Raid channel etiquete?

[DIRTsquirt]

I have a mb with onboard highpoint raid....
I am currently using it in raid 0
with each drive by itself.. as master on seperate channels.
each channel should be able to support 2 drives.. is there any way I can make use these additional 2 channels

I only ask because I was in compusa talkin to a sales dude.. and he has the exact same mobo as I and he said
he is running raid 0.. and he has both drives and one controller channel. and the other is empty...

I guess the bottom line is can i add more drives without adding a pci controller card

 

[Lord Evermore]

You can but it will reduce performance as far as I know. The two drives on one cable can't be operating independently, only one can be transferring data at a time, it's part of the ATA specification. So if you were striping, you'd end up with the performance of a single drive on the cable because each write would have to alternate between drives. With the drives on different cables, they can both be writing their share of data at the same time. To the best of my knowledge, the Promise, Highpoint, and other controller chips don't get rid of that limitation. This is one of the reasons Promise has a TX4 card line, with 4 independent channels.

Putting another drive on the channel that isn't part of the RAID array would reduce performance too, since anytime that drive is being used, the other drive on the channel has to alternate with it.

 

[DIRTsquirt]

Thanx for the Clarification... I thought it would go something like that..

 

[Bglad]

Not exactly true. Because a 4 drive raid array with 4 drives on the 2 raid channels is faster than a 2 drive array. Not twice as fast, the performance increase sees diminishing returns, but it is faster. If they had to completely alternate, the performance of 4 drives would be identical to that of a 2 drive array. I agree you definitely should not put both drives on the same channel but I'm not sure the argument is exactly correct.

 

[Lord Evermore]

The only thing that can be sent to the one drive while the other is active is commands, which of course is optimized by the fact that the controller knows what commands to send ahead of time, but still the actual data transfer can't occur at the same time. So essentially the two drives on one channel would result in performance similar to a single drive on the channel, depending on exactly how the array is configured. It can't be any faster though.

In cases with a prolonged period of random reads, the throughput might go slightly over the single drive throughput, because there can always be data coming from one drive while commands are sent to the other, so there's never a period of no data coming while a command is sent. But this would be unnoticeable, probably wouldn't even go outside the standard deviance of benchmarks, for the short periods most people have random reads. In the case of a sequential read, only one command needs to go to the drive to read an entire sequence, even for a large file (contiguous of course) or several files.

Possibly also there could be some performance gain due to the wait period allowing a cache to be filled so it can burst to the controller, rather than streaming from the disk. As one drive is transferring, the other could be loading up the cache. It probably wouldn't happen with every alternation, but possibly every other one.

Performance with a striped array on one channel would pretty much be exactly the same as a mirrored array, since with both types the drives alternate their data transfers.