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

[david]

On Wed, 26 Aug 2009, Pavel Machek wrote:

> > > > I don't object to making that general statement - "Don't hot unplug a
> > > > device with an active file system or actively used raw device" - but
> > > > would object to the overly general statement about ext3 not working on
> > > > flash, RAID5 not working, etc...
> > >
> > > You can object any way you want, but running ext3 on flash or MD RAID5
> > > is stupid:
> > >
> > > * ext2 would be faster
> > >
> > > * ext2 would provide better protection against powerfail.
> >
> > Not true in the slightest, you continue to ignore the ext2/3/4 developers
> > telling you that it will lose data.
>
> I know I will lose data. Both ext2 and ext3 will lose data on
> flashdisk. (That's what I'm trying to document). But... what is the
> benefit of ext3 journaling on MD RAID5? (On flash, ext3 at least
> protects you against kernel panic. MD RAID5 is in software, so... that
> additional protection is just not there).

the block device can loose data, it has absolutly nothing to do with the 
filesystem

> > > "ext3 works on flash and MD RAID5, as long as you do not have
> > > powerfail" seems to be the accurate statement, and if you don't need
> > > to protect against powerfails, you can just use ext2.
> >
> > Strange how your personal preference is totally out of sync with the
> > entire enterprise class user base.
>
> Perhaps noone told them MD RAID5 is dangerous? You see, that's exactly
> what I'm trying to document here.

a MD raid array that's degraded to the point where there is no redundancy 
is dangerous, but I don't think that any of the enterprise users would be 
surprised.

I think they will be surprised that it's possible that a prior failed 
write that hasn't been scrubbed can cause data loss when the array later 
degrades.

David Lang
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