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snapshot: improve qemu handling of reused snapshot targets

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The oVirt developers have stated that the real reasons they want
to have qemu reuse existing volumes when creating a snapshot are:
1. the management framework is set up so that creation has to be
done from a central node for proper resource tracking, and having
libvirt and/or qemu create things violates the framework, and
2. qemu defaults to creating snapshots with an absolute path to
the backing file, but oVirt wants to manage a backing chain that
uses just relative names, to allow for easier migration of a chain
across storage locations.

When 0.9.10 added VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT (commit
4e9953a4), it only addressed point 1, but libvirt was still using
O_TRUNC which violates point 2.  Meanwhile, the new qemu
'transaction' monitor command includes a new optional mode argument
that will force qemu to reuse the metadata of the file it just
opened (with the burden on the caller to have valid metadata there
in the first place).  So, this tweaks the meaning of the flag to
cover both points as intended for use by oVirt.  It is not strictly
backward-compatible to 0.9.10 behavior, but it can be argued that
the O_TRUNC of 0.9.10 was a bug.

Note that this flag is all-or-nothing, and only selects between
'existing' and the default 'absolute-paths'.  A more flexible
approach that would allow per-disk selections, as well as adding
support for the 'no-backing-file' mode, would be possible by
extending the <domainsnapshot> xml to have a per-disk mode, but
until we have a management application expressing a need for that
additional complexity, it is not worth doing.

* src/libvirt.c (virDomainSnapshotCreateXML): Tweak documentation.
* src/qemu/qemu_monitor.h (qemuMonitorDiskSnapshot): Add
parameters.
* src/qemu/qemu_monitor_json.h (qemuMonitorJSONDiskSnapshot):
Likewise.
* src/qemu/qemu_monitor.c (qemuMonitorDiskSnapshot): Pass them
through.
* src/qemu/qemu_monitor_json.c (qemuMonitorJSONDiskSnapshot): Use
new monitor command arguments.
* src/qemu/qemu_driver.c (qemuDomainSnapshotCreateDiskActive)
(qemuDomainSnapshotCreateSingleDiskActive): Adjust callers.
(qemuDomainSnapshotDiskPrepare): Allow qed, modify rules on reuse.
This commit is contained in:
Eric Blake 2012-03-20 15:03:45 -06:00
parent 0436d328f5
commit d42a2ffc07
6 changed files with 53 additions and 28 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
src/libvirt.c
View File

@ -17102,10 +17102,13 @@ virDomainSnapshotGetConnect(virDomainSnapshotPtr snapshot)
* VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY to be passed as well.
*
* By default, if the snapshot involves external files, and any of the
* destination files already exist as a regular file, the snapshot is
* rejected to avoid losing contents of those files. However, if
* @flags includes VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT, then existing
* destination files are instead truncated and reused.
* destination files already exist as a non-empty regular file, the
* snapshot is rejected to avoid losing contents of those files.
* However, if @flags includes VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT,
* then the destination files must already exist and contain content
* identical to the source files (this allows a management app to
* pre-create files with relative backing file names, rather than the
* default of creating with absolute backing file names).
*
* Be aware that although libvirt prefers to report errors up front with
* no other effect, some hypervisors have certain types of failures where

