16 Mar
2005
16 Mar
'05
2:17 p.m.
Scott wrote:
The phenomenon of failure from the outside in has
interesting
possibilities-
have software write a known pattern in the outermost section of the disc
and
pop up a warning when it starts to deteriorate. Hopefully this would give
enough time to move data to another disk.
It doesn't have to be a known pattern. There's so much error correction
going on that as soon as you start detecting read errors (i.e., so many
errors that the error correction fails), you know you've got problems.
Writing all zeros, all ones, alternating bits or bytes, or random bits
would all work equally well for this test, and if they read at all, there's
no point in comparing the data to known values.
The reason all zeros or all ones is a reasonable test is that the data
goes through a scrambling process and is recorded with a DC-free code.
No matter what your write, there ends up being a mix of pit lengths.
(You could construct a pathological case, but it would actually be
quite hard to do so and to write it with any guarantee that it comes
out the way you expect.)
Eric