Richard Scobie wrote:
Jeff Garzik wrote:
Mark Lord wrote:
SATA port multipliers (think, "hub") permit multiple drives
to be active simultaneously.
Quite true, although the host controller could artificially limit
this, giving the user a mistaken impression of their port multiplier
being limited to one-command-per-N-drives.
Interesting. I was basing my comments on what may well be a vested
interest slanted paper - see the sidebar on page 2.
http://www.xtore.com/Downloads/WhitePapers/SAS_SATAValue%20Whitepaper_final.pdf
For the modest extra cost of a non-RAID SAS HBA and JBOD enclosure with
SATA drives, over a port multiplied setup, there would seem to be some
advantages.
Or have I been taken in by the hype... :)
..
Here's the "hype" part from that biased paper:
Performance: Port Multipliers only support one active host
connection at a time, signi cantly degrading e ective
throughput. Each time communication is initiated with a drive
time-consuming drive reset must occur.
Data Integrity: PMs must close the connection to one drive
to open a new one to another. When a connection is closed
drive history (e.g., data source, destination drive, data &
command context) is lost; with each opened connection the
chance of misidentification and sending data to the wrong
drive is increased.
Fiction. Or rather, heavily biased.
Modern SATA hosts and PMs have no such issues.
The key SATA term to ask for is "FIS-based switching".
The biggest difference between SATA and SAS,
is the same one we previously had between ATA and SCSI:
Vendors like to position SAS/SCSI as a "premium" brand,
and therefore cripple SATA/ATA with lower spin-rates
(7200rpm max, or 10000rpm for WD Raptors, vs. 20000rpm
for high end SAS/SCSI).
There may be other firmware algorithm differences as well,
but "RAID edition" SATA/ATA drives have similar low-readahead
and fast-seek programming as their SAS/SCSI counterparts.
Simple spin-rate (RPM) is the most significant distinguishing
factor in nearly all scenarios. SAS/SCSI may also still win when
connecting a ridiculously large number of drives to a single port.
Cheers
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