Re: [patch] ext2/3: document conditions when reliable operation ispossible

[Zan Lynx]

Ric Wheeler wrote:
> Pavel Machek wrote:
> > Degraded MD RAID5 does not work by design; whole stripe will be
> > damaged on powerfail or reset or kernel bug, and ext3 can not cope
> > with that kind of damage. [I don't see why statistics should be
> > neccessary for that; the same way we don't need statistics to see that
> > ext2 needs fsck after powerfail.]
> >                                     Pavel
> >   
> What you are describing is a double failure and RAID5 is not double 
> failure tolerant regardless of the file system type....

Are you sure he isn't talking about how RAID must write all the data 
chunks to make a complete stripe and if there is a power-loss, some of 
the chunks may be written and some may not?

As I read Pavel's point he is saying that the incomplete write can be 
detected by the incorrect parity chunk, but degraded RAID-5 has no 
working parity chunk so the incomplete write would go undetected.

I know this is a RAID failure mode. However, I actually thought this was 
a problem even for a intact RAID-5. AFAIK, RAID-5 does not generally 
read the complete stripe and perform verification unless that is 
requested, because doing so would hurt performance and lose the entire 
point of the RAID-5 rotating parity blocks.

--
Zan Lynx
zlynx@xxxxxxx

"Knowledge is Power.  Power Corrupts.  Study Hard.  Be Evil."
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