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THE PCI STORAGE REVOLUTION
|June 25, 2009
HOW IT WORKS
Using a simple host adapter card plugged into a multilane PCI Express
slot in the host computer's mother board, we create an external PCIe
I/O port for cable attachment. This is not a bus bridge, it's just a
pass-through method of allowing cable attachment to the host PCIe bus.
We then expand the PCIe bus by an external cable to a data storage
system containing a PCIe-to-SAS/SATA adapter. Normally, that adapter is
a "hardware RAID" controller having its own high-speed processor and
enabling RAID functionality for high performance and fault tolerance.
The connection speed from the PCIe host to that storage system RAID
controller is the full bandwidth of the PCIe bus to which it is
connected, which ensures the utmost in data throughput rates.
First-generation PCIe has a bus speed of 2.5GHz and bandwidth of
500MB/s per "lane." Today, common PCIe slots are x4, x8 and x16, or
4-8-16 lanes. At 500MB/s per lane, the ubiquitous x8 (eight lane) slot
provides 4,000MB/s maximum transfer rates in theory. In practice, we
can achieve about 3,200MB/s maximum using real-world hardware.
Second-generation PCIe has a bus speed of 5.0GHz and double the
bandwidth, or 1GB/s per lane. Thus, a second-generation PCIe x8 slot
can provide up to 8,000MB/s theoretical transfers. This is faster than
can be provided by 8Gb/s Fibre Channel, 12X InfiniBand, 10GigEthernet,
or any other external bus architecture.
The resident hardware PCIe RAID controller, co-located with the storage
disk subsystem, provides disk drive management without burdening host
CPU resources like a software RAID system would.
By controlling small batches of disk devices, e.g., eight disks per
RAID controller, we can maximize controller performance (throughput) to
provide near theoretical maximum data transfer rates to and from the
disk devices themselves. Eight 7,200rpm SATA drives with 80MB/s maximum
transfer rates in a RAID provide 640 MB/s (max) rates.
NOW FOR A SWITCH
Since PCIe is serial switched architecture, the way to aggregate
connected devices is by the use of switches. The biggest difference
from Fibre Channel or Ethernet is the switches are running at higher
speeds and are located electrically closer to the CPU system.
We do this to either add "more" storage (capacity) or to improve data transfer rates by throughput aggregation — or both.
Replacing the single-port host card with a two-port PCIe switch (PCIe
card) allows connection to two PCIe RAID controllers to a slot, using
two cables. The two controllers may be in two different storage
shelves, or co-located in the same shelf, in two different I/O modules,
each addressing eight of the 16 resident drives. The aggregated data
transfers have increased from 640MB/s to ~1.2GB/s — all done with only
We can continue to add PCIe switches, which have minimal latency, to
aggregate bandwidth until the host PCIe bus is saturated. With PCIe 2.0
and 16-lane connections, this would occur at 16GB/s. At 1GB/s per (Gen.
2) PCIe lane and disks that can read and write at 80MB/s burst speeds,
this would take about 200 disk drives to accomplish. Using 1TB SATA
disks, this can build an array of 200TB at a system cost of about
$1/GB. 1.5TB SATA drives are now available and 2TB drives are on most
suppliers' roadmaps for mid-2009.
There isn't any other technology that accomplishes so much so
inexpensively. Expansion is easy and comes with virtually no hit in
OOPS, OUT OF SLOTS!
PCI Express is a simple switched architecture and devices are commonly
available to create bus expansion products. A simple rack-mount unit
contains a multi-slot PCIe backplane connected (by external PCIe cable)
to a host computer to provide additional PCIe slots, which are an
extension of the computer's internal bus. Performance is the same as
using the host computer's internal slots, except now they are external.
Any PCIe-compatible peripherals may be plugged in to such an expansion
chassis, just as though they were plugged in to the host.
This kind of product allows us to use additional PCIe switches to
connect more storage units, or allow plug-in HD-plus video capture
cards, 10GE cards or any other PCIe devices.
This is the great thing about PCIe: it's ubiquitous and already used
for just about everything that connects to modern computers. It's also
backwards compatible with original PCI devices so that software used
with original (parallel) PCI devices will also work with serial PCIe
hardware. PCIe devices are also "hot pluggable," capable of being added
or subtracted from systems without the need to power down the system.
PCI attached storage is the way of the future. It's high bandwidth,
high performance and low cost, and expands easily with virtually no hit
in performance. Scalable systems can handle hundreds of SD streams,
tens of HD streams and multiple 2K and 4K workflows at cost points
Steve Katz is an executive at JMR Electronics (www.jmr.com) in
Chatsworth, CA. His 12 years there follows a career in aerospace
applications engineering and field sales, as well time at HP in product