Re: 3.12: raid-1 mismatch_cnt question

[NeilBrown <neilb@suse.de>]

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On Mon, 11 Nov 2013 16:23:49 -0500 "John Stoffel" <john@stoffel.org> wrote:

> 
> I thought you could also get a mis-match count from open mmap'd files,
> which aren't completely written to one disk or another?  
> 

The only cause I can imagine for the mismatch count increasing is for a page
of memory to be change while it is being written out (so each device sees a
different value) and then for the page to be invalidated (so the dirty page
never gets written out again).

The only way to change a page while it is being written out is (I think)
through memory mapping (though this could have change, "write" might achieve
it).

But normally if you change a memory mapped page while it is being written it
will be marked 'dirty' and so will be written out again - the same to both
devices.

You could possibly modify a mem-mapped file, and then delete it before the
latest changes were written, but think that would be unlikely to do on
purpose.

Swap is the most likely cause.  If some pages in a process were written out
and changed during the writeout, and then the process was killed, you could
easily get a mismatch persisting.

But that doesn't seem to be the case here.

So I don't know what would be causing it.

NeilBrown

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