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Re: [PATCH v4 03/11] qemu: snapshot: Introduce helpers for creating overlays on <transient/> disks



On a Thursday in 2020, Peter Krempa wrote:
To implement <transient/> disks we'll need to install an overlay on top
of the original disk image which will be discarded after the VM is
turned off. This was initially implemented by qemu but libvirt never
picked up this option. With blockdev the qemu feature became unsupported
so we need to do this via the snapshot code anyways.

The helpers introduced in this patch prepare a fake snapshot disk
definition for a disk which is configured as <transient/> and use it to
create a snapshot (without actually modifying metada or persistent def).

Signed-off-by: Peter Krempa <pkrempa redhat com>
---
src/qemu/qemu_snapshot.c | 91 +++++++++++++++++++++++++++++++++++++++-
src/qemu/qemu_snapshot.h |  5 +++
2 files changed, 95 insertions(+), 1 deletion(-)

diff --git a/src/qemu/qemu_snapshot.c b/src/qemu/qemu_snapshot.c
index ca051071aa..d10e7b6b3a 100644
--- a/src/qemu/qemu_snapshot.c
+++ b/src/qemu/qemu_snapshot.c
@@ -986,6 +986,7 @@ qemuSnapshotDiskPrepareOne(virDomainObjPtr vm,
                           qemuSnapshotDiskDataPtr dd,
                           virHashTablePtr blockNamedNodeData,
                           bool reuse,
+                           bool updateConfig,
                           qemuDomainAsyncJob asyncJob,
                           virJSONValuePtr actions)
{
@@ -1008,7 +1009,8 @@ qemuSnapshotDiskPrepareOne(virDomainObjPtr vm,
        return -1;

    /* modify disk in persistent definition only when the source is the same */
-    if (vm->newDef &&
+    if (updateConfig &&
+        vm->newDef &&
        (persistdisk = virDomainDiskByTarget(vm->newDef, dd->disk->dst)) &&
        virStorageSourceIsSameLocation(dd->disk->src, persistdisk->src)) {

@@ -1116,6 +1118,54 @@ qemuSnapshotDiskPrepareActiveExternal(virDomainObjPtr vm,
                                       snapctxt->dd + snapctxt->ndd++,
                                       blockNamedNodeData,
                                       reuse,
+                                       true,
+                                       asyncJob,
+                                       snapctxt->actions) < 0)
+            return NULL;
+    }
+
+    return g_steal_pointer(&snapctxt);
+}
+
+
+static qemuSnapshotDiskContextPtr
+qemuSnapshotDiskPrepareDisksTransient(virDomainObjPtr vm,
+                                      virQEMUDriverConfigPtr cfg,
+                                      virHashTablePtr blockNamedNodeData,
+                                      qemuDomainAsyncJob asyncJob)
+{
+    g_autoptr(qemuSnapshotDiskContext) snapctxt = NULL;
+    size_t i;
+
+    snapctxt = qemuSnapshotDiskContextNew(vm->def->ndisks, vm, asyncJob);
+
+    for (i = 0; i < vm->def->ndisks; i++) {
+        virDomainDiskDefPtr domdisk = vm->def->disks[i];
+        g_autoptr(virDomainSnapshotDiskDef) snapdisk = g_new0(virDomainSnapshotDiskDef, 1);
+
+        if (!domdisk->transient)
+            continue;
+
+        /* validation code makes sure that we do this only for local disks
+         * with a file source */
+        snapdisk->name = g_strdup(domdisk->dst);
+        snapdisk->snapshot = VIR_DOMAIN_SNAPSHOT_LOCATION_EXTERNAL;
+        snapdisk->src = virStorageSourceNew();
+        snapdisk->src->type = VIR_STORAGE_TYPE_FILE;
+        snapdisk->src->format = VIR_STORAGE_FILE_QCOW2;
+        snapdisk->src->path = g_strdup_printf("%s.TRANSIENT", domdisk->src->path);

I'd prefer:
a) an empty line between these assignments and the if()
b) a lowercase name, but I guess standing out is important here as the
file is not meant to stay.

+        if (virFileExists(snapdisk->src->path)) {
+            virReportError(VIR_ERR_OPERATION_UNSUPPORTED,
+                           _("Overlay file '%s' for transient disk '%s' already exists"),
+                           snapdisk->src->path, domdisk->dst);
+            return NULL;
+        }
+
+        if (qemuSnapshotDiskPrepareOne(vm, cfg, domdisk, snapdisk,
+                                       snapctxt->dd + snapctxt->ndd++,
+                                       blockNamedNodeData,
+                                       false,
+                                       false,
                                       asyncJob,
                                       snapctxt->actions) < 0)
            return NULL;
@@ -1253,6 +1303,45 @@ qemuSnapshotCreateActiveExternalDisks(virDomainObjPtr vm,
}


+/**
+ * qemuSnapshotCreateDisksTransient:
+ * @vm: domain object
+ * @asyncJob: qemu async job type
+ *
+ * Creates overlays on top of disks which are configured as <transient/>. Note
+ * that the validation code ensures that <transient> disks have appropriate
+ * configuration.
+ */
+int
+qemuSnapshotCreateDisksTransient(virDomainObjPtr vm,
+                                 qemuDomainAsyncJob asyncJob)
+{
+    qemuDomainObjPrivatePtr priv = vm->privateData;
+    virQEMUDriverPtr driver = priv->driver;
+    g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(driver);
+    g_autoptr(qemuSnapshotDiskContext) snapctxt = NULL;
+    g_autoptr(virHashTable) blockNamedNodeData = NULL;
+
+    if (virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_BLOCKDEV)) {
+        if (!(blockNamedNodeData = qemuBlockGetNamedNodeData(vm, asyncJob)))
+            return -1;
+
+        if (!(snapctxt = qemuSnapshotDiskPrepareDisksTransient(vm, cfg,
+                                                               blockNamedNodeData,
+                                                               asyncJob)))
+            return -1;
+
+        if (qemuSnapshotDiskCreate(snapctxt, cfg) < 0)
+            return -1;
+    }
+
+    /* the overlays are established, so they can be deleted on a shutdown */

*on shutdown
sounds better to me

+    priv->inhibitDiskTransientDelete = false;
+
+    return 0;
+}
+
+
static int
qemuSnapshotCreateActiveExternal(virQEMUDriverPtr driver,
                                 virDomainObjPtr vm,

With the empty line added:
Reviewed-by: Ján Tomko <jtomko redhat com>

Jano

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