On Sun, May 25, 2008 at 10:47 AM, Mark Lord <liml@xxxxxx> wrote: ... > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
