some random Linux tips

• echo 1 > /proc/sys/vm/block_dump
  
  The above command sets a sysctl to cause the kernel to log all disk writes. Below is a sample of the output from it. Beware that there is a lot of data.
  
  Jan 10 09:05:53 aeon kernel: kjournald(1048): WRITE block XXX152 on dm-6
  Jan 10 09:05:53 aeon kernel: kjournald(1048): WRITE block XXX160 on dm-6
  Jan 10 09:05:53 aeon kernel: kjournald(1048): WRITE block XXX168 on dm-6
  Jan 10 09:05:54 aeon kernel: kpowersave(5671): READ block XXX384 on dm-7
  Jan 10 09:05:54 aeon kernel: kpowersave(5671): READ block XXX400 on dm-7
  Jan 10 09:05:54 aeon kernel: kpowersave(5671): READ block XXX408 on dm-7
  Jan 10 09:05:54 aeon kernel: bash(5803): dirtied inode XXXXXX1943 (block_dump) on proc
  


• Prefixing a bash command with ' ' will prevent a ! operator from running it. For example if you had just entered the command " ls -al /" then "!l" would not repeat it but would instead match the preceeding command that started with a 'l'. On SLES-10 a preceeding space also makes the command not appear in
  the history while on Debian/etch it does (both run Bash 3.1).
  


• LD_PRELOAD=/lib/libmemusage.so ls > /dev/null
  
  The above LD_PRELOAD will cause a dump to stderr of data about all memory allocations performed by the program in question. Below is a sample of the output.
  
  

  Memory usage summary: heap total: 28543, heap peak: 20135, stack peak: 9844
          total calls   total memory   failed calls
  malloc|         85          28543              0
  realloc|         11              0              0   (in place: 11, dec: 11)
  calloc|          0              0              0
    free|         21          12107
  Histogram for block sizes:
     0-15             29  30% ==================================================
    16-31              5   5% ========
    32-47             10  10% =================
    48-63             14  14% ========================
    64-79              4   4% ======
    80-95              1   1% =
    96-111            20  20% ==================================
   112-127             2   2% ===
   208-223             1   1% =
   352-367             4   4% ======
   384-399             1   1% =
   480-495             1   1% =
  1536-1551            1   1% =
  4096-4111            1   1% =
  4112-4127            1   1% =
  12800-12815           1   1% =