Redundant Power Supplies (RPSs)

A number of server chassis are designed to use, either as standard or as options, RPSs. Essentially, an RPS is two or more power supply modules plugged in to a common connection to the motherboard, drives, and other devices. If the primary module fails, the reserve module takes over.

Figure 14.4 illustrates a typical RPS designed to fit in place of a standard SSI power supply. Other types of RPS units have form factors suitable for use in various sizes of rack-mounted servers.

Figure 14.4. A typical redundant power supply that can be used in place of a standard SSI power supply.

Preconfigured servers from major vendors often include RPS capability as either a standard or optional feature in both pedestal and rack-mounted server form factors. However, you can also retrofit existing pedestal, tower, and rack-mounted servers with an RPS. Note that standard power supply vendors do not typically manufacture RPS designs. The following are some RPS vendors:

• CyberResearch (www.cyberresearch.com)

• Orbit Micro (www.orbitmicro.com)

• Rackmount.com (www.rackmount.com)

Some RPS units use a common AC power source for all modules. While this type of design provides protection against power supply module failure, it does nothing to protect against AC power interruptions. This type of RPS should be used only if a UPS unit is used with the server.

A better design, such as the one shown in Figure 14.4, uses a separate AC power source for each module. This type of design is especially valuable if you can connect each module to a different circuit. In the event of a power failure to one circuit, the RPS can continue to receive power from the other circuit and can continue to operate. For maximum protection, a separate UPS should be used for each AC power source.

Note

Hewlett-Packard offers a useful document that outlines best practices for providing power to a server that contains RPS units. You can download "Creating High-Availability Power Solutions for the ProLiant ML570 Server: Best Practices" from http://h200000.www2.hp.com/bc/docs/support/SupportManual/c00257380/c00257380.pdf.

You need to be sure that any RPS solution you use can be hot-swapped. Hot-swapping means that you can remove the failed RPS module and replace it with a spare without shutting down the server.

High-quality RPS modules usually indicate a failure via an alarm (audible, visual, or both). Most server management programs are designed to alert you to a failed RPS.

Power Supply Sizing

Whether you are using an RPS or a standard power supply in your server, it's important to make certain it provides enough power to operate the internal components inside your server as well as those that receive power from the server's USB or IEEE 1394 ports. The calculations used to determine the size of the power supply are similar to those used to determine the appropriate rating for a UPS unit (refer to Table 14.2, earlier in this chapter). The differences include the following:

• Power supplies do not need to provide power to displays or to most other external devices, such as tape backups or external SCSI RAID arrays, so the wattage requirements for those devices are not included.

• Power supplies do not always operate reliably under full load. For safety's sake, it's best to select a power supply that provides at least 20% more wattage than the calculated requirement. For example, if you determine that your server's components need 350 watts of power, a high-quality 425 watt or larger power supply would be recommended.

Table 14.3 provides a typical example of calculating the power supply requirements for a dual-processor server.

Table 14.3. An Example of a Dual-Processor Server's Power Supply Requirements

Component	Maximum Wattage
Motherboard	25w
Xeon 3.0GHz CPU (Socket 604)1	92w
Xeon 3.0GHz CPU (Socket 604)2	92w
3.5-inch floppy drive	5w
Processor fanCPU 1	3w
Processor fanCPU 2	3w
Keyboard and mouse	3w
PCI video	20w
DDR memory1	10w
DDR memory2	10w
DDR memory3	10w
DDR memory4	10w
SCSI hard disk drive1	25w
SCSI hard disk drive2	25w
SCSI hard disk drive3	25w
SCSI host adapter card	25w
Rewritable DVD drive	25w
PCI NIC	4w
120mm system fan1	2w
120mm system fan2	2w
Total wattage (TW)	408w
Wattage with 20% safety factor	490w

Note that each module in an RPS must provide the wattage rating needed. For example, in the calculation in Table 14.3, a RPS for the sample system would need two 490w or higher-rated modules. Note that standard power supply sizes are 300w, 350w, 400w, 425w, 450w, 500w, 510w, 550w, 600w, and higher. You should use the next higher-wattage power supply if your calculation does not exactly match a standard size.

Tip

To make power supply sizing easier, try these resources:

• PC Power and Cooling provides a list of wattage ranges for components at www.pcpowercooling.com/technology/power_usage/ and an interactive power supply selector at www.pcpowercooling.com/products/power_supplies/selector/.

• Tom's Hardware provides a list of typical internal devices and wattage ratings at www.tomshardware.com/2002/10/21/inadequate_and_deceptive_product_labeling/page3.html.