[PLUG] Issues With New External Hard Drive: Need SysAdmin Help

Keith Lofstrom keithl at gate.kl-ic.com
Tue Sep 11 20:58:04 UTC 2012


On Tue, Sep 11, 2012 at 06:03:44AM -0700, Rich Shepard wrote:
>   Gotcha'. Will do. I cannot believe the WD drives are failing when the
> common factor is the enclosure.

Sadly, there are many poorly designed bridge chipsets in external
enclosures.  Linux adheres to standards, and goes fast.  Windows
is slower, and often has tweaks or vendor-supplied drivers that
are nonstandard, but patch over flaws in the chipsets.

I had similar problems with some brands of USB2-to-IDE enclosures,
years ago.  See http://wiki.dirvish.org/USB2Drives  

I can only speculate why, but perhaps the interface chip designer
did not put in enough buffering between the clock domain of the
drive and the clock domain of the USB interface.  A small fraction
of the data transfers will line up wrong, and some of the bits of
a byte will transfer on clock cycle N and some on clock cycle N+1.
There are known ways to add buffering to reduce this problem to
infinitesimal error rates, but many inept digital designers don't
know about them, or how to do the analysis and the testing.  

Rsync-based dirvish keeps hard drives very busy.  So does an
external "write a lot / read a lot " program, which might be a
good way to test out the enclosures and their interface chips
without risking your backups.

Different kernels may have different error recovery behavior;
some smart USB driver programmer might add a retry patch for
one version of the kernel, and a subsequent clueless programmer
might improve average performance by a fraction of a percent in
a subsequent driver by taking the patch back out again.  Sadly,
there is a lot of "cruelty free" (untested) design out there.

So, find out what chipset the enclosure uses with "lsusb", and
try an enclosure with a different chipset, and report your 
findings here and on the dirvish wiki.  Sometimes, the only
way to deal with this stuff is to carefully document our
problems on the web so our Linux brethren can avoid them.

Keith

-- 
Keith Lofstrom          keithl at keithl.com         Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs



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