Re: end to end error recovery musings

[Ric Wheeler <ric@emc.com>]

Alan wrote:
>> 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