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Entry  09 Nov 2021, Francesco Renga, Forum, Issue in data writing speed 
    Reply  10 Nov 2021, Stefan Ritt, Forum, Issue in data writing speed 
    Reply  26 Jan 2022, Konstantin Olchanski, Forum, Issue in data writing speed 
       Reply  26 Jan 2022, Konstantin Olchanski, Forum, Issue in data writing speed 
Message ID: 2320     Entry time: 26 Jan 2022     In reply to: 2318
Author: Konstantin Olchanski 
Topic: Forum 
Subject: Issue in data writing speed 
> Francesco, when you say "writing an event is slow", do you mean it in the frontend
> or in the output data file?

Another explanation just occurred to me. We do not know your event size and we do not
know the size of your SYSTEM buffer. But if you have an unlucky combination,
this can happen:

Consider event size is 6 Mbytes, buffer size is 8 Mbytes, enough space for only 1 event.

First event is written quickly (buffer is empty).
Second event will be delayed, there is not enough free space in the buffer, we have
to wait for mlogger to finish reading the first event.

Same thing happens if event size is 3 Mbytes, the first 2 events will write quickly,
writing the 3rd event will be delayed until mlogger does it's thing.

The mlogger reads the SYSTEM buffer "fast" and "quickly", but it can be delayed
for a number of reasons, i.e. handling a history event, a delay writing to disk,
a delay writing to network connected storage, etc.

In general, it is best to size the SYSTEM buffer to hold about 1 second worth
of data (of average size, average rate). If your event size is 4 Mbytes, and you
record them at 10/sec, SYSTEM buffer should be at least 40 Mbytes big. (this is
set in ODB /Experiment/Buffer Sizes). (MIDAS event buffer size is limited to 2 GBytes).

K.O.
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