Re: end to end error recovery musings

Ric Wheeler
2007-02-26T15:18:22Z

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