On Fri, 10 Jan 2014 14:05:25 -0500, Frank Ch Eigler wrote:
fche> 1.1) pmlogger needs to learn to write its output with what IIRC kenj
fche> has referred to as "semantic units", ie., proper use sequencing of
fche> write(2), fdatasync(), to put interdependent data on disk correctly.
>>
>> This was the biggest bugbear of NAS Manager (for those who remember)
>> which tried to provide historical and live data using archives as main
>> source of information. [...]
fche> (A little more of the history would be great.)
NAS Manager had to provide "historical" data for IO performance
aggregated over time invervals, e.g. number of IO requests for each
hour during last 24 hours. And it had to be "true averages", not
decaying averages. And it had to be displayed on a web page which
meant single-shot requests (AJAX wasn't an option for various
non-technical reasons).
We wrote a daemon which was generating graphs (static images)
based on the information which has been scrapped from the PCP
archives. It was working OK for historical data but doing the
equivalent of "tail -f" with libpcp and growing archives didn't work.
After few attempts the idea has been canned and we switched to a PMDA
which provided historical data based on the time intervals encoded
in instance names. The PMDA would pre-load the data from historical
archives" and then switch to polling pmcd for the data it needed (it
was known as HUTA mode - Head Up The ...).
The whole thing may even exists assuming NAS Manager exists (hi, gnb).
max
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