Re: precise characterization of ext3 atomicity

[Mike Fedyk <mfedyk@matchmail.com>]

On Thu, Sep 04, 2003 at 10:37:10PM +0400, Hans Reiser wrote:
> Mike Fedyk wrote:
> 
> >On Thu, Sep 04, 2003 at 08:25:18PM +0400, Hans Reiser wrote:
> > 
> >
> >>In data=journal and data=ordered modes ext3 also guarantees that the 
> >>metadata will be committed atomically with the data they point to.  
> >>However ext3 does not provide user data atomicity guarantees beyond the 
> >>scope of a single filesystem disk block (usually 4 kilobytes).  If a 
> >>single write() spans two disk blocks it is possible that a crash partway 
> >>through the write will result in only one of those blocks appearing in 
> >>the file after recovery.
> >>   
> >>
> >
> >And how does reiser4 do this without changing the userspace apps?
> >
> We don't.  We just make the hovercraft, we don't force you to go over 
> the water.....

So by default with no user space modifications, reiser4 will be atomic for
each write() call, and ext3 will if it aligns withing a single page.

Is that correct?

Then you can go on to specify that you can have larger transactions if you
make some changes to the userspace apps.