Are those log files in /storage/sdcard0/log necessary? On my SGS2 (GT-I9100, Android 4.1.2), 1.2GB of log files piled up in the folder

/storage/sdcard0/log


. I have almost no space left on the internal SD card and a lot of the space is used by those log files, so I want to get rid of them once and for all.

To give you an idea what's there, here are two examples of file names stored there:

dmesg_cp_boot_201405280100_24.log
logcat_cp_boot_radio_20140219173_53.log


Is it normal that those logfiles get saved there? I can simply delete them, but then new ones are written there anyways. Can I prevent that?

UPDATE:

The logfiles seem to have started appearing about a year ago. The file with the oldest date in the file name is logcat_cp_boot_radio_201304060741_36.log.

Most of the files have names of the form logcat_cp_boot_radio_*.log, but some are also called dmesg_cp_boot_*.log or dmesg_cp_boot_done_*.log, the latter two being completely empty files (0 bytes length).
Usually the presence of logcat_cp_boot_radio*.log files point to an issue with the phone's modem.  It's possible that when you first rooted the device you flashed a ROM which contained a modem firmware that was either old, or slightly incompatible with your current carrier.  Besides amassing large amounts of these log files, you would see frequent disconnects and reconnects from cell and data services when the radio crashes and restarts.

While you can always keep deleting the log files every now and then to free up the memory, the larger issue will persist. In order to resolve the root cause of the RIL crash, you will need to install the proper radio firmware.  These differ based on geographic location, and sometimes on the carrier.

The first step is to find the modem version that is currently installed on your phone.  You can find this information in Settings -> About phone, under the "Baseband version." It should start with "i19100", and then followed by some letters and numbers.  The letters represent a geographic location:


  CE---> Belgium, Luxembourg, Netherlands
  DC---> Thailand
  DD---> India
  DX---> Indonesia, Malaysia, Philippines, Singapore, Vietnam
  DZ---> Malaysia, Singapore
  JA---> South Africa
  JC---> Algeria, Morocco, Nigeria, South Africa, Tunisia
  JP---> Arabic, Algeria, Egypt, Iran, Iraq, Kuwait, Morocco, Nigeria,
  Oman, Pakistan, Saudi Arabia, Syria
  JV---> Tunisia, Turkey
  UG---> North America
  UH---> Latin America, The Caribbean
  XE---> Bulgaria, Estonia, Kazakhstan, Latvia, Lithuania, Russia,
  Ukraine
  XX---> Austria, Belgium, France, Germany, Hungary, United Kingdom
  XW---> Austria, Belgium, France, Germany, Hungary, Italy, Nordic,
  Spain, United Kingdom
  ZC---> China, Hong Kong
  ZH---> Hong Kong
  ZS---> China, Hong Kong
  ZT---> Taiwan  


And the numbers represent the version, incremented alphabetically.  You should find the latest modem version based on the country you are in, and then flash it on your phone using either Odin, or Recovery (must be a custom recovery like ClockworkMod or TWRP, for example.)

A quick Google search yielded this site which contains ClockworkMod-formatted modems for the Galaxy S2 I9100 variant for various countries.  I'm not sure how recent it is, but this should start you off in the right direction.