On Sat, 2017-08-12 at 00:42 -0700, Christoph Hellwig wrote:
> And how are you going to write your data and checksum atomically when
> doing in-place updates?

Maybe I misunderstand something, but what's the big deal with not doing
it atomically (I assume you mean in terms of actually writing to the
pyhsical medium)? Isn't that anyway already a problem in case of a
crash?

And isn't that the case also with all forms of e.g. software RAID (when
not having a journal)?

And as I've said, what's the worst thing that can happen? Either the
data would not have been completely written - with or without
checksumming. Then what's the difference to try the checksumming (and
do it successfully in all non crash cases)?
My understanding was (but that may be wrong of course, I'm not a
filesystem expert at all), that worst that can happen is that data an
csum aren't *both* fully written (in all possible combinations), so
we'd have four cases in total:

data=good csum=good => fine
data=bad  csum=bad  => doesn't matter whether csum or not and whether atomic or 
not
data=bad  csum=good => the csum will tell us, that the data is bad
data=
good csum=bad  => the only real problem, data would be actually
        
              good, but csum is not

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