raid 0 6Gb/s?

scotty365 · 19 Dec 2009 at 00:40

Does two drive in raid 0 increase to double the transmissible rate of 6 Gb/s (3Gb/s standard). I'm reading conflicting views.
SATA2 is 3 Gb/s so using two drives (in raid 0) doubles the speed?

~Scotty

drunkenmaster · 19 Dec 2009 at 01:04

Not entirely sure what your question is, does using raid double the POSSIBLE transmission rate for the single drive as seen by the OS, yes. Each uses a separate 3gb/s sata connection, if both drives could max out the connection then with two drives you'd be at aroudn 6gb/s.

No current single drives really approach these speeds so its not a real issue.

THe only thing you really have to be worried about is the controllers maximum speed, it might have 6 sata 2 ports, but basically no controller can actually support 6 sata drives all accessing at full speed all the time. But again 2 in raid, any remotely normal drives you can buy(so excluding those £1k + drives with multiple drives all raided in a case) won't max out any controller around now, 3 is fine, 4 can start to push quite a lot of current controllers and I believe even Intel's best struggles with 4 and beyond drives in raid 0.

That doesn't mean you can't use all 6 ports, in general you rarely use more than 1/2 drives at full speed at the same time so its never an issue.

scotty365 · 19 Dec 2009 at 13:05

Cheers,
A family friend claimed that more than 2 SSD (M225) drives in raid 0 can max out a 3Gb/s SATA, so the question really is - Does it double the bandwidth due to being plugged in a raid 0 config. Or is he simply talking cobblers?

ChileanLlama · 19 Dec 2009 at 13:14

He's talking cobblers, 2 M225s will hit around 500MB/s maximum for reads on an Intel ICHxR RAID controller. The Intel controller cap's out at around 660MB/s for all channels combined, so you'd hit that with 3 drives. That's for an onboard controller, with a dedicated RAID controller as Drunkenmaster says you can avoid that limit.

So yes you are getting more bandwidth than a single drive, but it's not maxing it out with 2 drives, as the individual drives are not close to the 300MB/s (per port) limit of SATA2.

scotty365 · 19 Dec 2009 at 15:41

Cheers!

Zarf · 19 Dec 2009 at 18:49

scotty365 said:
Cheers,
A family friend claimed that more than 2 SSD (M225) drives in raid 0 can max out a 3Gb/s SATA, so the question really is - Does it double the bandwidth due to being plugged in a raid 0 config. Or is he simply talking cobblers?

In short, the throughput is doubled, but your friend is wrong about why and where. None of the individual capabilities or bandwidths change, but the Controller accesses two drives at the same time and shares the workload over them - hence you can get twice as much done.

RAID0 (Striping) combines two (or more) drives into one logical device, which has a throughput equal to the slowest physical drive or SATA link of the array multiplied by the number of drives in the array. This holds true up until we are limited by RAID controller processing speed or the link from the controller back to the northbridge/equivilent.

The architechture looks something like this

Code:

Drive --- 300MB/s SATA ---|Controller, | --- Link to northbridge, usually at least 1000MB/s
Drive --- 300MB/s SATA ---| 660MB/s max|
empty --- 300MB/s SATA ---| Throughput |
empty --- 300MB/s SATA ---|            |