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HD Upgrade: U320 SCSI or SAS?

 
 
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      04-03-2007, 12:46 PM
Hi All, it's time to swap out my hard drives and I'm wondering if
SAS would buy me any additional performance. Currently I'm
running an IC7-G MaxII with an Adaptec 29160, and three
non-raided Seagate ST336753LW's (one for the OS, one for
pagefile/data and the third for just data).

Looking at specs for Seagate's 15K.5 SAS drive, there doesn't
appear to be much (any?) difference in performance between it
and the equivalent U320 drive, and I'm also wondering if I'll run
into the 32-bit 33/MHz PCI speed limit (133MB/s) with SAS
drives? The IC7-G has built-in SAS, is it still subject to the
PCI limit?

Thanks in advance for any info.


 
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Paul
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      04-03-2007, 05:10 PM
(E-Mail Removed) wrote:
> Hi All, it's time to swap out my hard drives and I'm wondering if
> SAS would buy me any additional performance. Currently I'm
> running an IC7-G MaxII with an Adaptec 29160, and three
> non-raided Seagate ST336753LW's (one for the OS, one for
> pagefile/data and the third for just data).
>
> Looking at specs for Seagate's 15K.5 SAS drive, there doesn't
> appear to be much (any?) difference in performance between it
> and the equivalent U320 drive, and I'm also wondering if I'll run
> into the 32-bit 33/MHz PCI speed limit (133MB/s) with SAS
> drives? The IC7-G has built-in SAS, is it still subject to the
> PCI limit?
>
> Thanks in advance for any info.
>
>


I don't know anything about SAS, so I cannot comment on compatibility
with motherboards and the like.

http://en.wikipedia.org/wiki/Serial_Attached_SCSI

I can offer some comments about chipsets. The 875P and 865PE share
some features. The hub bus that connects Northbridge to Southbridge,
is limited to 266MB/sec. The storage devices connected directly to
the Southbridge, get to share the 266MB/sec. Which is why, if you
had an ICH5R, and a couple of Raptors, it was possible to get more
than 133MB/sec total bandwidth (but just barely). There was a picture
of a benchmark result on a private form, but the picture has disappeared,
so I cannot show you the result.

The 266MB/sec is shared for a number of functions. The storage in
the Southbridge all connects to it, but the PCI bus on the
Southbridge also shares that bandwidth.

The 32 bit 33MHz PCI bus coming from the Southbridge, is limited to
133MB/sec theoretical, and maybe 110-120MB/sec practical. If you have
a fancy controller plugged to the PCI bus, chances are the PCI bus
is the limiting factor. So you cannot even burst to the cache on
a U320 disk at full speed, as the PCI bus would limit the transfer
rate.

Even if you bought a SAS controller of some sort, and plugged it to the
PCI bus, you would still face that limit.

Motherboards with PCI Express slots are a much better alternative for
high speed storage experiments. For example, some motherboards with
SLI video setups, can take a video card in one big slot, and a
storage card in the other big slot. Some people use Areca RAID cards
for example, and get hundreds of megabytes per second in transfer
rate (with enough disks in an array).

Some motherboards also offer PCI Express x4 slots. You have to be a
bit careful with those, as some motherboard designs connect a full
four lanes (1GB/sec) to the slot, while other designs only connect
two lanes (500MB/sec). Still, that is a lot of bandwidth, compared
to the slowest version of PCI.

According to that Wiki page above, SAS is a point to point protocol.
Which means if you were using a RAID array of some sort, the bandwidth
of the drives could be added together, and there is no restriction
caused by the drive cabling. A U320 SCSI cable, would be cable limited
to 320MB/sec, for all the disks sharing on the cable. Since SAS is point to
point, each cable has its own bandwidth. But that only helps, until you
get to where the controller card meets the motherboard. If you have a
PCI Express x8 slot on the motherboard, that would be 2GB/sec, and a
controller could handle the bursts of 6 drives at 300MB/sec with that
kind of bandwidth. But whether real controllers can do that or not,
is open to question. You'd have to look for some benchmarks of
real SAS controllers, to decide whether they are that good or not.
(At least with SATA, there have been a couple of dismal performers
already, that were limited by internal chip issues, or driver latency
or interrupt issues, so this is an area where you want to see
benchmarks before you buy. The controller could be quite expensive,
and not something you purchase on a whim.)

There are also server or workstation style motherboards, which offer
more slot types. Some workstation boards offer PCI-X slots at 133Mhz
clock rate, and yet fit within an ATX computer case. So there are
plenty of possibilities, if you have a reason to build a high speed
array.

P5WDG2 WS Professional, 12" x 9.6", 2 PCI-X slots, 2 PCI-E x16 slots
http://www.asus.com.tw/products4.asp...3&l2=82&l3=249

Paul
 
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