Port Multiplication is a technology to connect multiple hard drives
to a single port on a PCI host card. This discussion is specific to SATA buses
and drives and current technologies using SATA Port Multipliers connected
to SATA PCI, PCI-X and PCI-Express host cards for Macs and PCs.
Port Multiplication is new to the realm of SATAII Specification Host
Bus
Adapters. ONLY a motherboard bus, Host Bus Adapter or RAID Card
that specifically supports Port Multiplication will successfully connect through
a
Port
Multiplier card. Mounted in your hard drive enclosure a single port multiplier
card
supports
up to 5 connected hard drives. Every
port on
the
PCI
host
card
or RAID controller can be connected to a port multiplier with 4 or 5 drives attached
making for
HUGE storage capacities with superior performance.
Port Multiplier capable hosts are available for
new
G5
Macs with PCI Express slots like the excellent Sonnet Tempo
E4P
host card. For PCI and PCI-X Macs there is the Sonnet Tempo X4P. For PCs we
offer the
Lycom 4 Port PCI-X card, a card with RAID built in as well.
We expect to see many more offerings this year from all major host card developers.
Installation is very simple, especially in external enclosures equipped
with a port multiplier. A PCI, PCI-X or PCIe port multiplier capable host card
is installed in the computer, a single data cable connects from that
card to the back of the
enclosure.
(eSATA connectors only) In the enclosure each drive has a data cable attaching
it to the port multiplier, up to 5 per port multiplier. Drives attached via
a port multiplier can be mounted, dismounted and formatted
the same as if they were individually connected. A portion of the drives or
all of them can be formatted into RAID arrays
or mixed as JBOD disks or even included in RAID arrays containing drives
on other port multipliers making for monster arrays at mind blowing speeds.
Cost efficiency is one of the overriding benefits to using port
multiplication. By using many smaller and less expensive hard drives to make
up large capacity storage, costs per GB go way
down as opposed to the limited one drive per port method. It is far cheaper
to build a 2 TB array using 8 x 250 GB drives (at 1/2 the per GB price), compared
to the same capacity using 4 x 500 GB drives. With 20 drives per
host card possible, smaller drives become practical and very cost effective.
We ran performance tests with identical drives attached both via
traditional individually connected drives as well as drives attached via the
port multiplier.
A single drive performs nearly identically. Obviously the 300 MB/sec bandwidth
of a single data cable through which the port multiplier connects will hold
back simultaneous transfers when you add enough drives to it. But in most
cases we found the performance very good to great even in a 5 drive striped
RAID array.
Striped RAID (Software RAID0, one of the simplest and fastest types
of RAID) performance is a little harder to compare. The first thing to consider
is that
as a hard
drive
fills
up it
looses
significant performance. Drives connected individually
are given access to full bandwidth all the time while drives connected via
a port multiplier will only be as fast as the maximum the port multiplier can
pass
through a single
data cable to the host card. We found that the maximum
a port multiplier can move with the current state of the art of its chipset
is closer to 225 - 230 MB/sec. While this will impact a 5 drive array that
is empty and at its fastest, it has much less if any effect on the array's
speed as it reaches 50 percent of capacity. In fact the effect is to start out
at 225 MB/sec and stay there until the speed of the array drops below that the
port multiplier is capable of.
A striped RAID with its drives directly connected, each on its own cable/channel,
will start out very fast and slow rapidly until it somewhere around 50 percent
of capacity matches the speed of the port multiplier array for the rest of its
total capacity.
For Video Storage there is no better solution. Even the most demanding
format, 10bit 1080i uncompressed HD, is easily handled by 8 or 10 drive port
multiplier arrays. Starting out at over 400 MB/sec these arrays will maintain
over 265 MB/sec for virtually the entire usable
capacity of the RAID. One of our customers, Adam Levine, has posted a
review
here with some insights on video performance.
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