On Fri, 7 Sep 2012, Michael Ryder wrote:
Sometimes you need FT, but are working with an application that
doesn't explicitly allow for clustering, or simply can't be clustered.
Personally, I've worked with both, and MSCS is such a pain in the
neck.... depending on the application, FT can be the better choice.
More in-depth information here:
http://www.vmware.com/files/pdf/resources/ft_virtualization_wp.pdf
"? vLockstep guarantees: the primary and secondary execute
exactly the same x86 instruction sequences."
executing the exact same x86 instruction sequences will give you different
random numbers
timing differences will result in different results when there are race
conditions, etc
non-deterministic events (input, I/O, etc) are copied from one machine to
the other and replayed. There is a window here where the primary is in a
different state then the backup, if the failure happens to hit at this
time, you can be in trouble.
This is still far better than trying to mirror the state of the system.
I'm also not disputing that there are cases where this is your only (or
even best) choice due to software not doing clustering well.
I'm just claiming that it's not going to be perfect the way that the
marketing material makes it sound.
This may not be a problem for you. It all depends on what the impact will
be if it's not perfect.
For a webserver, the worst case could be that some connections get broken
in the transition, not a big deal.
But I had an application that was responsible for changeing passwords on
production systems that was climing that this sort of thing provided
perfect reliability (even between a primary and backup system running on
the east and west coasts of the US). In that situation the worst case
would have been having the root password changed to something that nothing
and nobody knows, resulting in an unmaintainable system. In that case the
cost was not acceptable (especially since competing systems did real
clustering and were not dependant on this sort of vm syncing)
David Lang
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