UPS For Your SAN - Do You Have The Power? Page 2


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UPS provision needs to be an integral part of the SAN strategy, not an afterthought. Monitoring and control of the UPS by the network manager is essential. This can range from software which shuts down the SAN hardware in an orderly fashion, to a fully interactive Simple Network Management Protocol (SNMP) communications (which makes the UPS an intelligent part of the hardware). This allows the network or IT manager to monitor power conditions on the hardware and be alerted instantly to any potential problem areas. Such software should be easy to use, install and integrate seamlessly with existing network operating systems.

System Configuration

There are four classes of power solutions designed to meet the protection requirements of most SAN hardware. These are:
  • One-on-one.
  • Clustered protection.
  • Integrated protection.
  • Facility-wide protection.


One-on-one means that each SAN node which needs protection gets its own UPS. The server will have its own larger and more sophisticated UPS than the workstations or peripherals, but the one-on-one principle still applies.

Clustered Protection

Clustered protection protects all SAN nodes in a single room. In other words, it protects a "clustered" group of servers (and other network equipment) by means of a single larger UPS.

Integrated Protection

Integrated protection "integrates" the power protection within a cabinet enclosure or raised floor system. It is termed integrated because many protection functions are combined into one system including system control/monitoring, air conditioning, security, wire management, smoke and fire alarms.

Facility-Wide Protection

If nearly everyone has their own computer and SAN servers (and nodes are located throughout an office), the best solution is often a large-scale UPS, which can provide protection to an entire floor or facility. The ideal time to plan this is when a building is being newly built or restored.


UPS design can employ a number of different topologies. These include:
  • Offline.
  • Online.
  • Line interactive.


Most SAN hardware now uses what is known as switched-mode power supplies, which can ride out very short gaps in the main power supply, because they store a small amount of charge in their large capacitors. This means they may be able to use off-line UPS units, as there is inevitably a short switchover time (measured in milli seconds) between a mains failure and transfer to the UPS battery.

Used almost exclusively at the low end of the UPS power spectrum, offline UPSs are the simplest and most cost-effective option. They supply partially filtered main power to the load (that is, there is virtually no power conditioning), but keep a charged battery in reserve. When the main supply falls below a certain voltage level, a switch in the UPS connects the battery to an inverter, which then converts the DC of the battery to an AC supply which can be used by the SAN hardware's power supply. In terms of applications, offline UPSs are best used where the SAN hardware being protected have low power ratings and are not mission critical.

Online UPS

Online UPS, in contrast to offline units, ensure that there is never a break in the power supply even when there are power disturbances. To do this, they use a double-conversion technique, which continuously converts the AC main power supply to DC (which is also connected to the battery), before feeding it through an inverter to turn it back into AC for the SAN hardware load. This ensures there is no switchover time if there is a power failure; and, it also compensates for brownout (reduction in line voltage) problems which cannot be handled by offline designs. In many cases, the main power supply does not fail suddenly, but tail off over a number of cycles. As the main power supply voltage drops off, the slack is taken up by the battery on an online UPS. The inverter does not detect any difference, even though the supply is a mixture of DC derived from the main power supply and battery. Once the main power supply fails completely, all the power is supplied by the battery. As far as the SAN hardware is concerned, there is no change in the supply and the sine-wave output is always synchronized with the main power supply.

Line interactive

The line interactive UPS is hybrid of online and offline, which gives power protection, plus some line conditioning to inhibit spikes and waveform problems. In this design, battery charging is provided by operating the inverter in reverse during times when the input AC power is normal. If the input power fails, a transfer switch opens and the power flows from the battery to the UPS output. As the inverter is always connected to the output, it provides additional filtering, and eliminates the brownout switching found in off-line designs. These factors make the line interactive a better solution than offline UPS, and a cost-effective alternative to online, where power conditioning is not an issue.

Choosing The Right UPS

When choosing a UPS, match the unit's capacity to your needs. UPS vendors (like American Power Conversion (APC), Clary, Falcon Electric, Liebert (Emerson), MGE UOPS Systems, ONEAC, OPTI-UPS, Powerware, Tripp Lite, Tsi Power, etc.) provide volt-amperes (VAs) power ratings to indicate the maximum amount of power provided when AC power is present. Some UPSs might provide less power than their VA rating suggests, because of losses incurred during the conversion from AC to DC. So, it is also important to verify the UPS's output wattage. The wattage ratings listed on your SAN hardware might not accurately represent the amount of power your system uses. You should either measure your SAN hardware's actual power draw or select a UPS with a wattage rating that significantly exceeds the wattage rating on your equipment. The UPS's output wattage rating might not have any relevance to the unit's battery runtime under your projected power requirements. Carefully read the vendor's battery life specifications and consider the typical length of the power outages in your area.

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