Re: Linux Raid performance

[Richard Scobie <richard@sauce.co.nz>]

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