RE: Mechanism to safely force repair of single md stripe w/o hurting data integrity of file system

["David Lethe" <david@santools.com>]

It will. But that defeats the purpose.  I want to limit repair to only the raid stripe that utilizes a specifiv disk with a block that I know has a unrecoverable reas error.  

-----Original Message-----

From:  "Guy Watkins" <linux-raid@watkins-home.com>
Subj:  RE: Mechanism to safely force repair of single md stripe w/o hurting data integrity of file system
Date:  Sat May 17, 2008 3:28 pm
Size:  2K
To:  "'David Lethe'" <david@santools.com>; "'LinuxRaid'" <linux-raid@vger.kernel.org>; "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>

} -----Original Message----- 
} From: linux-raid-owner@vger.kernel.org [mailto:linux-raid- 
} owner@vger.kernel.org] On Behalf Of David Lethe 
} Sent: Saturday, May 17, 2008 3:10 PM 
} To: LinuxRaid; linux-kernel@vger.kernel.org 
} Subject: Mechanism to safely force repair of single md stripe w/o hurting 
} data integrity of file system 
}  
} I'm trying to figure out a mechanism to safely repair a stripe of data 
} when I know a particular disk has a unrecoverable read error at a 
} certain physical block (for 2.6 kernels) 
}  
} My original plan was to figure out the range of blocks in md device that 
} utilizes the known bad block and force a raw read on physical device 
} that covers the entire chunk and let the md driver do all of the work. 
}  
} Well, this didn't pan out. Problems include issues where if bad block 
} maps to the parity block in a stripe then md won't necessarily 
} read/verify parity, and in cases where you are running RAID1, then load 
} balancing might result in the kernel reading the bad block from the good 
} disk. 
}  
} So the degree of difficulty is much higher than I expected.  I prefer 
} not to patch kernels due to maintenance issues as well as desire for the 
} technique to work across numerous kernels and  patch revisions, and 
} frankly, the odds are I would screw it up.  An application-level program 
} that can be invoked as necessary would be ideal. 
}  
} As such, anybody up to the challenge of writing the code?  I want it 
} enough to paypal somebody $500 who can write it, and will gladly open 
} source the solution. 
}  
} (And to clarify why, I know physical block x on disk y is bad before the 
} O/S reads the block, and just want to rebuild the stripe, not the entire 
} md device when this happens. I must not compromise any file system data, 
} cached or non-cached that is built on the md device.  I have system with 
} >100TB and if I did a rebuild every time I discovered a bad block 
} somewhere, then a full parity repair would never complete before another 
} physical bad block is discovered.) 
}  
} Contact me offline for the financial details, but I would certainly 
} appreciate some thread discussion on an appropriate architecture.  At 
} least it is my opinion that such capability should eventually be native 
} Linux, but as long as there is a program that can be run on demand that 
} doesn't require rebuilding or patching kernels then that is all I need. 
}  
} David @ santools.com 
 
I thought this would cause md to read all blocks in an array: 
echo repair > /sys/block/md0/md/sync_action 
 
And rewrite any blocks that can't be read. 
 
In the old days, md would kick out a disk on a read error.  When you added 
it back, md would rewrite everything on that disk, which corrected read 
errors. 
 
Guy