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Libvirt provides a portable, long term stable C API for managing the virtualization technologies provided by many operating systems. It includes support for QEMU, KVM, Xen, LXC, bhyve, Virtuozzo, VMware vCenter and ESX, VMware Desktop, Hyper-V, VirtualBox and the POWER Hypervisor.
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Eric Blake b298025063 blockcommit: document semantics of committing active layer 
Now that qemu 2.0 allows commit of the active layer, people are
attempting to use virsh blockcommit and getting into a stuck
state, because libvirt is unprepared to handle the two-phase
commit required by qemu.

Stepping back a bit, there are two valid semantics for a
commit operation:

1. Maintain a 'golden' base, and a transient overlay. Make
changes in the overlay, and if everything appears to work,
commit those changes into the base, but still keep the overlay
for the next round of changes; repeat the cycle as desired.

2. Create an external snapshot, then back up the stable state
in the backing file. Once the backup is complete, commit the
overlay back into the base, and delete the temporary snapshot.

Since qemu doesn't know up front which of the two styles is
preferred, a block commit of the active layer merely gets
the job into a synchronized state, and sends an event; then
the user must either cancel (case 1) or complete (case 2),
where qemu then sends a second event that actually ends the
job.  However, until commit e6bcbcd, libvirt was blindly
assuming the semantics that apply to a commit of an
intermediate image, where there is only one sane conclusion
(the job automatically ends with fewer elements in the chain);
and getting stuck because it wasn't prepared for qemu to enter
a second phase of the job.

This patch adds a flag to the libvirt API that a user MUST
supply in order to acknowledge that they will be using two-phase
semantics.  It might be possible to have a mode where if the
flag is omitted, we automatically do the case 2 semantics on
the user's behalf; but before that happens, I must do additional
patches to track the fact that we are doing an active commit
in the domain XML.  Later patches will add support of the flag,
and once 2-phase semantics are working, we can then decide
whether to relax things to allow an omitted flag to cause an
automatic pivot.

* include/libvirt/libvirt.h.in (VIR_DOMAIN_BLOCK_COMMIT_ACTIVE)
(VIR_DOMAIN_BLOCK_JOB_TYPE_ACTIVE_COMMIT): New enums.
* src/libvirt.c (virDomainBlockCommit): Document two-phase job
when committing active layer, through new flag.
(virDomainBlockJobAbort): Document that pivot also occurs after
active commit.
* tools/virsh-domain.c (vshDomainBlockJob): Cover new job.
* src/qemu/qemu_driver.c (qemuDomainBlockCommit): Explicitly
reject active copy; later patches will add it in.

Signed-off-by: Eric Blake <eblake@redhat.com>
2014-06-10 11:11:16 -06:00
.gnulib@d55899fd2c maint: detect VPATH builds when checking for gnulib update 2014-06-04 16:06:55 -06:00
build-aux Require spaces around equality comparisons 2014-03-18 11:29:44 +01:00
daemon Don't use AI_ADDRCONFIG when binding to wildcard addresses 2014-06-02 17:12:01 +02:00
docs docs: fix a typo in hacking.html.in 2014-06-10 08:53:03 -06:00
examples Add a new example to illustrate domain migration 2014-04-30 14:29:07 +01:00
gnulib maint: update to latest gnulib 2014-01-01 06:02:47 -07:00
include blockcommit: document semantics of committing active layer 2014-06-10 11:11:16 -06:00
m4 m4: bhyve: Fix check for the required bhyve programs 2014-06-09 09:40:25 +02:00
po Add vbox_snapshot_conf struct 2014-06-10 15:07:36 +01:00
src blockcommit: document semantics of committing active layer 2014-06-10 11:11:16 -06:00
tests Add vbox_snapshot_conf struct 2014-06-10 15:07:36 +01:00
tools blockcommit: document semantics of committing active layer 2014-06-10 11:11:16 -06:00
.ctags maint: Make ctags work out of the box 2013-07-18 08:47:21 +02:00
.dir-locals.el build: avoid tabs that failed syntax-check 2012-09-06 09:43:46 -06:00
.gitignore Add a new example to illustrate domain migration 2014-04-30 14:29:07 +01:00
.gitmodules make .gnulib a submodule 2009-07-08 16:17:51 +02:00
.mailmap Autogenerate AUTHORS 2012-10-19 12:44:56 -04:00
AUTHORS.in Add Roman Bogorodskiy to the committers list 2014-02-20 20:51:08 +04:00
autobuild.sh Disable libvirtd by default when building on Win32 2014-04-29 11:30:32 +01:00
autogen.sh maint: detect VPATH builds when checking for gnulib update 2014-06-04 16:06:55 -06:00
bootstrap maint: update to latest gnulib 2014-01-01 06:02:47 -07:00
bootstrap.conf Add helpers for getting env vars in a setuid environment 2013-10-21 14:03:52 +01:00
cfg.mk maint: prohibit empty first lines 2014-06-06 11:19:50 +02:00
ChangeLog-old Fix typos in src/* 2014-04-21 16:49:08 -06:00
config-post.h build: fix build of virt-login-shell on systems with older gnutls 2013-10-22 09:41:50 -06:00
configure.ac Bump version to 1.2.6 for new dev cycle 2014-06-02 09:53:30 +08:00
COPYING maint: follow recommended practice for using LGPL 2013-05-20 14:15:21 -06:00
COPYING.LESSER maint: follow recommended practice for using LGPL 2013-05-20 14:15:21 -06:00
HACKING docs: fix a typo in hacking.html.in 2014-06-10 08:53:03 -06:00
libvirt.pc.in Add missing 'libvirt_lxc_api' variable in pkg-config file 2013-09-04 14:52:40 +01:00
libvirt.spec.in Add helper program to create custom leases 2014-06-02 11:45:10 +01:00
Makefile.am Add a new example to illustrate domain migration 2014-04-30 14:29:07 +01:00
Makefile.nonreentrant maint: use LGPL correctly 2013-05-20 14:03:48 -06:00
mingw-libvirt.spec.in conf: create common storage RNG grammar file 2014-04-14 07:07:01 -06:00
README Correct typos in the documentation (Atsushi SAKAI) 2008-01-24 10:15:13 +00:00
README-hacking docs: update README-hacking 2014-05-06 16:20:24 -06:00
run.in Use virFileFindResource to locate virtlockd daemon 2014-04-28 13:30:41 +01:00
TODO Update todo list file to point at bugzilla/website 2010-10-13 16:45:26 +01:00

README 

         LibVirt : simple API for virtualization

  Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.

Daniel Veillard <veillard@redhat.com>