42
src/qemu/qemu_driver.c
View File

@ -9726,6 +9726,12 @@ qemuDomainSnapshotDiskPrepare(virDomainObjPtr vm, virDomainSnapshotDefPtr def,
int external = 0;
qemuDomainObjPrivatePtr priv = vm->privateData;
if (allow_reuse && !qemuCapsGet(priv->qemuCaps, QEMU_CAPS_TRANSACTION)) {
qemuReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
_("reuse is not supported with this QEMU binary"));
goto cleanup;
}
for (i = 0; i < def->ndisks; i++) {
virDomainSnapshotDiskDefPtr disk = &def->disks[i];
@ -9756,8 +9762,8 @@ qemuDomainSnapshotDiskPrepare(virDomainObjPtr vm, virDomainSnapshotDefPtr def,
virReportOOMError();
goto cleanup;
}
} else if (STRNEQ(disk->driverType, "qcow2")) {
/* XXX We should also support QED */
} else if (STRNEQ(disk->driverType, "qcow2") &&
STRNEQ(disk->driverType, "qed")) {
qemuReportError(VIR_ERR_CONFIG_UNSUPPORTED,
_("external snapshot format for disk %s "
"is unsupported: %s"),
@ -9771,7 +9777,7 @@ qemuDomainSnapshotDiskPrepare(virDomainObjPtr vm, virDomainSnapshotDefPtr def,
disk->name, disk->file);
goto cleanup;
}
} else if (!(S_ISBLK(st.st_mode) || allow_reuse)) {
} else if (!(S_ISBLK(st.st_mode) || !st.st_size || allow_reuse)) {
qemuReportError(VIR_ERR_CONFIG_UNSUPPORTED,
_("external snapshot file for disk %s already "
"exists and is not a block device: %s"),
@ -9824,7 +9830,8 @@ qemuDomainSnapshotCreateSingleDiskActive(struct qemud_driver *driver,
virDomainSnapshotDiskDefPtr snap,
virDomainDiskDefPtr disk,
virDomainDiskDefPtr persistDisk,
virJSONValuePtr actions)
virJSONValuePtr actions,
bool reuse)
{
qemuDomainObjPrivatePtr priv = vm->privateData;
char *device = NULL;
@ -9846,23 +9853,25 @@ qemuDomainSnapshotCreateSingleDiskActive(struct qemud_driver *driver,
if (virAsprintf(&device, "drive-%s", disk->info.alias) < 0 ||
!(source = strdup(snap->file)) ||
(STRNEQ_NULLABLE(disk->driverType, "qcow2") &&
!(driverType = strdup("qcow2"))) ||
(STRNEQ_NULLABLE(disk->driverType, snap->driverType) &&
!(driverType = strdup(snap->driverType))) ||
(persistDisk &&
(!(persistSource = strdup(source)) ||
(STRNEQ_NULLABLE(persistDisk->driverType, "qcow2") &&
!(persistDriverType = strdup("qcow2")))))) {
(STRNEQ_NULLABLE(persistDisk->driverType, snap->driverType) &&
!(persistDriverType = strdup(snap->driverType)))))) {
virReportOOMError();
goto cleanup;
}
/* create the stub file and set selinux labels; manipulate disk in
* place, in a way that can be reverted on failure. */
fd = qemuOpenFile(driver, source, O_WRONLY | O_TRUNC | O_CREAT,
&need_unlink, NULL);
if (fd < 0)
goto cleanup;
VIR_FORCE_CLOSE(fd);
if (!reuse) {
fd = qemuOpenFile(driver, source, O_WRONLY | O_TRUNC | O_CREAT,
&need_unlink, NULL);
if (fd < 0)
goto cleanup;
VIR_FORCE_CLOSE(fd);
}
origsrc = disk->src;
disk->src = source;
@ -9884,7 +9893,8 @@ qemuDomainSnapshotCreateSingleDiskActive(struct qemud_driver *driver,
origdriver = NULL;
/* create the actual snapshot */
ret = qemuMonitorDiskSnapshot(priv->mon, actions, device, source);
ret = qemuMonitorDiskSnapshot(priv->mon, actions, device, source,
driverType, reuse);
virDomainAuditDisk(vm, disk->src, source, "snapshot", ret >= 0);
if (ret < 0)
goto cleanup;
@ -10005,6 +10015,7 @@ qemuDomainSnapshotCreateDiskActive(virConnectPtr conn,
bool persist = false;
int thaw = 0; /* 1 if freeze succeeded, -1 if freeze failed */
bool atomic = (flags & VIR_DOMAIN_SNAPSHOT_CREATE_ATOMIC) != 0;
bool reuse = (flags & VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT) != 0;
if (qemuDomainObjBeginJobWithDriver(driver, vm, QEMU_JOB_MODIFY) < 0)
return -1;
@ -10080,7 +10091,8 @@ qemuDomainSnapshotCreateDiskActive(virConnectPtr conn,
ret = qemuDomainSnapshotCreateSingleDiskActive(driver, vm,
&snap->def->disks[i],
vm->def->disks[i],
persistDisk, actions);
persistDisk, actions,
reuse);
if (ret < 0)
break;
}

