LTTng » LTTng-tools

It seems the minimum size of the LTTng sub-buffers is 4096 bytes (for either domain and either buffering scheme):

$ sudo -H lttng create s
Session s created.
Traces will be written in /root/lttng-traces/s-20130625-101152
$ sudo -H lttng enable-channel k1 -k --subbuf-size 131072
Kernel channel k1 enabled for session s
$ sudo -H lttng enable-channel k2 -k --subbuf-size 65536
Kernel channel k2 enabled for session s
$ sudo -H lttng enable-channel k3 -k --subbuf-size 32768
Kernel channel k3 enabled for session s
$ sudo -H lttng enable-channel k4 -k --subbuf-size 16384
Kernel channel k4 enabled for session s
$ sudo -H lttng enable-channel k5 -k --subbuf-size 8192
Kernel channel k5 enabled for session s
$ sudo -H lttng enable-channel k6 -k --subbuf-size 4096
Kernel channel k6 enabled for session s
$ sudo -H lttng enable-channel k7 -k --subbuf-size 2048
PERROR [20531/20631]: ioctl kernel create channel: Invalid argument (in kernel_create_channel() at kernel.c:144)
Error: Channel k7: Kernel create channel failed (session s)
Warning: Some command(s) went wrong

$ sudo -H lttng create ss
Session ss created.
Traces will be written in /root/lttng-traces/ss-20130625-101446
$ sudo -H lttng enable-channel k7 -k --subbuf-size 2048
PERROR [20531/20631]: ioctl kernel create channel: Invalid argument (in kernel_create_channel() at kernel.c:144)
Error: Channel k7: Kernel create channel failed (session ss)
Warning: Some command(s) went wrong
$ sudo -H lttng enable-channel u7 -u --subbuf-size 2048
Error: Channel u7: Invalid parameter (session ss)
Warning: Some command(s) went wrong
$ sudo -H lttng enable-channel u6 -u --subbuf-size 4096
UST channel u6 enabled for session ss

This should be mentioned in the man pages.