[kwlug-disc] USB3 expectations? (was: Re: USB3 on Linux)
unsolicited at swiz.ca
Fri Jun 18 14:31:21 EDT 2010
Khalid Baheyeldin wrote, On 06/18/2010 11:54 AM:
> Something I would be interested in is transfer speed from a hard drive. Is
> it really that different on USB3 from USB2, or will it be the same?
Help me understand the question.
If USB3 is faster than USB2, why would it be the same? Hw/sw not
catching up to the specs yet?
Or are you expecting the performance increase not worth the price?
Until this thread, I don't think I even knew there was such as thing
as USB 3. So I poked wikipedia for some info. Thieving from wikipedia
A new major feature is the "SuperSpeed" bus, which provides a fourth
transfer mode at 5.0 Gbit/s. The raw throughput is 4 Gbit/s, and the
specification considers it reasonable to achieve 3.2 Gbit/s (0.4
GByte/s or 400 MByte/s), or more, after protocol overhead.
When operating in SuperSpeed mode, full-duplex signaling occurs over
two differential pairs separate from the non-SuperSpeed differential
pair. This results in USB 3.0 cables containing two wires for power
and ground, two wires for non-SuperSpeed data, and four wires for
SuperSpeed data, and a shield that was not required in previous
specifications." (It goes on to talk about a new, but backwards
compatible, plug. Interesting.)
Hmm. OK, so USB2 480 Mbit/s, USB3 3.2 Gbit/s effective (if we ever hit
even that. And let's not forget about the whole sharing devices / the
bus, mixing USB1/2/3 issues. So, we can expect hardware to come with
even more individual USB ports, and a plethora of add in cards, to
come?) And USB3 ~= SATA2.
Curious, I then flipped over to SATA,
http://en.wikipedia.org/wiki/SATA2, "While even the fastest
conventional hard disk drives can barely saturate the original SATA
1.5 Gbit/s bandwidth," - huh. Interesting. It continues "Solid-State
Drives have already saturated the SATA 3 Gbit/s limit at 250 MB/s net
read speed. Ten channels of fast flash can reach well over 500 MB/s
with new ONFI drives, so a move from SATA 3 Gbit/s to SATA 6 Gbit/s
would benefit the flash read speeds. As for the standard hard disks,
the reads from their built-in DRAM cache will end up faster across the
new interface. SATA 6 Gbit/s hard drives and Motherboards are now
shipping from several suppliers."
So, now is there not only a USB3, there's a SATA3 (6Gbits/s, 600MB/s
effective)! And most drives can't keep up to SATA 1, begging the
Now it gets a little convoluted "communicate at a rate of 3 Gbit/s.
Taking 8b/10b encoding overhead into account, they have an actual
uncoded transfer rate of 2.4 Gbit/s (3000000000*8/10/1024/1024/8 ?
286.10 MiB/s). The theoretical burst throughput of SATA 3.0 Gbit/s is
roughly double that of PATA/133. Also, SATA devices offer enhancements
such as NCQ that improve performance in a multitasking environment."
(i.e. SATA is gradually getting some of the advantages of the SCSI
spec.) And later "However, high-performance flash drives are
approaching SATA 3 Gbit/s transfer rate, and this is being addressed
with the SATA 6Gb/s interoperability standard."
[Sorry, the SATA quotes above is not linear with the article.]
So what does it all mean to you and I, the home consumer, or to business?
- SCSI will become even less popular, given the higher cost than SATA,
as SATA performance improves, in (large?) RAID arrays / shelves?
- even fewer devices than I would wish, like laptops, will come with
eSata ports in favour of USB3?
- OpenWRT devices won't get eSata (typically?) or USB3, the processors
not (typically?) being able to keep up to the drive? Vis a vis NAS?
- the benefits of USB3 or SATA3 will largely only be felt by multiple
drives feeding multiple servers that feed multiple clients (e.g. SQL).
Even if the clients are all connected Gigabit, the net will still be
the limiting factor?
- even on a home connected web server, the limitation will be ISP
speed, not disk speed? IIRC, typically systems are disk bound, but
there's a whole lot of 'stuff' between the CPU and disk, preventing an
increase in disk speed from providing an equivalent increase in system
speed? (And this is even presuming sufficient requests are coming in
sufficiently often to "make 'it' worthwhile"?
- systems, even more so, will stop coming with moving parts hard
drives, in favour of much faster (and more reasonably smaller) flash
drives for the OS. Particularly blade centers?
- if drives are SATA, and the bus is USB3, why USB3 instead of just
staying with (e)SATA?
- why USB3? HD video streaming? (Limited, by ISP speeds?)
- NAS vs more drives in a current system, the question still goes
begging. Especially for the home, if it's not a GUI I don't understand
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