[svlug] Files That Will Not Delete

Michael Eager eager at eagercon.com
Wed Nov 5 08:11:36 PST 2014


On 11/04/14 21:50, Greg Lindahl wrote:
> On Tue, Nov 04, 2014 at 04:42:18PM -0800, Scottix wrote:
>
>> First is it tries to read every sector on the drive, through various
>> methods and won't give up after a long time
>> Second is a read and then write sectors back to disk.
>
> Uh, that's stupid. Modern systems retry reads a crapload of times
> before they give up. Given that the data is lost, the way to clear the
> problem is to write *without* reading. But what do I know, I only have
> 8,000 drives?  Fortunately, my friends have a lot more, and they agree
> with me.

Retry tries to re-read a failing block over and over, without
moving the head.  Partial (i.e., uncorrectable) data is discarded.
 From what Gibson described, SpinRite repositions the head from
different directions so that there is a possibility that it will
be positioned slightly differently over the bad block.  SpinRite
also collects the uncorrectable reads.

Failing uncorrectable reads are not all identical.  It may be
possible to reconstruct the actual data from multiple reads,
if they contain different results.

Writing the block without reading will overwrite the data in the
block, replacing it with whatever is written.  Not a good recovery.


-- 
Michael Eager	 eager at eagercon.com
1960 Park Blvd., Palo Alto, CA 94306  650-325-8077




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