From 14ec52d3dc332a02dcb7d95b5f5d21fd863fc99e Mon Sep 17 00:00:00 2001 From: Richard Jones Date: Sat, 13 Jun 2009 12:58:47 +0100 Subject: [PATCH] Documentation for the supermin appliance. --- README | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 56 insertions(+) diff --git a/README b/README index 5d340d3..92bfc07 100644 --- a/README +++ b/README @@ -171,6 +171,62 @@ On some systems, the chmod will not survive a reboot, and you will need to make edits to the udev configuration. +Supermin appliance +---------------------------------------------------------------------- + +If you configure with --enable-supermin then we will build a supermin +appliance (supermin = super-minimized). This is a very specialized +appliance which is built on-the-fly at runtime (specifically, when you +call guestfs_launch). + +The normal appliance is a self-contained Linux operating system, based +on the Fedora/RHEL/CentOS Linux distro. So it contains a complete +copy of all the libraries and programs needed, like kernel, libc, +bash, coreutils etc etc. + +The supermin appliance removes the kernel and all the executable +libraries and programs from the appliance. That just leaves a +skeleton of config files and some data files, which is obviously +massively smaller than the normal appliance. At runtime we rebuild +the appliance on-the-fly from the libraries and programs on the host +(eg. pulling in the real /lib/libc.so, the real /bin/bash etc.) + +Although this process of rebuilding the appliance each time sounds +slow, it turns out to be faster than using the prebuilt appliance. +(Most of the saving comes from not compressing the appliance - it +transpires that decompressing the appliance is the slowest part of the +whole boot sequence). On my machine, a new appliance can be built in +under a fifth of a second, and the boot time is several seconds +shorter. + +The big advantage of the supermin appliance for distributions like +Fedora is that it gets security fixes automatically from the host, so +there is no need to rebuild the whole of libguestfs for a security +update in some underlying library. + +There are several DISADVANTAGES: + +It won't work at all except in very narrow, controlled cases like the +Fedora packaging case. We control the dependencies of the libguestfs +RPM tightly to ensure that the required binaries are actually present +on the host. + +Furthermore there are certain unlikely changes in the packages on the +host which could break a supermin appliance, eg. an updated library +which depends on an additional data file. + +Also supermin appliances are subjected to changes in the host kernel +which might break compatibility with qemu -- these are, of course, +real bugs in any case. + +Lastly, supermin appliances really can't be moved between branches of +distributions (eg. built on Fedora 12 and moved to Fedora 10) because +they are not self-contained and they rely on certain libraries being +around. You shouldn't do this anyway. + +Use supermin appliances with caution. + + Notes on cross-architecture support ---------------------------------------------------------------------- -- 1.8.3.1