With all the whirl about containers I decided I could wait no longer to join the fray. Here is a log of some of the things I learned.<\/p>\n
Basic test<\/a>, e.g. what the heck is this?<\/strong><\/p>\n I spun up a cent7 vm several months back. Apparently my repos were a tad old and things didnt work until i did a yum update first, then reboot. Then I could systemctl enable docker, systemctl start docker<\/p>\n Note you MUST sudo in for docker commands to work<\/p>\n Also note: cent7 has docker in it natively – no need to wget install it (unless you want to use some of the new features like the networking module)<\/p>\n The following is your smoke test:<\/p>\n Test 2: Expanded sample<\/a><\/strong><\/p>\n This worked flawlessly\u2026<\/p>\n Test 3: Interactive sample<\/a><\/strong><\/p>\n Ditto, worked perfectly<\/p>\n Test 4: Do my own thing<\/strong><\/p>\n I made a \u201clooper.py\u201d app with the following code:<\/p>\n I then made the following Dockerfile<\/p>\n And create the image with <\/p>\n When i ran the build one of the things I noticed is that python was already installed in the centos image. I modified the Dockerfile by removing the RUN line, and one cool thing is that when i re-ran the build command, the python install layer was automatically removed, and everything else was basically a noop. In other words docker appears to do a good job at being efficient.<\/p>\n I then ran my looper:<\/p>\n Nothing happened\u2026 So i thought\u2026and though\u2026 and eventually decided to try and attach. Ubeknowest to me, by doing docker run i WAS attached, but nonetheless I learned a few things:<\/p>\n Once I did a docker attach to my container id, i saw nothing, still. I did a Ctrl-C and viola, my looper output appeared! I suspected buffering, which turned out to be the case. I modified looper as follows:<\/p>\n Then rebuilt, and re-ran, and it all worked.<\/p>\n Note that this only runs the command in the foreground. To run it in the background:<\/p>\n You can then docker ps, find the cid, docker attach to it. But\u2026 you cannot detach (without sending a SIGKILL)! The docks say Ctrl-P + Ctrl+Q will detach, but this appears to only work if you use the following command when running it:<\/p>\n Where t<\/strong> means create a tty, and i<\/strong> means \u201ckeep stdin open even if not attached\u201d. This works well.<\/p>\n Note that each time i make changes to looper, when i rebuild it takes at most 20 seconds.. if no changes, docker takes milliseconds\u2026<\/p>\n Test 5: layers<\/strong><\/p>\n What if the container modifies a file?<\/p>\n I modified looper.py to write to stdout and a file:<\/p>\n For fun i created a file named \u201cmyfile\u201d, then build the image, then ran the container. When it runs i can do a docker diff:<\/p>\n And to be clear, this means the file was added in the image. Docker wont let the app reach into my own version of \u201cmyfile\u201d.<\/p>\n What if i want to see the file? In older versions of docker, apparently you had a few options, such as running ssh, or making a snapshot, but now its easy:<\/p>\n You can then just cat the file, etc. If you actually want to copy the files out, Test 6: CPU limit<\/strong><\/p>\n You can do a couple things:<\/p>\n 1) Limit the share of cpu usage across multiple containers. This is done by specifying a relative weighting (with -c)<\/p>\n 2) Pin the process to certain cpus with \u2014cpuset-cpus= I havent been able to find an equivalent to the simple \u201climit to N processors\u201d idea on virtual machines. The weighting is fairly close.<\/p>\n","protected":false},"excerpt":{"rendered":" With all the whirl about containers I decided I could wait no longer to join the fray. Here is a log of some of the things I learned. Basic test, e.g. what the heck is this? I spun up a cent7 vm several months back. Apparently my repos were a tad old and things didnt … Continue reading Containers for Fun and Profit<\/span> \r\nsudo docker run hello-world \r\n<\/pre>\n
\r\n#!\/usr\/bin\/python\r\nimport time\r\nwhile True:\r\n\tprint "Hi! ", time.time()\r\n\ttime.sleep(1)\r\n<\/pre>\n
\r\nFROM docker.io\/centos:latest\r\nRUN yum install python -y\r\nCOPY looper.py \/\r\nCMD \/looper.py\r\n<\/pre>\n
\r\n\tdocker build -t looper .\r\n<\/pre>\n
\r\n\tdocker run looper\r\n<\/pre>\n
\n
\r\n#!\/usr\/bin\/python\r\nimport time\r\nimport sys\r\nwhile True:\r\n\tprint "Hi! ", time.time()\r\n\tsys.stdout.flush()\r\n\ttime.sleep(1)\r\n<\/pre>\n
\r\n\tdocker run -d looper\r\n<\/pre>\n
\r\n\tdocker run -tdi looper\r\n<\/pre>\n
\r\n#!\/usr\/bin\/python\r\nimport time\r\nimport sys\r\nwhile True:\r\n\tmsg = "Hi! " + str(time.time())\r\n\tprint msg\r\n\twith open('myfile','a') as f:\r\n\t\tf.write(msg + '\\n')\r\n\tsys.stdout.flush()\r\n\ttime.sleep(1)\r\n<\/pre>\n\r\n# docker diff f04849645523\r\nA \/myfile\r\n<\/pre>\n
\r\n\tdocker exec -t -i <cid> \/bin\/bash\r\n<\/pre>\n
\nyou can export the whole filesystem (docker export \r\ndocker cp <cid>:<src> <dest>\r\n<\/pre>\n