Re: Driver retries disk errors.
From: Bill Davidsen
Date: Wed Sep 01 2004 - 10:36:05 EST
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@xxxxxxx)
"The secret to procrastination is to put things off until the
last possible moment - but no longer" -me
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