README.md

   1 # lxcfs
   2
   3 ## Introduction
   4 LXCFS is a small FUSE filesystem written with the intention of making Linux
   5 containers feel more like a virtual machine. It started as a side-project of
   6 `LXC` but is useable by any runtime.
   7
   8 LXCFS will take care that the information provided by crucial files in `procfs`
   9 such as:
  10
  11 ```
  12 /proc/cpuinfo
  13 /proc/diskstats
  14 /proc/meminfo
  15 /proc/stat
  16 /proc/swaps
  17 /proc/uptime
  18 /sys/devices/system/cpu/online
  19 ```
  20
  21 are container aware such that the values displayed (e.g. in `/proc/uptime`)
  22 really reflect how long the container is running and not how long the host is
  23 running.
  24
  25 Prior to the implementation of cgroup namespaces by Serge Hallyn `LXCFS` also
  26 provided a container aware `cgroupfs` tree. It took care that the container
  27 only had access to cgroups underneath it's own cgroups and thus provided
  28 additional safety. For systems without support for cgroup namespaces `LXCFS`
  29 will still provide this feature but it is mostly considered deprecated.
  30
  31 ## Upgrading `LXCFS` without restart
  32
  33 `LXCFS` is split into a shared library (a libtool module, to be precise)
  34 `liblxcfs` and a simple binary `lxcfs`. When upgrading to a newer version of
  35 `LXCFS` the `lxcfs` binary will not be restarted. Instead it will detect that
  36 a new version of the shared library is available and will reload it using
  37 `dlclose(3)` and `dlopen(3)`. This design was chosen so that the fuse main loop
  38 that `LXCFS` uses will not need to be restarted. If it were then all containers
  39 using `LXCFS` would need to be restarted since they would otherwise be left
  40 with broken fuse mounts.
  41
  42 To force a reload of the shared library at the next possible instance simply
  43 send `SIGUSR1` to the pid of the running `LXCFS` process. This can be as simple
  44 as doing:
  45
  46     kill -s USR1 $(pidof lxcfs)
  47
  48 ### musl
  49
  50 To achieve smooth upgrades through shared library reloads `LXCFS` also relies
  51 on the fact that when `dlclose(3)` drops the last reference to the shared
  52 library destructors are run and when `dlopen(3)` is called constructors are
  53 run. While this is true for `glibc` it is not true for `musl` (See the section
  54 [Unloading libraries](https://wiki.musl-libc.org/functional-differences-from-glibc.html).).
  55 So users of `LXCFS` on `musl` are advised to restart `LXCFS` completely and all
  56 containers making use of it.
  57
  58 ## Building
  59 Build lxcfs as follows:
  60
  61     yum install fuse fuse-lib fuse-devel
  62     git clone git://github.com/lxc/lxcfs
  63     cd lxcfs
  64     ./bootstrap.sh
  65     ./configure
  66     make
  67     make install
  68
  69 ## Usage
  70 The recommended command to run lxcfs is:
  71
  72     sudo mkdir -p /var/lib/lxcfs
  73     sudo lxcfs /var/lib/lxcfs
  74
  75 A container runtime wishing to use `LXCFS` should then bind mount the
  76 approriate files into the correct places on container startup.
  77
  78 ### LXC
  79 In order to use lxcfs with systemd-based containers, you can either use
  80 LXC 1.1 in which case it should work automatically, or otherwise, copy
  81 the `lxc.mount.hook` and `lxc.reboot.hook` files (once built) from this tree to
  82 `/usr/share/lxcfs`, make sure it is executable, then add the
  83 following lines to your container configuration:
  84 ```
  85 lxc.mount.auto = cgroup:mixed
  86 lxc.autodev = 1
  87 lxc.kmsg = 0
  88 lxc.include = /usr/share/lxc/config/common.conf.d/00-lxcfs.conf
  89 ```
  90
  91 ## Using with Docker
  92
  93 ```
  94 docker run -it -m 256m --memory-swap 256m \
  95       -v /var/lib/lxcfs/proc/cpuinfo:/proc/cpuinfo:rw \
  96       -v /var/lib/lxcfs/proc/diskstats:/proc/diskstats:rw \
  97       -v /var/lib/lxcfs/proc/meminfo:/proc/meminfo:rw \
  98       -v /var/lib/lxcfs/proc/stat:/proc/stat:rw \
  99       -v /var/lib/lxcfs/proc/swaps:/proc/swaps:rw \
 100       -v /var/lib/lxcfs/proc/uptime:/proc/uptime:rw \
 101       ubuntu:18.04 /bin/bash
 102  ```
 103
 104  In a system with swap enabled, the parameter "-u" can be used to set all values in "meminfo" that refer to the swap to 0.
 105
 106  sudo lxcfs -u /var/lib/lxcfs
 107
 108 ## Swap handling
 109 If you noticed LXCFS not showing any SWAP in your container despite
 110 having SWAP on your system, please read this section carefully and look
 111 for instructions on how to enable SWAP accounting for your distribution.
 112
 113 Swap cgroup handling on Linux is very confusing and there just isn't a
 114 perfect way for LXCFS to handle it.
 115
 116 Terminology used below:
 117  - RAM refers to `memory.usage_in_bytes` and `memory.limit_in_bytes`
 118  - RAM+SWAP refers to `memory.memsw.usage_in_bytes` and `memory.memsw.limit_in_bytes`
 119
 120 The main issues are:
 121  - SWAP accounting is often opt-in and, requiring a special kernel boot
 122    time option (`swapaccount=1`) and/or special kernel build options
 123    (`CONFIG_MEMCG_SWAP`).
 124
 125  - Both a RAM limit and a RAM+SWAP limit can be set. The delta however
 126    isn't the available SWAP space as the kernel is still free to SWAP as
 127    much of the RAM as it feels like. This makes it impossible to render
 128    a SWAP device size as using the delta between RAM and RAM+SWAP for that
 129    wouldn't account for the kernel swapping more pages, leading to swap
 130    usage exceeding swap total.
 131
 132  - It's impossible to disable SWAP in a given container. The closest
 133    that can be done is setting swappiness down to 0 which severly limits
 134    the risk of swapping pages but doesn't eliminate it.
 135
 136 As a result, LXCFS had to make some compromise which go as follow:
 137  - When SWAP accounting isn't enabled, no SWAP space is reported at all.
 138    This is simply because there is no way to know the SWAP consumption.
 139    The container may very much be using some SWAP though, there's just
 140    no way to know how much of it and showing a SWAP device would require
 141    some kind of SWAP usage to be reported. Showing the host value would be
 142    completely wrong, showing a 0 value would be equallty wrong.
 143
 144  - Because SWAP usage for a given container can exceed the delta between
 145    RAM and RAM+SWAP, the SWAP size is always reported to be the smaller of
 146    the RAM+SWAP limit or the host SWAP device itself. This ensures that at no
 147    point SWAP usage will be allowed to exceed the SWAP size.
 148
 149  - If the swappiness is set to 0 and there is no SWAP usage, no SWAP is reported.
 150    However if there is SWAP usage, then a SWAP device of the size of the
 151    usage (100% full) is reported. This provides adequate reporting of
 152    the memory consumption while preventing applications from assuming more
 153    SWAP is available.