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 /proc/slabinfo
  19 /sys/devices/system/cpu/online
  20 ```
  21
  22 are container aware such that the values displayed (e.g. in `/proc/uptime`)
  23 really reflect how long the container is running and not how long the host is
  24 running.
  25
  26 Prior to the implementation of cgroup namespaces by Serge Hallyn `LXCFS` also
  27 provided a container aware `cgroupfs` tree. It took care that the container
  28 only had access to cgroups underneath it's own cgroups and thus provided
  29 additional safety. For systems without support for cgroup namespaces `LXCFS`
  30 will still provide this feature but it is mostly considered deprecated.
  31
  32 ## Upgrading `LXCFS` without restart
  33
  34 `LXCFS` is split into a shared library (a libtool module, to be precise)
  35 `liblxcfs` and a simple binary `lxcfs`. When upgrading to a newer version of
  36 `LXCFS` the `lxcfs` binary will not be restarted. Instead it will detect that
  37 a new version of the shared library is available and will reload it using
  38 `dlclose(3)` and `dlopen(3)`. This design was chosen so that the fuse main loop
  39 that `LXCFS` uses will not need to be restarted. If it were then all containers
  40 using `LXCFS` would need to be restarted since they would otherwise be left
  41 with broken fuse mounts.
  42
  43 To force a reload of the shared library at the next possible instance simply
  44 send `SIGUSR1` to the pid of the running `LXCFS` process. This can be as simple
  45 as doing:
  46
  47     rm /usr/lib64/lxcfs/liblxcfs.so # MUST to delete the old library file first
  48     cp liblxcfs.so /usr/lib64/lxcfs/liblxcfs.so # to place new library file
  49     kill -s USR1 $(pidof lxcfs) # reload
  50
  51 ### musl
  52
  53 To achieve smooth upgrades through shared library reloads `LXCFS` also relies
  54 on the fact that when `dlclose(3)` drops the last reference to the shared
  55 library destructors are run and when `dlopen(3)` is called constructors are
  56 run. While this is true for `glibc` it is not true for `musl` (See the section
  57 [Unloading libraries](https://wiki.musl-libc.org/functional-differences-from-glibc.html).).
  58 So users of `LXCFS` on `musl` are advised to restart `LXCFS` completely and all
  59 containers making use of it.
  60
  61 ## Building
  62
  63 In order to build LXCFS install fuse and the fuse development headers according
  64 to your distro. LXCFS prefers `fuse3` but does work with new enough `fuse2`
  65 versions:
  66
  67     git clone git://github.com/lxc/lxcfs
  68     cd lxcfs
  69     meson setup -Dinit-script=systemd --prefix=/usr build/
  70     meson compile -C build/
  71     sudo meson install -C build/
  72
  73 ## Usage
  74 The recommended command to run lxcfs is:
  75
  76     sudo mkdir -p /var/lib/lxcfs
  77     sudo lxcfs /var/lib/lxcfs
  78
  79 A container runtime wishing to use `LXCFS` should then bind mount the
  80 approriate files into the correct places on container startup.
  81
  82 ### LXC
  83 In order to use lxcfs with systemd-based containers, you can either use
  84 LXC 1.1 in which case it should work automatically, or otherwise, copy
  85 the `lxc.mount.hook` and `lxc.reboot.hook` files (once built) from this tree to
  86 `/usr/share/lxcfs`, make sure it is executable, then add the
  87 following lines to your container configuration:
  88 ```
  89 lxc.mount.auto = cgroup:mixed
  90 lxc.autodev = 1
  91 lxc.kmsg = 0
  92 lxc.include = /usr/share/lxc/config/common.conf.d/00-lxcfs.conf
  93 ```
  94
  95 ## Using with Docker
  96
  97 ```
  98 docker run -it -m 256m --memory-swap 256m \
  99       -v /var/lib/lxcfs/proc/cpuinfo:/proc/cpuinfo:rw \
 100       -v /var/lib/lxcfs/proc/diskstats:/proc/diskstats:rw \
 101       -v /var/lib/lxcfs/proc/meminfo:/proc/meminfo:rw \
 102       -v /var/lib/lxcfs/proc/stat:/proc/stat:rw \
 103       -v /var/lib/lxcfs/proc/swaps:/proc/swaps:rw \
 104       -v /var/lib/lxcfs/proc/uptime:/proc/uptime:rw \
 105       -v /var/lib/lxcfs/proc/slabinfo:/proc/slabinfo:rw \
 106       ubuntu:18.04 /bin/bash
 107  ```
 108
 109  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.
 110
 111  sudo lxcfs -u /var/lib/lxcfs
 112
 113 ## Swap handling
 114 If you noticed LXCFS not showing any SWAP in your container despite
 115 having SWAP on your system, please read this section carefully and look
 116 for instructions on how to enable SWAP accounting for your distribution.
 117
 118 Swap cgroup handling on Linux is very confusing and there just isn't a
 119 perfect way for LXCFS to handle it.
 120
 121 Terminology used below:
 122  - RAM refers to `memory.usage_in_bytes` and `memory.limit_in_bytes`
 123  - RAM+SWAP refers to `memory.memsw.usage_in_bytes` and `memory.memsw.limit_in_bytes`
 124
 125 The main issues are:
 126  - SWAP accounting is often opt-in and, requiring a special kernel boot
 127    time option (`swapaccount=1`) and/or special kernel build options
 128    (`CONFIG_MEMCG_SWAP`).
 129
 130  - Both a RAM limit and a RAM+SWAP limit can be set. The delta however
 131    isn't the available SWAP space as the kernel is still free to SWAP as
 132    much of the RAM as it feels like. This makes it impossible to render
 133    a SWAP device size as using the delta between RAM and RAM+SWAP for that
 134    wouldn't account for the kernel swapping more pages, leading to swap
 135    usage exceeding swap total.
 136
 137  - It's impossible to disable SWAP in a given container. The closest
 138    that can be done is setting swappiness down to 0 which severly limits
 139    the risk of swapping pages but doesn't eliminate it.
 140
 141 As a result, LXCFS had to make some compromise which go as follow:
 142  - When SWAP accounting isn't enabled, no SWAP space is reported at all.
 143    This is simply because there is no way to know the SWAP consumption.
 144    The container may very much be using some SWAP though, there's just
 145    no way to know how much of it and showing a SWAP device would require
 146    some kind of SWAP usage to be reported. Showing the host value would be
 147    completely wrong, showing a 0 value would be equallty wrong.
 148
 149  - Because SWAP usage for a given container can exceed the delta between
 150    RAM and RAM+SWAP, the SWAP size is always reported to be the smaller of
 151    the RAM+SWAP limit or the host SWAP device itself. This ensures that at no
 152    point SWAP usage will be allowed to exceed the SWAP size.
 153
 154  - If the swappiness is set to 0 and there is no SWAP usage, no SWAP is reported.
 155    However if there is SWAP usage, then a SWAP device of the size of the
 156    usage (100% full) is reported. This provides adequate reporting of
 157    the memory consumption while preventing applications from assuming more
 158    SWAP is available.