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snapshot: allow reuse of existing files in disk snapshot

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When disk snapshots were first implemented, libvirt blindly refused
to allow an external snapshot destination that already exists, since
qemu will blindly overwrite the contents of that file during the
snapshot_blkdev monitor command, and we don't like a default of
data loss by default.  But VDSM has a scenario where NFS permissions
are intentionally set so that the destination file can only be
created by the management machine, and not the machine where the
guest is running, so that libvirt will necessarily see the destination
file already existing; adding a flag will allow VDSM to force the file
reuse without libvirt complaining of possible data loss.

https://bugzilla.redhat.com/show_bug.cgi?id=767104

* include/libvirt/libvirt.h.in (virDomainSnapshotCreateFlags): Add
VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT.
* src/libvirt.c (virDomainSnapshotCreateXML): Document it.  Add
note about partial failure.
* tools/virsh.c (cmdSnapshotCreate, cmdSnapshotCreateAs): Add new
flag.
* tools/virsh.pod (snapshot-create, snapshot-create-as): Document
it.
* src/qemu/qemu_driver.c (qemuDomainSnapshotDiskPrepare)
(qemuDomainSnapshotCreateXML): Implement the new flag.
This commit is contained in:
Eric Blake
2012-01-09 11:57:46 -07:00
parent 529e4a5006
commit 4e9953a426
5 changed files with 44 additions and 8 deletions
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16
tools/virsh.pod
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@@ -2024,7 +2024,7 @@ used to represent properties of snapshots.
=over 4
=item B<snapshot-create> I<domain> [I<xmlfile>] {[I<--redefine> [I<--current>]]
| [I<--no-metadata>] [I<--halt>] [I<--disk-only>]}
| [I<--no-metadata>] [I<--halt>] [I<--disk-only>] [I<--reuse-external]}
Create a snapshot for domain I<domain> with the properties specified in
I<xmlfile>. Normally, the only properties settable for a domain snapshot
@@ -2061,14 +2061,19 @@ treat the snapshot as current, and cannot revert to the snapshot
unless I<--redefine> is later used to teach libvirt about the
metadata again).
If I<--reuse-external> is specified, and the snapshot XML requests an
external snapshot with a destination of an existing file, then the
existing file is truncated and reused; otherwise, a snapshot is refused
to avoid losing contents of the existing files.
Existence of snapshot metadata will prevent attempts to B<undefine>
a persistent domain. However, for transient domains, snapshot
metadata is silently lost when the domain quits running (whether
by command such as B<destroy> or by internal guest action).
=item B<snapshot-create-as> I<domain> {[I<--print-xml>]
| [I<--no-metadata>] [I<--halt>]} [I<name>] [I<description>]
[I<--disk-only> [[I<--diskspec>] B<diskspec>]...
| [I<--no-metadata>] [I<--halt>] [I<--reuse-existing>]} [I<name>]
[I<description>] [I<--disk-only> [[I<--diskspec>] B<diskspec>]...]
Create a snapshot for domain I<domain> with the given <name> and
<description>; if either value is omitted, libvirt will choose a
@@ -2091,6 +2096,11 @@ results in the following XML:
<source file='/path/to,new'/>
</disk>
If I<--reuse-external> is specified, and the domain XML or I<diskspec>
option requests an external snapshot with a destination of an existing
file, then the existing file is truncated and reused; otherwise, a
snapshot is refused to avoid losing contents of the existing files.
If I<--no-metadata> is specified, then the snapshot data is created,
but any metadata is immediately discarded (that is, libvirt does not
treat the snapshot as current, and cannot revert to the snapshot