On Fri, 2010-06-11 at 13:58 +0400, Andrey Kuzmin wrote:
> # dd if=/dev/zero of=/dev/rdsk/cXtYdZs0 bs=512
>
> I did a test on my workstation a moment ago and got about 21k
> IOPS from my sata drive (iostat).
> The trick here of course is that this is sequentail write with
> no other workload going on and a drive should be able to
> nicely coalesce these IOs and do a sequential writes with
> large blocks.
>
>
> Exactly, though one might still wonder where the coalescing actually
> happens, in the respective OS layer or in the controller. Nonetheless,
> this is hardly a common use-case one would design h/w for.
No OS layer coalescing happens. The most an OS will ever do is "sort"
the IOs to make them advantageous (e.g. avoid extra seeking), but the
I/Os are still delivered as individual requests to the HBA.
I'm not aware of any logic in an HBA to coalesce either, and I would
think such a thing would be highly risky.
That said, caching firmware on the drive itself may effectively
(probably!) cause these transfers to happen as a single transfer if they
are naturally contiguous, and if they are arriving at the drive firmware
faster than the firmware can flush them to media.
- Garrett
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