At 10:32 -0500 21/1/13, RavenManiac wrote:
I just read an interesting article stating that many mechanical
drives aren’t fast enough to take advantage of a SATA 6Gbit/s
speeds, which everyone seems to be promoting these days. It was
actually pretty good find as I almost replaced the 3Gbit/s drives
I’m installing in my new NAS.
That’s probably why Apple installs 3Gbit/s drives in most of their
new computers, even though many of its systems are capable of
6Gbit/s speeds.
Learn more here:
http://blog.synology.com/blog/?p=1130#sata6gbps
The absolute maximum throughput to/from a rotating drive is bound to
be the rate that the bit go past under the head. This depends on the
speed of rotation and the bit density in bits per track. That maximum
could only be achieved for transfers to/from consecutive sectors on
the same track so no seek time was lost. In practice each sector has
lead-in, lead-out, self address, and a gap on the end, so it’s even
lower than that.
If you connected a 6Gb/s SATA drive to a 6Gb/s interface in your Mac
(Pro) then data could travel between the cache on the drive and the
buffer on the Mac SATA card at 6Gb/s, but the speed to memory and
platter may/will be lower and delayed.
You’re not going to get even the best your drive can manage from a
NAS box. The NAS box CPU has to read the data from the disc into
memory and then serve it out again virtually, as if it were from
some, possibly different, filesystem. A faster drive in your NAS, up
to that that the NAS system can handle may appear faster than a
slower drive, but nowhere near the 3Gb/s rate. A SAN based drive
system that presents block level data to the host doesn’t have the
file locking overhead that a NAS does because the data isn’t shared,
but it still has to be buffered in the subsystem.
The data also has to get in to/out of the Mac memory. USB used Mac
CPU power to do that. Firewire and direct SATA use DMA where the
interface steals memory cycles from the CPU, but if that rate gets
too high then whatever the Mac CPU is trying to do wil stutter. I
don’t suppose that’s a problem yet, but with solid state drives it
may become one.
David
–
David Ledger - Freelance Unix Sysadmin in the UK.
email@hidden
www.ivdcs.co.uk
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