14
src/qemu/qemu_monitor.c
View File

@ -2657,12 +2657,13 @@ int qemuMonitorDeleteSnapshot(qemuMonitorPtr mon, const char *name)
* at file. */
int
qemuMonitorDiskSnapshot(qemuMonitorPtr mon, virJSONValuePtr actions,
const char *device, const char *file)
const char *device, const char *file,
const char *format, bool reuse)
{
int ret;
VIR_DEBUG("mon=%p, actions=%p, device=%s, file=%s",
mon, actions, device, file);
VIR_DEBUG("mon=%p, actions=%p, device=%s, file=%s, format=%s, reuse=%d",
mon, actions, device, file, format, reuse);
if (!mon) {
qemuReportError(VIR_ERR_INVALID_ARG, "%s",
@ -2671,11 +2672,12 @@ qemuMonitorDiskSnapshot(qemuMonitorPtr mon, virJSONValuePtr actions,
}
if (mon->json) {
ret = qemuMonitorJSONDiskSnapshot(mon, actions, device, file);
ret = qemuMonitorJSONDiskSnapshot(mon, actions, device, file, format,
reuse);
} else {
if (actions) {
if (actions || STRNEQ(format, "qcow2") || reuse) {
qemuReportError(VIR_ERR_INVALID_ARG, "%s",
_("actions not supported with text monitor"));
_("text monitor lacks several snapshot features"));
return -1;
}
ret = qemuMonitorTextDiskSnapshot(mon, device, file);

4
src/qemu/qemu_monitor.h
View File

@ -505,7 +505,9 @@ int qemuMonitorDeleteSnapshot(qemuMonitorPtr mon, const char *name);
int qemuMonitorDiskSnapshot(qemuMonitorPtr mon,
virJSONValuePtr actions,
const char *device,
const char *file);
const char *file,
const char *format,
bool reuse);
int qemuMonitorTransaction(qemuMonitorPtr mon, virJSONValuePtr actions)
ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2);

6
src/qemu/qemu_monitor_json.c
View File

@ -3105,7 +3105,8 @@ cleanup:
int
qemuMonitorJSONDiskSnapshot(qemuMonitorPtr mon, virJSONValuePtr actions,
const char *device, const char *file)
const char *device, const char *file,
const char *format, bool reuse)
{
int ret;
virJSONValuePtr cmd;
@ -3115,6 +3116,9 @@ qemuMonitorJSONDiskSnapshot(qemuMonitorPtr mon, virJSONValuePtr actions,
"blockdev-snapshot-sync",
"s:device", device,
"s:snapshot-file", file,
"s:format", format,
reuse ? "s:mode" : NULL,
reuse ? "existing" : NULL,
NULL);
if (!cmd)
return -1;

4
src/qemu/qemu_monitor_json.h
View File

@ -228,7 +228,9 @@ int qemuMonitorJSONDeleteSnapshot(qemuMonitorPtr mon, const char *name);
int qemuMonitorJSONDiskSnapshot(qemuMonitorPtr mon,
virJSONValuePtr actions,
const char *device,
const char *file);
const char *file,
const char *format,
bool reuse);
int qemuMonitorJSONTransaction(qemuMonitorPtr mon, virJSONValuePtr actions);
int qemuMonitorJSONArbitraryCommand(qemuMonitorPtr mon,