Storage Works product line.) For a more in-depth discussion of SAN
technology, please see
our DataCore product page.
