Re: precise characterization of ext3 atomicity

[Hans Reiser]

Andrew Morton wrote:

> Hans Reiser <reiser@xxxxxxxxxxx> wrote:
>  
>
> > Perhaps the following is correct?
> >
> >     By contrast, ext3 in data=journal and data=ordered modes only guarantees the atomicity of a single write 
> > that does not span a page boundary, and it guarantees that its internal 
> > metadata will not be corrupted even if your application's data is 
> > corrupted after the crash (due to the application spreading what should be committed atomically across more than one block).
> >    
>
> Correct != comprehensible ;)
Touche!

Let's try this then:

Ext3 guarantees that its metadata will be comitted sufficiently 
atomically that after a crash it will be consistent with itself.

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.



--
Hans


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