Re: end to end error recovery musings

[Andreas Dilger <adilger@clusterfs.com>]

On Feb 23, 2007  16:03 -0800, H. Peter Anvin wrote:
> Ric Wheeler wrote:
> >   (1) read-ahead often means that we will  retry every bad sector at 
> >least twice from the file system level. The first time, the fs read 
> >ahead request triggers a speculative read that includes the bad sector 
> >(triggering the error handling mechanisms) right before the real 
> >application triggers a read does the same thing.  Not sure what the 
> >answer is here since read-ahead is obviously a huge win in the normal case.
> 
> Probably the only sane thing to do is to remember the bad sectors and 
> avoid attempting reading them; that would mean marking "automatic" 
> versus "explicitly requested" requests to determine whether or not to 
> filter them against a list of discovered bad blocks.

And clearing this list when the sector is overwritten, as it will almost
certainly be relocated at the disk level.  For that matter, a huge win
would be to have the MD RAID layer rewrite only the bad sector (in hopes
of the disk relocating it) instead of failing the whiole disk.  Otherwise,
a few read errors on different disks in a RAID set can take the whole
system offline.  Apologies if this is already done in recent kernels...

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.