UPS For Your SAN - Do You Have The Power?
November 30, 2002 By John Vacca
Your enterprise cannot afford to lose its most precious resource: stored data hardware. The fact is, that power problems are the largest cause of data loss (about 56.4%) on storage area networks (SANs) hardware: Other problems consist of:
Power outages interrupt operations at 83% of US enterprises.
44.8% of U.S. enterprises have had business operations interrupted because of lightning storms.
42% of computer hardware outages are the result of power failures.
Like any other piece of network equipment, SAN hardware needs to be protected by an Uninterruptible Power Supply (UPS) system. With the preceding in mind, Part I of this two part article first discusses why you need a UPS; the different types of UPS that are available; how a UPS is rated; and, how you can evaluate what size of UPS system is needed (or how to calculate volt-ampere (VA) to get the required Kilovolt-Ampere (kVA) size). Part II, continues the UPS for SAN hardware theme by discussing a categorized and prioritized collection of SAN hardware powering needs and problems; avoiding costs from over-sizing a UPS system; UPS lifecycle cost imperatives; UPS rack powering options; power-monitoring software; UPS adaptability/scalability, availability, manageability, and serviceability imperatives; and, next generation UPS systems for SAN hardware.
Before starting the discussion of why you need a UPS, you need to know what it is and how it works. Lets take a look.
What Is A UPS
Power interruptions can cause severe repercussions for business computer systems. The problem is especially difficult in places where power outages and fluctuations frequently occur.
Electricity suppliers distribute what is known as alternating current (AC), at a voltage of 230 volts and a frequency of 50 Hertz. The only way of storing electricity, in a battery for example, is to convert the alternating current from the mains power supply into what is known as direct current (DC). Simply speaking, a UPS is a buffer-battery located between the power socket and the computer. It converts the AC mains power supply into direct current in order to charge the battery and then reconverts it back (to AC) in order to supply the load, in this case a computer and/or SAN hardware.
In other words, a UPS provides a backup battery and power inverter circuitry to insulate systems and data from power outages. In situations where momentary power fluctuations occur, a UPS provides constant power to keep your systems running. During extended power failures, a UPS provides backup power to keep your SAN hardware running long enough so that you can gracefully power down. The transfer time to backup is typically very short (usually rated in milliseconds) to maintain system and data integrity. Many UPSs provide filtration circuitry to eliminate voltage spikes, and some also deliver a constant voltage to your systems when the utility voltage surges or sags.
So what types of UPS are available? How do you choose the right UPS for your SAN hardware?
Types Of UPS
The technology employed in the modern UPS has evolved dramatically in the past few years and there is now a wide choice of UPS to suit every type of environment and SAN hardware system. It is no longer simply a choice between online and offline topologies. Line interactive devices have been developed which offer a hybrid solution.
There are a number of key areas which the IT manager needs to consider to decide which type of UPS will give he or she the optimal level of protection. These include:
Criticality.
Load Size/Battery Time.
Communications.
Configuration.
UPS Topology.
Criticality
The first question to be addressed by the IT manager is how critical the SAN hardware is which is to be protected? Or what would be the consequences to the enterprise if there was a power cut or disruption to the mains power supply which caused the SAN hardware to go down? The answer to this will depend on what type of SAN hardware is being protected and which storage applications are being run on the system. The loss of some storage applications could effectively paralyze a enterprise, while others may just cause a temporary inconvenience. For mission-critical uses, an on-line UPS is the preferred choice. However, protecting the SAN server is not the total solution. Hubs, routers, workstations, PCs and peripherals need protection as well. Even if their use is not mission-critical, an unexpected failure could cause a domino effect that brings down the whole storage area network (SAN). Using line-interactive UPSs to protect some SAN nodes can be an economical alternative, providing not only outage protection but also greater resistance to brownout conditions.
Load Size And Battery
Constructing an efficient UPS system requires far more than totaling the wattage of the equipment to be protected. Every piece of SAN hardware has its own power needs. A UPS which is ideal for a few PCs is probably not the right choice for an industrial storage application. Likewise, an enterprise-wide SAN with hundreds of nodes needs a different kind of protection than an in-bound telemarketing operation. The size and shape of a SAN can change significantly over the course of a year. Contingency needs to be built into the UPS plan from the start to cope with increasing loads. To calculate the load to be protected, companies should take the VA ratings from the back of all SAN hardware to be protected, add them up and select a UPS with a VA rating of at least 1.5 times the total. The extra capacity of the UPS is needed to handle any additional power demand when the SAN hardware is turned on.
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