[Discuss] ddrescue, anyone?

[dbarrett at blazemonger.com (Daniel Barrett)]

I used ddrescue (http://www.gnu.org/software/ddrescue/ddrescue.html) for
the first time yesterday, as my boot disk developed errors and I needed to
copy the contents to a fresh drive.

Can anyone explain why the examples in the manual all run ddrescue twice?

  http://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html#Examples

The pattern seems to be:

  $ ddrescue -f -n /dev/one /dev/two logfile
  $ ddrescue -d -f -r3 /dev/one /dev/two logfile

but the reasoning is never explained, and plenty of other sources on the
say simply to run it once as "ddrescue /dev/one dev/two logfile".

I believe that the first invocation avoids problematic areas of the disk
(-n), whereas the second invocation retries errors up to 3 times. But I
don't understand if these two invocations are independent, i.e., they will
both take the full 10 hours for a 1 gig drive, or does the second invocation
somehow make use of the result from the first run and go more quickly?

I ran only the first invocation, e2fsck'ed all partitions, and booted, and
everything worked fine.

Thanks for any advice/understanding.

--
Dan Barrett
dbarrett at blazemonger.com