SATAII And PCIe RAID

[matonb]

I may be wrong and I'm sure you let me know if I am

As far as I know, SATA drives each require a PCIe lane.

So my question is if a 16 SATA Port PCIe RAID Controller has 8 PCIe Lanes, how does it manage 16 SATA drives without causing a bottleneck if it's sharing lanes?

 

[Vorador2]

Err, only PCIe slots for video cards have a maximun of 16 lanes. The other slots have a maximun of 4 lanes. And you're wrong, since there's no relationship between the number of SATA drives and the PCIe lanes, even if the controller is on a PCIe slot.

 

[Wolfshadw]

A PCI-E RAID Controller Card, whether it's x1, x4, or x8 will utilize the bandwidth available from the slot it's inserted into. So long as the card is physically compatible with the slot (for instance, a PCI-Ex4 card that's physically compatible with a x16 slot) will utilize the bandwidth available from the slot up to the card's maximum limits.

As Vorador2 already mentioned, the number of drives plugged into the controller card is irrelevant.

-Wolf sends

 

[matonb]

Not talking about a Desktop PC Hence the 8x.

If as you say the PCIe lanes are completly independent from the SATA drives (which makes perfect sense) then the theoretical maximum through put for 16 SATAII drives will exceed the available bandwidth on 8 PCIe Lanes

8x PCie 40Gbs
16x SATAII 48Gbs

Granted in a real world application it's very unlikely that you'll ever reach bandwidth saturation as the bottleneck will be IOPS.

Still, if you were to drive it to the hilt with a sequential read or write you can't meet the discs maximum performance...

(I'm asking and answering myself questions as I go along... #-o )

 

[TeraMedia]

You won't hit sequential Read speeds anywhere near 3 Gbps on a single SATA drive, even with an SSD running parallel flash cards. You'll only hit that speed if you repeatedly read and write the same small block, so that you're always hitting the drive's 8 MB or 16 MB RAM cache... at which point the O/S would likely cache that data in RAM anyway.

Unless you start to talk SCSI / SAS, the best drive sequential read speeds out there are < 100 MB / s (figure 1.0 Gbps if 10 bits per byte w/ low-transition codes over SATA and PCIe). So a 4x card at 20 Gbps will still be enough for 16 SATA drives at max sequential read speed.

 

[matonb]

Interesting performance figures TeraMedia.

I've not had much reason before now to bother with SATA performance.
Do you do much in the way of SATA benchmarking ?

We're getting one of these shortly, although we've changed the configuration to 16 x 500GB SATAII Drives to increase the spindle count.

It's cheap as attached storage goes (Test Environment), however I don't expect to see more than 1,000 IOPS out of it.