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

[Neil Brown]

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.

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