Re: Drive performance bottleneck

[Andrew Morton]

Lincoln Dale <ltd@xxxxxxxxx> wrote:
>
> sg_dd uses a window into a kernel DMA window.  as such, two of the four 
> memory acccesses are cut out (1. DMA from HBA to RAM, 2. userspace 
> accessing data).
> 1.6Gbps / 2 = 800MB/s -- or roughly what Ian was seeing with sg_dd.

Right.  That's a fancy way of saying "cheating" ;)

But from the oprofile output it appears to me that there is plenty of CPU
capacity left over.  Maybe I'm misreading it due to oprofile adding in the
SMP factor (25% CPU on a 4-way means we've exhausted CPU capacity).

> DIRECT_IO should achieve similar numbers to sg_dd, but perhaps not quite as 
> efficient.

Probably so.

There are various tools in
http://www.zip.com.au/~akpm/linux/patches/stuff/ext3-tools.tar.gz which are
more useful than dd, btw.  `odread' and `odwrite' are usful for this sort
of thing.


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/