Re: [xfsprogs PATCH 2/2] xfs_io: add a new 'log_writes' command

[Eric Sandeen <sandeen@sandeen.net>]

On 11/17/17 2:48 PM, Ross Zwisler wrote:
> On Fri, Nov 17, 2017 at 02:39:07PM -0600, Eric Sandeen wrote:
>> On 11/17/17 2:25 PM, Ross Zwisler wrote:
>>> Add a new 'log_writes' command to xfs_io so that we can add dm-log-writes
>>> log marks via the external 'dmsetup' executable.  It's helpful to allow
>>> users of xfs_io to adds these marks from within xfs_io instead of waiting
>>> until after xfs_io exits because then they are able to replay the
>>> dm-log-writes log up to immediately after another xfs_io operation such as
>>> mwrite.  This isolates the log replay from other operations that happen as
>>> part of xfs_io exiting (file handles being closed, mmaps being torn down,
>>> etc.).  This also allows users to insert multiple marks between different
>>> xfs_io commands.
>>>
>>> Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
>>> Suggested-by: Dave Chinner <david@fromorbit.com>
>>
>> Without reviewing in detail, what is the advantage of wrapping dmsetup
>> into xfs_io?  My first inclination is that there is none at all, and
>> xfstests can call dmsetup as easily as they can call xfs_io.  No?
>>
>> -Eric
> 
> I commented on this a bit in the changelog for the 2nd patch:
> 
> It's helpful to allow users of xfs_io to adds these marks from within xfs_io
> instead of waiting until after xfs_io exits because then they are able to
> replay the dm-log-writes log up to immediately after another xfs_io operation
> such as mwrite.  This isolates the log replay from other operations that
> happen as part of xfs_io exiting (file handles being closed, mmaps being torn
> down, etc.).  This also allows users to insert multiple marks between
> different xfs_io commands.
> 
> I agree that the shell-out to dmsetup isn't awesome...  For the current test I
> have written I think we can get away with just assuming that the xfs_io exit
> stuff won't interact too heavily with the dm-log-writes log, and we could
> potentially move the dmsetup call back into the fstest.  This is how I
> initially had it, and moved it into the C program via shell-out in response to
> Amir's feedback:

Sorry, terrible of me to not have read that.  :(  Ok, so next question - 
DM_TARGET_MSG seems to be public, can we just invoke the ioctl directly
instead of shelling out to dmsetup?

I'm checking w/ the dm folks too, to make sure that's expected to work.  As
long as the use isn't too tricky it seems like that might be better.

-Eric