Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)

[Jeff Garzik <jeff@garzik.org>]

Ric Wheeler wrote:
> Theodore Tso wrote:
>> On Thu, Jan 17, 2008 at 04:31:48PM -0800, Bryan Henderson wrote:
>>> But I heard some years ago from a disk drive engineer that that is a 
>>> myth just like the rotational energy thing.  I added that to the 
>>> discussion, but admitted that I haven't actually seen a disk drive 
>>> write a partial sector.
>>
>> Well, it would be impossible or at least very hard to see that in
>> practice, right?  My understanding is that drives do sector-level
>> checksums, so if there was a partially written sector, the checksum
>> would be bogus and the drive would return an error when you tried to
>> read from it.
> 
> There is extensive per sector error correction on each sector written. 
> What you would see in this case (or many, many other possible ways 
> drives can corrupt media) is a "media error" on the next read.

Correct.


> You would never get back the partially written contents of that sector 
> at the host.

Correct.


> Having our tools (fsck especially) be resilient in the face of media 
> errors is really critical. Although I don't think the scenario of a 
> partially written sector is common, media errors in general are common 
> and can develop over time.

Agreed.

	Jeff