On Fri, Sep 11, 2009 at 01:47:45PM -0500, Michael Stowe wrote:
> Errr... Do you mean that USB 2.0 would be *faster* then eSATA for random
> access, or do you mean that USB 2.0 would always slower than eSATA, but
> not as much slower for random access?
Obviously, USB2 is slower than SATA, and is slower than anything else
short of USB1 or floppy disk.
I'm surprised by the "especially for random access" bit.
> In either case, you're likely to be incorrect, there's a study here:
>
> http://www.rt.db.erau.edu/655s08/655webUSBSAT/analysis.htm
I don't see anything there that talks about random access. It shows
that USB is slower than SATA when transferring a single 80MB file, and
that USB continues to be slower than SATA when transferring a single
256MB file.
> > random access times are dominated primarily by disk head seek time,
> > which is gonna be the same no matter what the transport to the drive is.
> > So the slower transport won't matter nearly as much with random I/O as
> > it will with sequential.
>
> This is not quite correct,
i never said it was completely "correct", i said "dominated primarily"
and "won't matter nearly as much".
> because each round trip to the drive controller
> experiences additional latency, and the round trip latency adds up.
I don't know what kind of additional latency USB has vs. SATA, a quick
web search didn't show me anything useful.
I'm sure there's some, but I'd be surprised to see numbers that showed
that the extra latency for USB is significant compared to the average
time-to-access inherent in the disk mechanism (avg seek + avg rotational
latency). Let's guess that it adds 10% to the total latency of a request.
Maybe i'm way off-base here, feel free to provide data.
My own real-world measurements showed, with the same target hard disk,
25 MB/sec bulk throughput via USB2 and 40MB/sec throughput via eSATA.
I was surprised at how slow the eSATA throughput was, actually, but did
not investigate further.
The test was done on a linux system, dd'ing from a fast disk array to
the target hard disk.
> It's probably worth keeping in mind that a USB 2.0 attached drive is
> actually attached to either a SATA or an IDE controller; so you're either
> comparing USB+SATA or USB+IDE to eSATA.
sure, and it's easy to remove this source of error by using the same
sata hard disk for each set of measurements.
danno
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