Re: [BackupPC-users] usb slow for random access? (was Re: Using rsync fo

Subject:

Re: [BackupPC-users] usb slow for random access? (was Re: Using rsync for blockdevice-level synchronisation of BackupPC pools)

From:

dan <dandenson AT gmail DOT com>

To:

"General list for user discussion, questions and support" <backuppc-users AT lists.sourceforge DOT net>

Date:

Mon, 14 Sep 2009 13:20:51 -0600

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 <backuppc.lists AT tisc DOT de> 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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