RAID battery-backed cache - necessary?

Eric Rostetter rostetter at mail.utexas.edu
Wed Feb 10 21:57:30 CST 2010


Quoting Adam Nielsen <adam.nielsen at uq.edu.au>:

> Ahh, okay, so it's not there for pure write speed as such, it's there so
> that software can be told "yes, the data you just wrote is now on disk
> no matter what" even though the actual disk write may happen at a later
> time.

Yes, all true.  But it is also used by some to do write combining or write
ordering or such (try a google search like "raid5 small write problem" for
info).  And it is also there so that it can be used for parity calculations
on raid sets that use parity.  And it is also there so that if you lose
power while writing out a parity stripe, such that the data is written but
the parity isn't written when the power goes out, it doesn't create a
"corrupted" stripe that can't be recovered on a disk failure (try a google
search on "raid write hole"  for info).

And I'd bet there are other reasons too.

> I guess although a UPS would stop the OS cache from going away, it can't
> help if the kernel crashes.

Or when the UPS stops on a prolonged power outage.  RAID batteries will
generally go for weeks or months without power...  UPS batteries usually
go for minutes or maybe a few hours without power...  Sometimes power is
out for a while...

> Well that answers my question - thanks :-)

At least partially... :)  There's usually more to these things than
meets the eyes...

> Cheers,
> Adam.

-- 
Eric Rostetter
The Department of Physics
The University of Texas at Austin

Go Longhorns!



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