I'm putting together a SAN with Promise V-Trak E-class RAID units. I can use 1TB drives if I go with SATA units but only 450GB drives if I go SAS. Why is this? Why would I want to use more than twice as many SAS drives over the larger SATA drives?
There is no single SATA HDD that can sustain 200MB/second,
and no SATA/6G HDDs are currently available.
And, there has always been a speed/storage trade-off.
> Why would I want to use more than twice as many SAS drives over the larger SATA drives?
... to achieve maximum speed on the entire storage subsystem,
rather than a hybrid approach i.e. a careful mixture of large/slow storage
and small/fast storage.
To illustrate with a very simple yet realistic example,
drive image files of your OS partition do NOT need to be
stored on THE FASTEST HDDs in your storage subsystem:
they can be best archived on slower, "green" HDDs
that will do the job of restoring your OS partition
whenever the need arrives, but that task may not
need to execute at the fastest possible speed.
... because SATA/6G SSDs are coming -- in 6-9 months (maybe sooner).
Those options are yet another reason favoring a hybrid storage policy.
There is no escaping the raw data rate directly below the read/write
heads of spinning platters: those rates are not even close to saturating
the SATA/3G (300 MB/second) interface bandwidth.
SATA/6G SSDs should easily exceed 300MB/second, once they are available.
SATA/3G SSDs are already doing READS near 260 MB/second, which rate
reflects the interface maximum + controller overhead.
So, plan on implementing controllers that will fully exploit the
extra bandwidth soon to appear with SAS/6G and SATA/6G drives,
without any compromises like the "PCIe x1 Gen 2" solution
described here (only 500MB/second = NOT "full 6G support"):