fapifta commented on pull request #1322:
URL: https://github.com/apache/hadoop-ozone/pull/1322#issuecomment-674766274
Hi @avijayanhwx,
thank you for starting this work, and posting an inital version of the core
code for the upgrade support.
I have a few general questions and concerns, also added a few comments after
a quick review.
In HDFS the layoutversion is a monotonically decreasing number, we chose to
use monotonically increasing version numbers, I am unsure why HDFS chose the
negative numbers, can there be some hidden considerations, we might go through,
before committing to the positive layoutversions and monotonic increase? I
haven't found one.
Also, in HDFS one layoutversion covers one feature, would it be better to do
not let two feetures associated with one layoutversion number? What is the
benefit of having two features mapped to the same layout version? I don't feel
good about it, though I don't have a specific example yet where it can cause
trouble.
LayoutVersionManager is implemented in a way that it is fully static. I am
unsure if this is a good design, I understand the intent that there has to be
only one instance of this per component, and seeing it this way it is a
reasonable choice to use static a implementation, but it can fire back later
when we want to implement tests that involves changing something in this logic,
and we can not freely and easily change the behaviour in tests as I fear, also
it can introduce invisible interdependencies later that might be hard to
detect/factor out. Implementing it in a non-static way does not seem to cause
any drawback, even we can be fine with multiple instances for the same
component, as the used values will be anyway hardcoded in the real system. What
do you think, I would consider making it non-static, as I think it has more
possibilities and less limitations later.
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