Virtual Worlds: Virtualization Layers In The SAN - Industry Trend or Event

Computer Technology Review, Nov, 2000 by Christine Chudnow

Saving money in both personnel and hardware

SAN virtualization makes physical storage devices act and appear as logical volumes in a storage area network. IT administrators can then allocate the volumes on demand to application servers without resorting to awkward host zoning and LUN mapping procedures.

Samuel Tam, CEO of Vicom said, "You need to be able to have centralized services, and those services now include host zoning, LUN masking, and being able to map storage resources in a secure manner to all the users and servers attached to the network. Our idea is that we would take it to level of granularity, allowing a very small chunk of storage partition to be available, to be dynamically allocated to any users of servers. We're talking about true virtualization, where it's done at each individual block level."

This method of SAN virtualization, which represents a distinct layer in the storage stack, can save money in both personnel and hardware. It impacts personnel by allowing servers to access centralized storage while IT administrators manage the storage as a single logical entity. Since storage is growing 100% a year in many cases and additional headcount is unlikely, easier storage administration is welcome news. In fact, IDC reported that single storage administrators can manage 7.5 times more data when it is centralized than if it is dispersed. The hardware budget improves because IT administrators can allocate storage much more efficiently. It's also relatively easy to reassign storage on the fly, whether for an emergency need or temporary disk space for migration, testing, or financial closes.

Although some products have offered virtualization types for years, SAN virtualization as a separate layer from a server virtualization layer is just now coming into its own. It offers two different functions. The first is the access function, providing virtual volumes for security, storage consolidation, storage on demand, and SAN file systems. Control functions offer SAN services for serverless backup, archiving, migration, and virtual devices for Storage Resource Management (SRM).

SAN Virtualization Layer

Since software server components divide storage space into volumes, the physical layout of the actual storage devices is irrelevant. This contrasts with the present method for dynamically allocating storage, which is combining LUN zoning with port zoning. The method offers a high level of security, but there are distinct problems connected with the technology as the data is held captive in layers and within different operating systems.

According to DataCore, the top three technical issues in dynamic reallocation are:

1. Hosts communicate with physical storage devices using SCSI protocol over Fibre Channel. The destination address is a combination of a Target I.D. and a LUN.

2. Some hosts take ownership of any visible LUNs on attached fibers.

3. Two or more hosts connected to the same LUN will automatically compete for the same storage device.

To get around technical limitations, IT often zones switches to block other servers from connecting to the same storage device. "LUN Masking" relies on each application server to ignore the neighboring servers' LUNs by using special host-based device drivers to provide a filtered view of the storage network. However, some hosts cannot accept a LUN Masking installation so they often cannot share storage assets. Both workarounds are time-consuming, which affects downtime, backup and capacity chores, and allows large areas of storage to remain unused.

In contrast, the IT administrator can create, expand, remove, move, and selectively present a virtual volume to users, independent of its storage subsystem. It can consist of storage space in different subsystems, even subsystems with different characteristics. This results in a highly efficient space-assigning operation, saving money both on added storage hardware and on scarce man-hours and storage personnel. LDC reports that consolidating data in one location can lower data management costs from 55% to 15% of the total storage budget.

Killer Apps?

Investment firm Morgan Keegan lists several top applications for SAN virtualization and different architecture types (See Table).

Storage sharing. IT administrators must regulate storage access to prevent storage sharing conflicts. They do it now with a combination of port and LUN zoning, but SAN virtualization makes it much easier to map servers to storage as virtual volumes. This would produce an integrated access framework spanning all SAN resources in an organization.

Data sharing. Port and LUN zoning enables storage sharing, but no data sharing as access to each LUN is restricted. Managers usually use NAS servers at the logical layer to solve the problem, the NAS devices acting as intermediaries between data stored in specific LUNs and the application servers. This helps, but you then have the problem of an additional latency of a NAS and increased file transactions over the LAN.

To answer this problem, a SAN File Systems approach is emerging. The IT administrator installs a network file system such as NFS or CIFS onto each application server to capture and forward file operations over TCP/IP. Using SAN virtualization as a key enabler, the file server processes the operation and sends the data traffic to the application server over a SAN instead of the LAN. A similar procedure can handle clustered file systems.