[libvirt] [RFC PATCH] Add new migration flag VIR_MIGRATE_DRY_RUN

[Jim Fehlig]

A dry run can be used as a best-effort check that a migration command
will succeed. The destination host will be checked to see if it can
accommodate the resources required by the domain. DRY_RUN will fail if
the destination host is not capable of running the domain. Although a
subsequent migration will likely succeed, the success of DRY_RUN does not
ensure a future migration will succeed. Resources on the destination host
could become unavailable between a DRY_RUN and actual migration.

Signed-off-by: Jim Fehlig <jfehlig at suse.com>
---

If it is agreed this is useful, my thought was to use the begin and
prepare phases of migration to implement it. qemuMigrationDstPrepareAny()
already does a lot of the heavy lifting wrt checking the host can
accommodate the domain. Some of it, and the remaining migration phases,
can be short-circuited in the case of dry run.

One interesting wrinkle I've observed is the check for cpu compatibility.
AFAICT qemu is actually invoked on the dst, "filtered-features" of the cpu
are requested via qmp, and results are checked against cpu in domain config.
If cpu on dst is insufficient, migration fails in the prepare phase with
something like "guest CPU doesn't match specification: missing features: z y z".
I was hoping to avoid launching qemu in the case of dry run, but that may
be unavoidable if we'd like a dependable dry run result.

Thanks for considering the idea!

(BTW, if it is considered useful I will follow up with a V1 series that
includes this patch and and impl for the qemu driver.)

 include/libvirt/libvirt-domain.h | 12 ++++++++++++
 src/qemu/qemu_migration.h        |  3 ++-
 tools/virsh-domain.c             |  7 +++++++
 tools/virsh.pod                  | 10 +++++++++-
 4 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/include/libvirt/libvirt-domain.h b/include/libvirt/libvirt-domain.h
index fdd2d6b8ea..6d52f6ce50 100644
--- a/include/libvirt/libvirt-domain.h
+++ b/include/libvirt/libvirt-domain.h
@@ -830,6 +830,18 @@ typedef enum {
      */
     VIR_MIGRATE_TLS               = (1 << 16),
 
+    /* Setting the VIR_MIGRATE_DRY_RUN flag will cause libvirt to make a
+     * best-effort attempt to check if migration will succeed. The destination
+     * host will be checked to see if it can accommodate the resources required
+     * by the domain. For example are the network, disk, memory, and CPU
+     * resources used by the domain on the source host also available on the
+     * destination host. The dry run will fail if libvirt determines the
+     * destination host is not capable of running the domain. Although a
+     * subsequent migration will likely succeed, the success of dry run does
+     * not ensure a future migration will succeed. Resources on the destination
+     * host could become unavailable between a dry run and actual migration.
+     */
+    VIR_MIGRATE_DRY_RUN           = (1 << 16),
 } virDomainMigrateFlags;
 
 
diff --git a/src/qemu/qemu_migration.h b/src/qemu/qemu_migration.h
index e12b6972db..b0e2bc689b 100644
--- a/src/qemu/qemu_migration.h
+++ b/src/qemu/qemu_migration.h
@@ -57,7 +57,8 @@
      VIR_MIGRATE_AUTO_CONVERGE | \
      VIR_MIGRATE_RDMA_PIN_ALL | \
      VIR_MIGRATE_POSTCOPY | \
-     VIR_MIGRATE_TLS)
+     VIR_MIGRATE_TLS | \
+     VIR_MIGRATE_DRY_RUN)
 
 /* All supported migration parameters and their types. */
 # define QEMU_MIGRATION_PARAMETERS \
diff --git a/tools/virsh-domain.c b/tools/virsh-domain.c
index 372bdb95d3..46f0f44917 100644
--- a/tools/virsh-domain.c
+++ b/tools/virsh-domain.c
@@ -10450,6 +10450,10 @@ static const vshCmdOptDef opts_migrate[] = {
      .type = VSH_OT_BOOL,
      .help = N_("use TLS for migration")
     },
+    {.name = "dry-run",
+     .type = VSH_OT_BOOL,
+     .help = N_("check if migration will succeed without actually performing the migration")
+    },
     {.name = NULL}
 };
 
@@ -10694,6 +10698,9 @@ doMigrate(void *opaque)
     if (vshCommandOptBool(cmd, "tls"))
         flags |= VIR_MIGRATE_TLS;
 
+    if (vshCommandOptBool(cmd, "dry-run"))
+        flags |= VIR_MIGRATE_DRY_RUN;
+
     if (flags & VIR_MIGRATE_PEER2PEER || vshCommandOptBool(cmd, "direct")) {
         if (virDomainMigrateToURI3(dom, desturi, params, nparams, flags) == 0)
             ret = '0';
diff --git a/tools/virsh.pod b/tools/virsh.pod
index 86c041d575..715fa3887f 100644
--- a/tools/virsh.pod
+++ b/tools/virsh.pod
@@ -1845,7 +1845,7 @@ I<domain> I<desturi> [I<migrateuri>] [I<graphicsuri>] [I<listen-address>] [I<dna
 [I<--compressed>] [I<--comp-methods> B<method-list>]
 [I<--comp-mt-level>] [I<--comp-mt-threads>] [I<--comp-mt-dthreads>]
 [I<--comp-xbzrle-cache>] [I<--auto-converge>] [I<auto-converge-initial>]
-[I<auto-converge-increment>] [I<--persistent-xml> B<file>] [I<--tls>]
+[I<auto-converge-increment>] [I<--persistent-xml> B<file>] [I<--tls>] [I<--dry-run>]
 
 Migrate domain to another host.  Add I<--live> for live migration; <--p2p>
 for peer-2-peer migration; I<--direct> for direct migration; or I<--tunnelled>
@@ -1937,6 +1937,14 @@ Providing I<--tls> causes the migration to use the host configured TLS setup
 the migration of the domain. Usage requires proper TLS setup for both source
 and target.
 
+I<--dry-run> can be used as a best-effort check that the migration command
+will succeed. The destination host will be checked to see if it can
+accommodate the resources required by the domain. I<--dry-run> will fail if
+the destination host is not capable of running the domain. Although a
+subsequent migration will likely succeed, the success of dry run does not
+ensure a future migration will succeed. Resources on the destination host
+could become unavailable between a dry run and actual migration.
+
 Running migration can be canceled by interrupting virsh (usually using
 C<Ctrl-C>) or by B<domjobabort> command sent from another virsh instance.
 
-- 
2.18.0