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Wikser
Asked: 2010-04-13 11:53:35 +0800 CST

Benefits of a RAID BBU in addition to a double UPS + PS system

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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!

cache ups raid write bbu

3 Answers

  1. 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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  2. Best Answer

    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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  3. 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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