Re: [RFC][PATCH] dm: add dm-power-fail target

[Chris Mason <clm@fb.com>]

On Mon, Nov 24, 2014 at 3:15 PM, Josef Bacik <jbacik@fb.com> wrote:
> On 11/24/2014 02:57 PM, Zach Brown wrote:
> 
> That is way complicated, I was just going to take two devices, one 
> that's a linear mapping and the other that's the log, and then write 
> to the log the sector+data that was written in order that it 
> completes, and then have userspace do the replay.  So basically do 
> the flush tracking like I am, then write out chunks to the log device 
> to keep a semblance of how the flushing would have affected stuff, 
> something like this
> 
> write a, write b, a complete, flush, b complete, flush complete
> 
> would log out
> 
> wrote a, flush, write b, <other writes>, <next flush>
> 
> and then we have a userspace thing that could do something like 
> replay all writes to a flush, do fs consistency and data consistency 
> checks, walk to the next flush, rinse repeat, and that way we could 
> be sure that we always have a consistent fs.  This would make it 
> easier to check complex fs operations (like btrfs's balance) without 
> having to come up with special hacks in those operations to check 
> them.  I like this better because it's less DM code which means less 
> swearing printks, but whichever we think will be the best thing for 
> this sort of testing.  Thanks,

I vote for whatever is the easiest to fit in our little programmer 
brains.  The more complex the tool, the less we'll trust it.

-chris