Lukas Beeler's IT Blog

When talking about hardware, the main difference between a “PC” and a “Server” is the amount of hardware redundancy the manufacturer has incorporated into it’s design.

• Disk redundancy
  Also called RAID (almost everywhere) or Disk Protection (in the System i world). Disk redundancy ensures that the loss of a single disk drive doesn’t result in loss of data. There are many ways in which can raid be implemented – starting with purely software solutions provided by the operating system (like in Windows Server and all Linux distributions), with solutions that use a part BIOS/part driver solution (SOHO/Consumer equipment because Windows Clients lack software RAID), to full blown hardware solutions incorporating a co-processor for checksum calculation (for RAID5/6) and a battery backed write cache.
  I would go as far and say “if it doesn’t have some form of disk protection, it’s not a server”. With software RAID, all you need are a few more disks.
• Memory protection
  Mostly called ECC / Checksum Memory / ChipKill memory. ECC ensures that defective memory can’t cause silent data corruption or system crashes. I don’t know of any server manufacturer which doesn’t ship their servers with ECC memory – i consider it an absolute must. ECC can usually recover from single bit errors (and write to the logfiles) and it can halt the system in case of a multiple bit error (and write to the management CPU log).
  There are newer technologies out like Memory Mirroring, which allows of whole banks of memory to fail, and recover without any downtime. This latter feature usually needs twice the memory, and is thus prohibitively expensive.
• Power redundancy
  Multiple power supplies are available as soon as you leave the lowest priced server segment. Power redundancy is a good thing for a variety of reasons. Having a day of downtime because of a blown power supply is not funny – a second power supply can help. A second power supply also helps you if your UPS has a problem – this is actually the most common situation where a 2nd PSU helped me – with broken UPSs happening more than power downs (at least here), a second PSU is an insurance that a defective UPS can’t bring down your production server. Of course this doesn’t work if you pull the second PSU into the same UPS – plug it into the wall, or into another UPS.
• Cooling redundancy
  Some server manufacturers ship their secondary PSU together with a redundant cooling kit. Redundant cooling is as important as a second power supply, because downtime because of a single blown fan is embarassing. Most fans are hot pluggable, allowing you to keep the server up and running even when replacing the broken one with a spare.
• CPU redundancy
  This is a nice add-on feature. Most 2 CPU machines support an automatic reboot to 1 CPU when one of the CPU fails – of course you don’t buy a second CPU just for this, but it’s really nice to have if you have 2 CPUs anyway.

There are many more hardware redundancy techniques, but most of them are not meant to be used in a small business. Things like Multipath IO, fail over blades, etc. are just far too expensive.