Messages in this thread | | | Date | Mon, 29 May 2006 11:01:53 +0800 | From | Wu Fengguang <> | Subject | Re: [PATCH 00/32] Adaptive readahead V14 |
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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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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