http://sata-io.org/portmultiplier.asp
While it is possible to connect up to 15 drives to each SATA PM port
via a port multiplier, drive connectivity is practically limited to the
maximum available bandwidth on the 3Gb/s link. Sustained I/O rates from
the drives are kept to within the 3Gb/s host port connection limit for
maximum efficiency and performance.
This is sounding more and more like SAS
By using port multipliers, a single host adapter occupying a single PCI
slot is able to connect four times as many drives with no performance
degradation on a 3Gb/s line.
umm
SATA PM requires that controllers support either command-based
switching [bad] or FIS (Frame Information Structure)-based switching [good] in order
to use port multiplication. Each paradigm offers unique capabilities
suited for particular environments.
FIS (SATA) –> SAS
Command-based switching, conceptually similar to a
mechanical A/B switch, limits the host to issue commands to only one
drive at a time. Commands to other drives will not be issued until the
command queue is completed for the prior transaction. Since
command-based switching only accesses one drive at a time, it does not
take advantage of the higher speed 3Gb/s host link. Therefore,
command-based switching is ideal for simple drive expansion where
capacity is more important than performance.
FIS–based
switching offers high performance storage connections to multiple
drives simultaneously. The host issues and completes commands to drives
at any time. The port multiplier will direct data to any drive ready
for I/O. An arbitration algorithm ensures a balanced data flow. Unlike
Command-based switching, FIS-based switching allows aggregation of
reads to fully use the higher bandwidth of the 3Gb/s host link and
takes full advantage of the performance benefits of Native Command
Queuing (NCQ) on the port multiplier, resulting in aggregated
throughput of up to 300MB/second.
CoolGear CGS-3726-A1 (IFS)
host : IFS :: device : NCQ ?
ONLY a motherboard bus, Host Bus Adapter or RAID Card that specifically supports Port Multiplication will successfully connect through a Port Multiplier card. 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 quick speeds (300 MB/sec shared pool). The maximum
a port multiplier can move with the current state of the art of its chipset
is closer to 225 – 230 MB/sec.
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.
