virDomainMigrate: Introduce VIR_MIGRATE_NON_SHARED_SYNCHRONOUS_WRITES flag

Non-shared storage migration of guests which are disk I/O intensive and
have fast local storage may actually never converge if the guest happens
to dirty the disk faster than it can be copied.

This patch introduces a new flag
'VIR_MIGRATE_NON_SHARED_SYNCHRONOUS_WRITES' which will instruct
hypervisors to synchronize local I/O writes with the writes to remote
storage used for migration so that the guest can't overwhelm the
migration. This comes at a cost of decreased local I/O performance for
guests which behave well on average.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

docs/manpages/virsh.rst

@@ -3180,7 +3180,7 @@ migrate
       [--postcopy-bandwidth bandwidth]
       [--parallel [--parallel-connections connections]]
       [--bandwidth bandwidth] [--tls-destination hostname]
-      [--disks-uri URI]
+      [--disks-uri URI] [--copy-storage-synchronous-writes]
 
 Migrate domain to another host.  Add *--live* for live migration; <--p2p>
 for peer-2-peer migration; *--direct* for direct migration; or *--tunnelled*
@@ -3202,6 +3202,10 @@ images on source host to the images found at the same place on the destination
 host. By default only non-shared non-readonly images are transferred. Use
 *--migrate-disks* to explicitly specify a list of disk targets to
 transfer via the comma separated ``disk-list`` argument.
+With *--copy-storage-synchronous-writes* flag used the disk data migration will
+synchronously handle guest disk writes to both the original soure and the
+destination to ensure that the disk migration converges at the price of possibly
+decreased burst performance.
 
 *--change-protection* enforces that no incompatible configuration changes will
 be made to the domain while the migration is underway; this flag is implicitly

include/libvirt/libvirt-domain.h

@@ -850,6 +850,16 @@ typedef enum {
      */
     VIR_MIGRATE_PARALLEL = (1 << 17),
 
+     /* Force the guest writes which happen when copying disk images for
+      * non-shared storage migration to be synchronously written to the
+      * destination. This ensures the storage migration converges for VMs
+      * doing heavy I/O on fast local storage and slow mirror.
+      *
+      * Requires one of VIR_MIGRATE_NON_SHARED_DISK, VIR_MIGRATE_NON_SHARED_INC
+      * to be present as well.
+      */
+    VIR_MIGRATE_NON_SHARED_SYNCHRONOUS_WRITES = (1 << 18),
+
 } virDomainMigrateFlags;
 
 

src/libvirt-domain.c

@@ -3567,6 +3567,10 @@ virDomainMigrate(virDomainPtr domain,
                              VIR_MIGRATE_PARALLEL,
                              error);
 
+    VIR_REQUIRE_FLAG_GOTO(VIR_MIGRATE_NON_SHARED_SYNCHRONOUS_WRITES,
+                          VIR_MIGRATE_NON_SHARED_DISK | VIR_MIGRATE_NON_SHARED_INC,
+                          error);
+
     if (flags & VIR_MIGRATE_OFFLINE) {
         rc = VIR_DRV_SUPPORTS_FEATURE(domain->conn->driver, domain->conn,
                                       VIR_DRV_FEATURE_MIGRATION_OFFLINE);
@@ -3760,6 +3764,10 @@ virDomainMigrate2(virDomainPtr domain,
                              VIR_MIGRATE_PARALLEL,
                              error);
 
+    VIR_REQUIRE_FLAG_GOTO(VIR_MIGRATE_NON_SHARED_SYNCHRONOUS_WRITES,
+                          VIR_MIGRATE_NON_SHARED_DISK | VIR_MIGRATE_NON_SHARED_INC,
+                          error);
+
     if (flags & VIR_MIGRATE_OFFLINE) {
         rc = VIR_DRV_SUPPORTS_FEATURE(domain->conn->driver, domain->conn,
                                       VIR_DRV_FEATURE_MIGRATION_OFFLINE);
@@ -3966,6 +3974,14 @@ virDomainMigrate3(virDomainPtr domain,
                              VIR_MIGRATE_NON_SHARED_INC,
                              error);
 
+    VIR_REQUIRE_FLAG_GOTO(VIR_MIGRATE_NON_SHARED_SYNCHRONOUS_WRITES,
+                          VIR_MIGRATE_NON_SHARED_DISK | VIR_MIGRATE_NON_SHARED_INC,
+                          error);
+
+    VIR_REQUIRE_FLAG_GOTO(VIR_MIGRATE_NON_SHARED_SYNCHRONOUS_WRITES,
+                          VIR_MIGRATE_NON_SHARED_DISK | VIR_MIGRATE_NON_SHARED_INC,
+                          error);
+
     if (flags & VIR_MIGRATE_PEER2PEER) {
         virReportInvalidArg(flags, "%s",
                             _("use virDomainMigrateToURI3 for peer-to-peer "
@@ -4137,6 +4153,10 @@ int virDomainMigrateUnmanagedCheckCompat(virDomainPtr domain,
                             VIR_MIGRATE_NON_SHARED_INC,
                             -1);
 
+    VIR_REQUIRE_FLAG_RET(VIR_MIGRATE_NON_SHARED_SYNCHRONOUS_WRITES,
+                         VIR_MIGRATE_NON_SHARED_DISK | VIR_MIGRATE_NON_SHARED_INC,
+                         -1);
+
     if (flags & VIR_MIGRATE_OFFLINE) {
         rc = VIR_DRV_SUPPORTS_FEATURE(domain->conn->driver, domain->conn,
                                       VIR_DRV_FEATURE_MIGRATION_OFFLINE);

tools/virsh-domain.c

@@ -10543,6 +10543,10 @@ static const vshCmdOptDef opts_migrate[] = {
      .type = VSH_OT_BOOL,
      .help = N_("migration with non-shared storage with incremental copy (same base image shared between source and destination)")
     },
+    {.name = "copy-storage-synchronous-writes",
+     .type = VSH_OT_BOOL,
+     .help = N_("force guest disk writes to be synchronously written to the destination to improve storage migration convergence")
+    },
     {.name = "change-protection",
      .type = VSH_OT_BOOL,
      .help = N_("prevent any configuration changes to domain until migration ends")
@@ -10949,6 +10953,15 @@ doMigrate(void *opaque)
     if (vshCommandOptBool(cmd, "copy-storage-inc"))
         flags |= VIR_MIGRATE_NON_SHARED_INC;
 
+    if (vshCommandOptBool(cmd, "copy-storage-synchronous-writes")) {
+        if (!(flags & VIR_MIGRATE_NON_SHARED_DISK) &&
+            !(flags & VIR_MIGRATE_NON_SHARED_INC)) {
+            vshError(ctl, "'--copy-storage-synchronous-writes' requires one of '--copy-storage-all', 'copy-storage-inc'");
+            goto out;
+        }
+        flags |= VIR_MIGRATE_NON_SHARED_SYNCHRONOUS_WRITES;
+    }
+
     if (vshCommandOptBool(cmd, "change-protection"))
         flags |= VIR_MIGRATE_CHANGE_PROTECTION;