[Lustre-devel] global epochs [an alternative proposal, long and dry].

[Nikita Danilov]

Alex Zhuravlev writes:
 > Nikita Danilov wrote:
 > > The question was about SC being the single point of failure. This can be
 > > eliminated by replicating stability messages to a few nodes.
 > 
 > more complexity to workaround initial problem?

More optional optimizations that are easy to implement later should they
prove necessary.

 > 
 > > But "works" always means at least "meet requirements". There is no such
 > > thing as efficient (or scalable), but incorrect program. Ordinary Lustre
 > > recovery was implemented years ago and it is still has problems. I bet
 > > it looked very easy in the beginning, so it was tempting to optimize it.
 > 
 > then we can just proceed with synchronous IO if scalability isn't a requirement.
 > and BKL is much better because of simplicity.

Precisely. If Linus decided to do an initial Linux SMP implementation
based on a fine grained locking the Linux kernel would have been
as... some other Free Beautifully Scalable kernel with a Daemon (slow,
un-scalable, and buggy). :-)

 > > Suppose that transaction groups with these updates commit at the same
 > > time and servers are ready to send information to each other. What
 > > information each server sends and where?
 > 
 > I'll prepare a detailed description in a separate mail.

Thanks.

 > 
 > >  > yes, it's well written and proven thing. the point is different - if it's clear that
 > >  > in some cases it doesn't work well (see sync requirement), what the proof does?
 > > 
 > > It assures you that it _works_. Maybe sub-optimally, but it does. The
 > > program that is lighting fast, consumes zero memory and scales across
 > > the galaxy is useless if it is incorrect.
 > 
 > interesting point. sounds like it's absolutely impossible to prove (somehow)
 > another approach. having something "proved" doesn't mean we shouldn't try
 > another approach to avoid sub-optimal but important cases?

We definitely should try, but I think much much more formal and rigorous
treatment than we are accustomed to is necessary for such fundamental
thing as recovery.

 > 
 > 
 > thanks, Alex

Nikita.