[dm-devel] thin_dump cannot dump the correct data mapping of thin pools

[Mike Snitzer]

On Fri, Oct 04 2013 at  3:01am -0400,
Teng-Feng Yang <shinrairis at gmail.com> wrote:

> Hi folks,
> 
> I have been working on an incremental backup tools of thin volumes
> created by dm-thin for a couple of months.
> This tool relies on the data mappings dumped by thin_dump from the
> thin-provisioing-tools.
> As I update thin_dump from version 0.1.5 to 0.2.7, it cannot work
> correctly as the earlier 0.1.5 version.
> 
> The following things have been observed:
> 1. thin_dump-0.2.7 cannot take user-specified "metadata_snap" as
> v0.1.5. It will always return "bad checksum in superblock".
> 2. If "metadata_snap" is not given by users, thin_dump-0.2.7 only
> dumps the out-dated data mappings which can be recognized by observing
> the wrong "transaction id".
> 
> Since it looked like thin_dump failed to get the most up-to-date
> metadata blocks in version 0.2.7, I dig a little deeper into the
> source code and find something that might cause the problem.
> The open flag used on metadata device has been changed from "O_DIRECT
> | O_SYNC" to "0" in commit 4deb1751a650010ce9c15910a064be1348dec8ba .
> After I changed this back to "O_DIRECT | O_SYNC" in version 0.2.7, it
> works as expected once again.
> 
> So here is my question.
> Can I change this flag back to "O_DIRECT | O_SYNC" in any newer version?
> I am worry about that I might accidentally mess something up by
> changing this flag.
> Also, what's the purpose to get rid of the "O_DIRECT" and "O_SYNC"
> flags when we migrate to version 0.2.x?

Getting rid of O_DIRECT provided a fix for this issue:
https://bugzilla.redhat.com/show_bug.cgi?id=960284

It is strange that buffered IO breaks thin_dump though.  Joe and/or
Heinz, any ideas?