Alan wrote:

quoted

the new location.  I believe this should be always true, so presumably
with all modern disk drives a write error should mean something very
serious has happend. 

Not quite that simple.

I think that write errors are normally quite serious, but there are exceptions 
which might be able to be worked around with retries.  To Ted's point, in 
general, a write to a bad spot on the media will cause a remapping which should 
be transparent (if a bit slow) to us.

If you write a block aligned size the same size as the physical media
block size maybe this is true. If you write a sector on a device with
physical sector size larger than logical block size (as allowed by say
ATA7) then it's less clear what happens. I don't know if the drive
firmware implements multiple "tails" in this case.

On a read error it is worth trying the other parts of the I/O.

I think that this is mostly true, but we also need to balance this against the 
need for higher levels to get a timely response.  In a really large IO, a naive 
retry of a very large write could lead to a non-responsive system for a very 
large time...

ric