Re: 3ware hardware raid with battery backup and the impact on barrier and no write cache options.

[Michael Monnerie <michael.monnerie@is.it-management.at>]

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 
happen etc.
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.

mfg zmi
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