Re: Driver retries disk errors.

[Bill Davidsen <davidsen@tmr.com>]

Alan Cox wrote:
> On Maw, 2004-08-31 at 18:00, Erik Mouw wrote:
> 
>>>For non hard disk cases many devices do want and need retry.
>>
>>And many others do not. CompactFlash readers are usually implemented as
>>a USB storage device, which on its turn is implemented as a SCSI
>>"disk". So far I haven't seen a CompactFlash which could be "fixed" by
>>retries.
> 
> 
> It does no harm trying. It does real harm not being conservative and
> losing peoples data. You recover people's data after its lost, the
> IDE layer's job is to make sure it doesn't get lost in the first place.
> 
> 
>>(1) Imagine an application doing a linear read on a file with an 8
>>block read ahead and the last block being bad. The kernel will try to
>>read that bad block 16 times, but because the IDE driver also has 8
>>retries, the kernel will try to read that bad block *64* times. It
>>usually takes an IDE drive about 2 seconds to figure out a block is
>>bad, so the application gets stuck for 2 minutes in that single bad
>>block.
> 
> 
> Right now I know of no way to tell which is readahead for a failed
> command or of telling the block layer to forget them. Fix this at the
> block layer and IDE can abort the readahead sequence happily enough
> because IDE is too dumb to have issued further commands to the drive at
> this point.

If would probably be good to retry "read what you were asked, nothing 
more" on error, to avoid passing back errors caused by readahead. I 
suspect this would avoid some issues reading data off CD as well, where 
one software can read clean and another ends with a short image and error.

-- 
    -bill davidsen (davidsen@tmr.com)
"The secret to procrastination is to put things off until the
  last possible moment - but no longer"  -me