Re: [PATCH 00/32] Adaptive readahead V14

[Jens Axboe <axboe@suse.de>]

On Mon, May 29 2006, Wu Fengguang wrote:
> 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?

Depends entirely on the driver, and that point we've typically lost the
fact that this is a read-ahead request and could just be tossed. In
fact, the entire request may consist of read-ahead as well as normal
read entries.

For ide-cd, it tends do only end the first part of the request on a
medium error. So you may see a lot of repeats :/

-- 
Jens Axboe

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