Re: [testcase] test your fs/storage stack (was Re: [patch] ext2/3:document conditions when reliable operation is possible)

[Ric Wheeler]

On 08/29/2009 05:49 AM, Pavel Machek wrote:
>    
> > > So instead of fixing or at least documenting known software deficiency
> > > in Linux MD stack, you'll try to surpress that information so that
> > > people use more of raid5 setups?
> > >
> > > Perhaps the better documentation will push them to RAID1, or maybe
> > > make them buy an UPS?
> > >        
> > people aren't objecting to better documentation, they are objecting to
> > misleading documentation.
> >      
> Actually Ric is. He's trying hard to make RAID5 look better than it
> really is.
>
>    

I object to misleading and dangerous documentation that you have 
proposed. I spend a lot of time working in data integrity, talking and 
writing about it so I care deeply that we don't misinform people.

In this thread, I put out a draft that is accurate several times and you 
have failed to respond to it.

The big picture that you don't agree with is:

(1) RAID (specifically MD RAID) will dramatically improve data integrity 
for real users. This is not a statement of opinion, this is a statement 
of fact that has been shown to be true in large scale deployments with 
commodity hardware.

(2) RAID5 protects you against a single failure and your test case 
purposely injects a double failure.

(3) How to configure MD reliably should be documented in MD 
documentation, not in each possible FS or raw device application

(4) Data loss occurs in non-journalling file systems and journalling 
file systems when you suffer double failures or hot unplug storage, 
especially inexpensive FLASH parts.

ric



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