Drew wrote:
The RAID array is 16 devices attached to a port expander in turn attached to
a SAS controller. At a most simplistic level, surely the SAS controller has
overhead attached to which drive is being addressed.
Don't forget that with a port expander is still limited to the bus
speed of the link between it and the HBA. It doesn't matter how many
drives you hang off an expander, you will still never exceed the rated
speed (1.5/3/6Gb/s) for that one port on the HBA.
I'm not sure if you are thinking of a port multiplier instead of a port
expander.
In any case, in my setup, there are 4 x 3Gb/s lanes connecting the HBA
to the port expander and each drive is connected to it's own port on the
expander at 3Gb/s ( obviously each drive is not streaming at 3Gb/s).
So there is plenty of bandwidth there.
Say you have four drives behind an expander on a 6Gb/s link. Each
drive in the array could still bonnie++ at the full 6Gb/s but once you
try to write to all four drives simultaneously (RAID-5/6), the best
you can get out of each is around 1.5Gb/s.
4 x 100MB/s (average outer edge speed of SATA drive) = 3.2Gb/s which is
less than 6Gb and works out to 0.8Gb/s to each drive, so I'm not sure
what you mean here.
Regards,
Richard
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