Consider the modes of deployment. Host and appliance-based storage virtualization is typically installed in-band or out-of-band, while fabric-based virtualization is implemented in storage area network ( SAN) switches. There are advantages and disadvantages to each approach. In-band deployments actually sit in the active data stream to handle virtualization. This is often the most straightforward approach and requires no software agents to collect and process information, but the presence of a virtualization server in the data path can easily become a traffic bottleneck. Out-of-band virtualization uses servers or appliances that sit on the network but not directly in the data path. This will not impair traffic performance but often requires agent software installed on each platform being virtualized -- increasing the maintenance burden for IT staff. Implementing virtualization within the fabric is increasingly popular because of the good performance, broad interoperability and centralized management potential achieved through intelligent switches.
Ensure that the storage virtualization hardware and software support each other. This is not an issue for array-based or appliance-based virtualization where the hardware and software have already been mated for you. This is a critical concern with host-based and fabric-based virtualization where you need to choose compatible platforms. For example, you might select a Cisco MDS9000 SSM to run EMC Corp.'s Invista software in the switch. Server and switch hardware must meet the software's requirements and provide enough processing headroom to accommodate some amount of scaling into the future.
Consider the connectivity and protocol support. When evaluating hardware, it's also important to determine how it will actually connect to your local area network (LAN) or SAN and to ensure that it supports the protocols currently being used by the network. As an example, the FM5500 file manager from Attune Systems Inc. offers ten 1 Gigabit Ethernet (GigE) ports. By comparison, the iQ2280 storage system from iQStor provides four 4 Gbit Fibre Channel (FC) ports. Protocol support can include CIFS, NFS, TCP, UDP, SNMP and so on.
Upfront planning is essential. Virtualization should not be attempted without a comprehensive deployment plan that addresses hardware preparation, software installation, physical installation and interconnection, setup and configuration, and testing. The virtualization vendor can generally assist with these considerations prior to purchase. Remember that physical installation and subsequent setup will cause some downtime and service interruptions for your users, so installation may be reserved for evening or weekend hours. Have a fallback plan in place, so that the virtualization can be removed or reversed if unexpected problems arise.
Evaluate disruption levels. Upgrades and changes to virtual storage assignments sometimes result in service disruptions for users and applications. For example, an application may be temporarily unavailable while allocated disk space is being increased. An increasing number of storage virtualization products are focusing on nondisruptive techniques for upgrades and changes. The Maestro File Manager appliance from Attune is one product that claims transparent upgrades, expansions and migrations. Use lab testing to determine the actual disruptions that occur during file server migrations, upgrades and other maintenance. Excessive disruption may prompt you to select an alternate product.
Consider the security features. Your storage virtualization platforms should always be protected by security features, such as usernames and passwords or other forms of authentication, such as Kerberos. Products like the V-Switch 3800 from Sanrad Inc. supports ACLs, SRP, RADIUS and iSNS. Security features protect data within the SAN and prevent unauthorized changes to storage allocation or service levels -- especially in the fabric where switching can impact all network traffic. Evaluate the security features of your virtualization product and be sure to proactively implement security. Part of any security should also include a comprehensive change policy to help storage administrators prioritize and manage changes to virtual storage resources.
Evaluate performance and latency. Storage virtualization platforms can sometimes experience latency when accessing directories -- especially directories with many small files. Some storage virtualization products attempt to compensate for this latency by using file acceleration techniques. Others may handle performance issues using tactics like volume rebalancing and migrating lightly used files to other storage resources on a lower tier or another virtual volume with lower utilization. Ultimately, it's important to test any virtualization system and see how it performs under actual load conditions.
The storage virtualization hardware product specifications page in this chapter covers the following products: