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SATA bandwidth vs. SCSI bandwidth

Last response: in Storage
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July 26, 2004 12:44:06 AM

When one is talking about a multi drive configuration - employing more than one HDD in one's rig - performance may be a critical factor. We all know that in, say, a 4 SCSI RAID 5 array, the RAID controller can communicate with all the HDD's simulataneously. In other words, the bandwidth is being shared at the same time. But what about a SATA RAID configuration? I'm pretty sure that each drive has their own bandwidth, due to one channel per drive. But if a SATA RAID controller (software or hardware) can communicate with each device simulataneously, then the bandwidth would be shared. Basically, I'm trying to find out if SATA HDD bandwidth can be shared, a la SCSI HDDs.

Thank you in advance for any input.
July 26, 2004 3:41:30 AM

Since each drive currently gets its own channel, the bandwidth of all channels is routed to the controller simultaniously. So the controller is the limiting factor, or the bus it's on, depending on which is smaller.

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July 26, 2004 1:29:13 PM

In other words, if one is running a 4 HDD SATA RAID, via hardware, on PCI-X, each HDD can read/write a theoretical 528 MB/s, regardless if all 4 or just 1 of the drives is busy. As opposed to a 4 HDD SCSI RAID implementation, where the 528 MB/s transfer would be divided among the four if all four are busy at the same time.
July 26, 2004 8:59:05 PM

With SATA, 2 SATA 150 drives could theoretically transfer 150MB/s EACH, so the in theory a Level 0 array of 2 drives could tranfer 300MB/s. But theory is different from practice, where SATA150 drives seem capable of only 102MB/s max. So 2 would still get 200MB/s combined, in theory, from a Level 0 array. Except that Level 0 isn't 100% efficient, it really only gives you about a 50% performance boost. So in reality both drives would act as one drive with 150MB/s transfer.

So theory vs reality, in theory 2 SATA150 drives in level 0 would act like 1 300MB/s drive. But in practice, they'd really only transfer 150MB/s. Still a 50% boost from 1 drive which only transfers 100MB/s.

That would likely make your RAID0 array with 4 drives around 300MB/s.

If PATA drives were available with the same mechanical specs as Raptors, 2 drives would give you 150MB/s as well, but 4 drives would also give you 150MB/s, because you're dealing with 2 channels. Hence the advantage of SATA having separate channels for each drive.

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