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!


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

how about ugly OS crash because of something... bug in drivers, bug in kernel of the os?

they happen rarely but still.


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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