Re: end to end error recovery musings

[Ric Wheeler <ric@emc.com>]

Jeff Garzik wrote:
> Theodore Tso wrote:
>> Can someone with knowledge of current disk drive behavior confirm that
>> for all drives that support bad block sparing, if an attempt to write
>> to a particular spot on disk results in an error due to bad media at
>> that spot, the disk drive will automatically rewrite the sector to a
>> sector in its spare pool, and automatically redirect that sector to
>> the new location.  I believe this should be always true, so presumably
>> with all modern disk drives a write error should mean something very
>> serious has happend.  
> 
> 
> This is what will /probably/ happen.  The drive should indeed find a 
> spare sector and remap it, if the write attempt encounters a bad spot on 
> the media.
> 
> However, with a large enough write, large enough bad-spot-on-media, and 
> a firmware programmed to never take more than X seconds to complete 
> their enterprise customers' I/O, it might just fail.
> 
> 
> IMO, somewhere in the kernel, when we receive a read-op or write-op 
> media error, we should immediately try to plaster that area with small 
> writes.  Sure, if it's a read-op you lost data, but this method will 
> maximize the chance that you can refresh/reuse the logical sectors in 
> question.
> 
>     Jeff

One interesting counter example is a smaller write than a full page - say 512 
bytes out of 4k.

If we need to do a read-modify-write and it just so happens that 1 of the 7 
sectors we need to read is flaky, will this "look" like a write failure?

ric