USB is slower because
a) there is an additional protocol translation to/from USB
b) USB chipsets must hand off data to the CPU for processing which causes
each piece of data to have additional latency going through the CPU once as
raw USB packets to be translated by the driver and then again by whatever
app is processing that data. SATA/SAS/IDE all have DMA so they can dump the
usable data to memory and the CPU can process it once from there.
c)because USB packets (for storage devices) are fairly simple packets to
decode, its the mhz that matter as its how fast the packet can be pushed
through. Improving controller design can only have a marginal impact on
performance unless a high speed controller is used specifically for storage
devices(i dont believe there are any on the market).
d)USB devices rely on a driver to process the raw USB packets into
scsi/ide/ata packets. SATA/IDE controllers require a driver only to read
packets already in the appropriate format. More processing is done in the
driver and software tends to have more latency that hardware.
to break that down to a sign phrase. USB requires multiple levels of data
processing to get the data delivered to the OS while specialize storage
interfaces do most of the work in a hardware chip before handing data to the
OS.
On Mon, Sep 14, 2009 at 5:23 AM, Tino Schwarze <[email protected]>wrote:
> Hi Dan,
>
> On Fri, Sep 11, 2009 at 01:40:02PM -0400, Dan Pritts wrote:
>
> > > I'd say: Replace that USB 2.0 disk by something else like something
> > > connected via Firewire or eSATA. USB 2.0 is very, very slow, especially
> > > for random access.
> >
> > do you have empirical results that show this?
>
> I did not do benchmarks. It's just my personal experience that I've yet
> to see an USB-attached disk which feels fast. Remember: Disks do not
> speak USB, they are adressed via IDE or SATA. So, if you use USB, you
> get an additional translation layer.
>
> Apart from that it looks like USB is not optimized for fast transfer and
> low latency. SATA et al are designed for adressing hard disks, they
> don't care about input devices etc. So there is less overhead.
>
> Tino.
>
> --
> "What we nourish flourishes." - "Was wir nähren erblüht."
>
> www.lichtkreis-chemnitz.de
> www.craniosacralzentrum.de
>
>
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