Messages in this thread | | | Date | Tue, 30 May 2006 11:23:10 +0200 | From | Jens Axboe <> | Subject | Re: [PATCH 00/32] Adaptive readahead V14 |
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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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