RAFT and CTDB

[Martin Schwenke]

On Mon, 8 Dec 2014 17:56:53 +0100, Michael Adam <obnox at samba.org> wrote:

> On 2014-12-08 at 16:26 +1100, Martin Schwenke wrote:

> > The summary is that you can't race through and simply confirm that the
> > test prints the correct data_increment value when running with -rw.
> > 
> > For the recovery lock to work you need to run the non -rw version and
> > actually confirm that *the locking rate drops dramatically*.  If it
> > doesn't then it is *not* working!
> 
> This is not necessarily true!

Then we need a better test and/or better documentation... and also more
hours in each day to make that happen.  ;-)

> For instance I remember that a few years ago, for GFS2 with the default
> configuration, I observed a constant lock rate until I reached 5
> nodes or so. This was due to the fact, that GFS' lock manager by
> default restricted locks to 100/second. Only if you removed that
> limit, you could see that dramatic drop.
> 
> Also the drop will not be as dramatic with every file system,
> since file systems seem to have different levels of optimization
> when only one node is involed.
> 
> I also remember (I think also with GFS), that initial lock rate
> was pretty high for 1 node (with custom config), and dropped
> drastically when I added a node. But when I removed the but-last
> node, the rate did not raise as drastically as it initially
> dropped, i.e. not to the orignal high lock rate.
> The explanation was that the lock manager stayed in the special
> mode for a single locking node only until a second locking node
> was added, but it did not revert back to the special scheme
> after the last had left (presumably based on a heuristic that
> probably more lockers would come back later).
> 
> So I'd say that ping_pong without -rw is generally good for
> seing possible lock rates, but if you want to verify real
> behaviour, then you should test with -rw (of course only if
> the file system implements coherence of data operations under
> locks, which hopefully all file systems that we can seriously
> take into account do...). :-)

Ah, but in the OCFS2 case the -rw test works, while the "without -rw"
test does not work!  ;-)

People definitely need to run both.  It seems that just running with
-rw is not good enough.

peace & happiness,
martin
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