[lttng-dev] "lttng enable-channel --tracefile-size" bug

[lttng-dev] "lttng enable-channel --tracefile-size" bug

[John Smith]

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Hi,
it looks like for the "enable-channel" command, "tracefile-size" option
doesn't take effect, bellow are all the options, the resulted files have
the actual size the same as the size specified by subbuf-size option:

# lttng -V
lttng (LTTng Trace Control) 2.7.1 - Herbe à Détourne

$ lttng enable-channel chan_name -u -s session_name --discard --num-subbuf
8
            --subbuf-size 32M --tracefile-size 8K --tracefile-count 8
--buffers-pid

thanks, John
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[lttng-dev] "lttng enable-channel --tracefile-size" bug

[Julien Desfossez]

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> Hi,
> it looks like for the "enable-channel" command, "tracefile-size" option
> doesn't take effect, bellow are all the options, the resulted files have
> the actual size the same as the size specified by subbuf-size option:
>  
> # lttng -V
> lttng (LTTng Trace Control) 2.7.1 - Herbe à Détourne
> 
> $ lttng enable-channel chan_name -u -s session_name --discard
> --num-subbuf 8 
>             --subbuf-size 32M --tracefile-size 8K --tracefile-count 8
> --buffers-pid

Yes, that is the expected behaviour, we work with packets (sub-buffers),
so the smallest unit we can store is the subbuf-size.

With 2.7.3 I get this warning when I try your command:
Warning: Tracefile size rounded up from (8192) to subbuffer size (33554432)

Julien

[lttng-dev] "lttng enable-channel --tracefile-size" bug

[John Smith]

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Julien,
thanks, I was mislead by the 2.7 documentation saying:
"...parameters of enable-channel are --tracefile-sizeand --tracefile-count,
which respectively limit the size of each trace file and the their count
for a given channel. "
In this case the tracefile-size option should be removed, will it become
useful in the future releases or has a more subtle use?

thanks, John

On Sat, May 28, 2016 at 6:21 AM, Julien Desfossez <jdesfossez at efficios.com>
wrote:

> > Hi,
> > it looks like for the "enable-channel" command, "tracefile-size" option
> > doesn't take effect, bellow are all the options, the resulted files have
> > the actual size the same as the size specified by subbuf-size option:
> >
> > # lttng -V
> > lttng (LTTng Trace Control) 2.7.1 - Herbe à Détourne
> >
> > $ lttng enable-channel chan_name -u -s session_name --discard
> > --num-subbuf 8
> >             --subbuf-size 32M --tracefile-size 8K --tracefile-count 8
> > --buffers-pid
>
> Yes, that is the expected behaviour, we work with packets (sub-buffers),
> so the smallest unit we can store is the subbuf-size.
>
> With 2.7.3 I get this warning when I try your command:
> Warning: Tracefile size rounded up from (8192) to subbuffer size (33554432)
>
> Julien
>
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[lttng-dev] "lttng enable-channel --tracefile-size" bug

[Julien Desfossez]

> Julien,
> thanks, I was mislead by the 2.7 documentation saying:
> "...parameters
> of |enable-channel| are |--tracefile-size|and |--tracefile-count|, which
> respectively limit the size of each trace file and the their count for a
> given channel. "
> In this case the tracefile-size option should be removed, will it become
> useful in the future releases or has a more subtle use? 

No, the tracefile-size options does what it says, the limitation is that
the smallest unit it can work with is the subbuf-size. So the smallest
tracefile size you can have is the subbuf-size.

If you are using 4k subbuffers and you limit the size of the tracefiles
to 4M, then you will have at most 1000 full packets in each tracefile.
But if you have subbuffers of 8M, the smallest tracefile size you can
have is 8M.

Julien