Re: [PATCH v3 10/13] fstests: crash consistency fsx test using dm-log-writes

[Josef Bacik <josef@toxicpanda.com>]

On Mon, Nov 27, 2017 at 08:23:49AM -0600, Ashlie Martinez wrote:
> Amir,
> 
> Just to throw in what I believe I've found about dm-log-writes (though
> Josef would know more about this as it's just what I've concluded
> after looking at  the code): dm-log-writes logs at IO completion,
> meaning disks that ignore flush operations could exhibit bugs where
> there are none, and dm-log-writes does synchronous marks though this
> may not line up with the stream of IO operations (due to buffering)
> unless you just did a sync. The CrashMonkey team wanted to make
> something similar to the "mark" operation dm-log-writes has and we
> concluded that the only place we could guarantee a mark would line up
> with the stream of IO operations the user had performed was right
> after a call to sync as that would force cached updates to disk. If
> you call mark without a sync, then you could insert a mark after a
> write(2) returns, but before the delayed allocation for that write(2)
> actually allocates blocks and changes the extent tree on disk.
> 

Yeah dm-log-writes gladly hands you the loaded gun, it's your job to not shoot
yourself with it.  This fsx test is specifically set up to only do the mark
_after_ a successful call of fsync(), which means that the mark will be after
where we expect our data to be consistent.  Thanks,

Josef