Re: [PATCH 008 of 8] md/bitmap: Change md/bitmap file handling touse bmap to file blocks.

[Andrew Morton]

Neil Brown <neilb@xxxxxxx> wrote:
>
> On Saturday May 13, akpm@xxxxxxxx wrote:
> > Paul Clements <paul.clements@xxxxxxxxxxxx> wrote:
> > >
> > > Andrew Morton wrote:
> > > 
> > > > The loss of pagecache coherency seems sad.  I assume there's never a
> > > > requirement for userspace to read this file.
> > > 
> > > Actually, there is. mdadm reads the bitmap file, so that would be 
> > > broken. Also, it's just useful for a user to be able to read the bitmap 
> > > (od -x, or similar) to figure out approximately how much more he's got 
> > > to resync to get an array in-sync. Other than reading the bitmap file, I 
> > > don't know of any way to determine that.
> > 
> > Read it with O_DIRECT :(
> 
> Which is exactly what the next release of mdadm does.
> As the patch comment said:
> 
> : With this approach the pagecache may contain data which is inconsistent with 
> : what is on disk.  To alleviate the problems this can cause, md invalidates
> : the pagecache when releasing the file.  If the file is to be examined
> : while the array is active (a non-critical but occasionally useful function),
> : O_DIRECT io must be used.  And new version of mdadm will have support for this.

Which doesn't help `od -x' and is going to cause older mdadm userspace to
mysteriously and subtly fail.  Or does the user<->kernel interface have
versioning which will prevent this?


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