Re: Unbelievably _good_ write performance: RHEL5.4 mirror

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>
> So, you're saying that the MD layer is doing it's own buffering?  Are
> you sure?  With the system cache disabled and the drive (and block
> device driver) cache disabled, there should be no reason to require
> synchronous I/O, unless, as you suggest, the MD layer is broken.
>
> You're saying that both O_DIRECT and O_SYNC must be used to disable
> cache effects. Why then are there two separate flags and not just one?
>  Synchronous is a very different behavior that is not necessary for
> this test and put additional requirements that are not needed for this
> test.
>
> Chris
>

Reading the manual page it seems O_DIRECT explicitly minimizes any
attempts at extra copying; not explicitly disabling buffers, merely
not adding more.  In another mail thread this tweak-able was
discussed:

echo 0 > /sys/block/md*/md/stripe_cache_size

Which would should (I think) disable any cache involved with the md layer.
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