Re: [PATCH V3 17/18] Btrfs: Full direct I/O and AIO read implementation.

[jim owens <owens6336@gmail.com>]

sorry, this time with cc to list

Andi Kleen wrote:
> > On Tue, Mar 23, 2010 at 05:40:00PM -0400, jim owens wrote:
>> >> Andi Kleen wrote:
>> >>
>> >> With checksums enabled, uncompressed reads aligned on the 4k block
>> >> are classic direct IO to user memory except at EOF.
> > 
> > Hmm, but what happens if the user modifies the memory in parallel?
> > Would spurious checksum failures be reported then?

It does put a warning in the log but it does not fail the read
because I circumvent that by doing the failed-checksum-retry as
a buffered read and retest.  The checksum passes and we copy
the data to the user memory (where they can then trash it again).
I was going to put a comment about that but felt my comment
density was already over the btrfs style guide limit.  :) 

> > Same for writing I guess (data end up on disk with wrong checksum)?

Well we don't have any code done yet for writing and that was
just one interesting challenge that needed to be solved.

> > Those both would seem like serious flaws to me.

Agree, so the write design needs to prevent bad checksums.

Read is already correct and if people do not want a log warning
that the application is misbehaving that can be eliminated.

jim