Re: [PATCH 00/32] Adaptive readahead V14

[Wu Fengguang]

On Sun, May 28, 2006 at 11:23:33PM +0400, Michael Tokarev wrote:
> Wu Fengguang wrote:
> > 
> > It's not quite reasonable for readahead to worry about media errors.
> > If the media fails, fix it. Or it will hurt read sooner or later.
> 
> Well... In reality, it is just the opposite.
> 
> Suppose there's a CD-rom with a scratch/etc, one sector is unreadable.
> In order to "fix" it, one have to read it and write to another CD-rom,
> or something.. or just ignore the error (if it's just a skip in a video
> stream).  Let's assume the unreadable block is number U.
> 
> But current behavior is just insane.  An application requests block
> number N, which is before U. Kernel tries to read-ahead blocks N..U.
> Cdrom drive tries to read it, re-read it.. for some time.  Finally,
> when all the N..U-1 blocks are read, kernel returns block number N
> (as requested) to an application, successefully.
> 
> Now an app requests block number N+1, and kernel tries to read
> blocks N+1..U+1.  Retrying again as in previous step.
> 
> And so on, up to when an app requests block number U-1.  And when,
> finally, it requests block U, it receives read error.
> 
> So, kernel currentry tries to re-read the same failing block as
> many times as the current readahead value (256 (times?) by default).

Good insight... But I'm not sure about it.

Jens, will a bad sector cause the _whole_ request to fail?
Or only the page that contains the bad sector?

> This whole process already killed my cdrom drive (I posted about it
> to LKML several months ago) - literally, the drive has fried, and
> does not work anymore.  Ofcourse that problem was a bug in firmware
> (or whatever) of the drive *too*, but.. main problem with that is
> current readahead logic as described above.
> 
> With that logic, an app also becomes unkillable (at least for some
> time) -- ie, even when I knew something's wrong and the CDrom should
> not behave like it was, I wasn't able to stop it until I powered the
> machine off (just unplugged the power cable) - but.. too late.
> 
> Yes, bad media is just that - a bad thing.  But it's not a reason to
> force power unplug to stop the process, and not a reason to burn a
> drive (or anything else).  And this is where readahead comes into
> play - it IS read-ahead logic who's responsible for the situation.
> 
> And there's alot of scratched/whatever CD-Rom drives out there -
> unreadable CDrom (or a floppy which is already ancient, or some
> other media) - you can't just say to every user out there that
> linux isn't compatible with all people's stuff and those people
> should "fix" it before ever trying to insert it into their linux
> machine...
> 
> Thanks.
> 
> /mjt
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