On Montag 02 November 2009 Justin Piszcz wrote:
> I am also curious, I have a 16 drive RAID-6 configuration on a
> 9650SE-16ML and using -o nobarrier or mounting normal the
> speed/benchmarks seemed to be the same. Either barriers are not
> enabled by default for 3ware RAID arrays or they make no difference
> in performance?
I'd say a RAID controller with it's own write cache enabled + BBM will
effectively turn barriers off, even if you can use them as a mount
options. What happens with barriers, is that it writes
1,2,3,barrier,4,5,barrier,6 etc. so, that 123 are sure on disk before 45
The RAID controller will happily tell you it did flush everything,
because as soon as data is in it's cache, it's claimed sure that the
data gets written, and therefore it will tell that the barrier is
already done. And that's why it's a *must* to turn off disk cache
writes, because the filesystem got it's barrier ACK and believes it, the
controller has it's cache written to disk, powerfail.... all gone. That
will do a bigger damage than any single disk could have done.
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