On Sun, May 25, 2008 at 10:47 AM, Mark Lord <liml@rtr.ca> wrote:
...
quoted text
> Sure, if the bus and memory are fast enough.  You'll hit the Linux/libata
> transactions/sec limit at some point, but I don't know what that is.
>
> Eg. Here's a 4X (I think) card in my PCIe video slot:
>
> beefy:~# mdadm --create /dev/md0 --chunk=256 -l 0 --raid-devices=4
> /dev/sd[bcde]
> mdadm: array /dev/md0 started.
>
> beefy:~# hdparm -t --direct /dev/sd[bcde] /dev/md0
>
> /dev/sdb:
> Timing O_DIRECT disk reads:  218 MB in  3.01 seconds =  72.46 MB/sec
>
> /dev/sdc:
> Timing O_DIRECT disk reads:  256 MB in  3.01 seconds =  85.00 MB/sec
>
> /dev/sdd:
> Timing O_DIRECT disk reads:  192 MB in  3.01 seconds =  63.79 MB/sec
>
> /dev/sde:
> Timing O_DIRECT disk reads:  256 MB in  3.01 seconds =  85.07 MB/sec
>
> /dev/md0:
> Timing O_DIRECT disk reads:  768 MB in  3.00 seconds = 255.69 MB/sec

The bottle neck here is with the four disks.
Modern 1TB 7200 rpm disks have raw throughput rates up to around
100MB/s. Anything in the last 4-5 years should do better than 60MB/s.

In theory, ~16 disks (4 disks per 7042 port using Port Mulitpliers),
one should be able to hit 200+ MB/s per 7042 port and approach 900MB/s
total. Getting more than 900Mb/s will depend on the chipset. IIRC, the
MMRBC of 128 bytes will result in about 15% protocol overhead in the
PCI-e link.
Bigger PCI-e "burst" will result in dramatically less overhead (IIRC,
256Byte == 9% overhead.)

hth,
grant
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