[Gllug] Hardware comparisons

[Tethys]

"Martin A. Brooks" writes:

>t.clarke wrote:
>> Sorry,  I am curious - what are the merits of having a battery-backed up
>> cache on the RAID controller ?
>
>That's the only safe way to be doing write caching.  Sensible RAID 
>controllers don't let you turn it on unless there's a BBU present.

To elaborate on that a bit, when data is written, the controller
doesn't write it to disk directly, but to a (usually volatile) cache.
This increases performance because the controller can then report the
write as being complete without having to wait until the arm is over
the correct spot on the disk platter. The downside, of course, is that
in the event of a power failure, the contents of the cache are lost,
despite the controller having reported to the OS that they've been
successfully written to disk. The point of a BBU is simply that it
provides enough power to flush the contents of the cache to disk in
the event of power failure, so you don't lose any data. Thus you get
the best of both worlds -- high performance under normal usage, and
data integrity when something unexpected happens.

Tet
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