Nehalem Mac Pro:
How Fast is Internal RAID 0?
Posted Tuesday, April 17th, 2009, by rob-ART morgan, mad scientist
One of our strategic partners was experimenting with a Solid State Disk (SSD) RAID set. He noticed that the speed was lower on the 2009 Nehalem Mac Pro than on the 2008 'Harpertown' Mac Pro. So we did some experimentation of our own and learned some fascinating things.
First, the data...
LEGEND OF GRAPHS
Harper = 'early 2008' Mac Pro 3.2GHz 8-core 'Harpertown'
Nehalem = 'early 2009' Mac Pro 2.93GHz 8-core 'Nehalem'
SSD = Intel X25-E 32G Solid State Drive
x2, x3, x4, x6 = number of drives in the RAID 0 set
1. The maximum average transfer speeds of internal SATA RAID 0 sets are limited on the both the 2009 'Nehalem' Mac Pro and previous Mac Pros. This may be due to the fact that the I/O hub has only 1000MB/s to
disribute among all I/O devices including USB and FireWire. In our testing, we saw a max 725MB/s READ no matter how many SSDs we threw at it. Keep in mind that the Intel X25-Es that we used clocked 268MB/s READ as single drives, so theoretically, four should achieve 1000MB/+. Six have the potential to go 1500MB/s.
I know what you are thinking. "How did you mount six SSDs inside the Mac Pro?" MaxUpgrades makes custom sleds for mounting 2.5" drives in the four drive bays for all models of Mac Pro including the 2009. As for the fifth and sixth SSDs, in the case of the Nehalem, we popped out the dual optical drive module and used the combined data and power cables to connect to fifth and sixth SSDs. MaxUpgrades makes a special kit for permanently mounting two of them in the lower optical bay. More on that later.
2. Three SSDs can READ faster than either four or six. Huh?
3. The 2009 Nehalem is indeed was slower than the 2008 Harpertown when it came to sustained WRITE speeds. With six SSDs, the Nehalem was slower than with four SSDs. The Harpertown, on the other hand, kept climbing in write speed as drives were added up to six. Huh?
We happen to know that a different I/O hub chip is being used in the Nehalem than what is used in the 2006-2008 Mac Pros, but, according to the specs, it should be just as fast as the old one. But that's not what our tests show.
4. Who needs more than 725MB/s? Some would say that if you used normal 7K HDDs, they would never hit a "brick wall" like we did with the multiple SSDs. That may be true but Mac power users are always pushing the limits with extreme setups like an SSD RAID 0 set (e.g. for use as a very fast Photoshop scratch volume).
5. This was not a new discovery. When I shared my findings with a professional who builds custom Nehalelm servers running Vista 64, he said, "Everybody in the Windows world knows about that bandwidth issue. If you want to go faster, get a PCIe RAID host adapter!" But there's a problem there, too.
Apple has introduced a new Pro RAID card that's trick. It does not require a mini-SAS cable connection to the motherboard like last year's RAID card. It signals the motherboard of its presence in slot 4 and takes over the control of the internal drives.
That's "green" as Ruby Rhod would say in Fifth Element, but, according to Apple's own info, it's limited to 550MB/s -- even slower than the internal SATA controller. Arghh. There are third party SATA/SAS host adapters with the Intel IOP348 and internal mini-SAS ports that go up to 1200MB/s, but they CAN'T be used in the Nehalem Mac Pro -- at least until Apple shares the secret to their cable-free trick.
So that leaves you with one choice at the present. If you need to go faster than 725MB/s READ and 608MB/s WRITE, get a PCIe SAS or SATA host adapter with external ports. One example is the HighPoint RocketRAID 4322 we recently tested on the Nehalem connected to an 8 drive SATA enclosure. It achieved 800MB/s with "ordinary" 7K SATA HDDs and 1000MB/s with 15K SAS HDDs.
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