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Today I roughly measured the benefits of enabling write-back on the RAID controller on a server at work. It got no RAID battery-backup-unit (BBU) so the write-cache is currently disabled. As the server is not used to capacity (by far), the results in most test were spectacular, e.g.:

  • Database CRUD: before 35s, after 4s
  • Saving a 1MB Excel file: before: 20s (!), after: 0.5s

Of course having a BBU is always recommended, but what are the main benefits of installing a BBU to a system, which got redundant power supplies and is attached to UPSs? Does this depend on the type of the system (database, file, terminal)?

What is a realistic fail scenario which could be prevented by a BBU?

Thanks in advance!

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3 Answers

up vote 3 down vote accepted

A RAID battery back-up protects you from:

  • Building power failures (not as big an issue with dual-UPS)
  • PSU Failures (not as big an issue with dual-PS)
  • Utility failures (Do you have a generator? What happens when your UPS runs flat?)
  • Generator failures (I've seen these happen; again: What happens when your UPS runs flat?)
  • Breaker trips (single if you only have one power leg; double less likely)
  • Fat Feet tripping over your power cords
  • "Oops, I pulled the wrong breaker!"
  • "Oops, dropped my screwdriver onto the power bus!"

etc. etc. etc.

How likely any of these are in your environment is something only you can determine. I have seen every single one of them in a production scenario though, so they're all possible :)

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haha @ Ooops, dropped my screwdriver onto the power bus! ... I would also add transfer switch blowing up when it tries to switch over or UPS batteries catching on fire ... i've seen all of the above as well. –  Zypher Apr 12 '10 at 20:10
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Or the Emergency Power Off feature of your fire-system dropping power from the entire room for safety reasons. I haven't seen an EPO actually trigger, but I've been in datacenters that had them. –  sysadmin1138 Apr 12 '10 at 20:17
    
Good point on the EPO -- I've been in a lot of datacenters that have them but never been (un)fortunate enough to see one go off. (I've also never seen the UPS batteries or ATS blow up/catch fire, but I have a wonderful story about a generator and 5-foot flames from the exhaust... :) –  voretaq7 Apr 12 '10 at 21:59
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how about ugly OS crash because of something... bug in drivers, bug in kernel of the os?

they happen rarely but still.

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From my experience, unless the server is receiving power from different PDUs, I can't really consider redundant power supplies a complete solution.

The write cache does not commit data instantly to the disk, so if there is a power failure you will lose all the data in the cache without a BBU.

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