Hi,
Recently, Nathan pointed out to me that pmchart does not use
PM_MODE_BACK when replaying archives in reverse but, rather, uses
PM_MODE_INTERP with negative time offsets. It was pointed out that
this should be investigated with regard to the multi-archive context
work that I've been doing. With that in mind, I've spent some time
looking at pmchart, the way it behaves currently with respect to
multiple archives and what can be done to support multiple archives
in a single contexts.
If you already know how pmchart works with multiple archives today,
then you can skim this next bit or, even better, correct me where my
understanding is incorrect.
pmchart currently allows the specification of more than one archive
on the command line via multiple -a options and via additional
untagged sources on the command line. However, it treats each
specified archive as a separate source of metrics. When a view is
opened, the user is asked which source the view should be applied
to. Each archive is therefore opened using a separate context. So
while metrics from more than one archive can be viewed,
multiple-archives cannot be viewed as a single source of metrics
within a single view. One limitation of this is that metrics cannot
currently be scaled across multiple archives within the same view
(PM_MODE_INTERP).
I've thought about ways to allow pmchart to display multiple
archives as a single source.
- My first idea was to change the current behaviour, so that
multiple archives would be automatically treated as a single
source. It didn't take me long to realize that, while this might
be valuable, it would invalidate some current use scenarios
which are equally valid. For example, viewing separate archives
which are not related in any way in separate views using tabs.
Automatically treating multiple archives as a single source
would require that all of the archives are compatible for this
purpose.
- Another idea is to simply allow more than one archive to be
selected in the UI when opening a view. This would give the user
explicit control over which of the archives specified on the
command line to combine in the given view. Single archive views
would be supported as before and multi-archive views would also
now be possible. If a directory is named on the command line,
then the user could be given the option to include all of the
archives within it as a whole or to choose individual archives
from within it.
I would appreciate thoughts about this second approach as well as
ideas on other possible approaches.
Thanks,
Dave
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