virDomainBlockCopy: Introduce VIR_DOMAIN_BLOCK_COPY_SYNCHRONOUS_WRITES flag

In cases when the destination storage is slower than the normal VM
storage and the VM does intensive I/O to the disk a block copy job may
never converge.

Switching it to synchronous mode will ensure that all writes done by the
guest are propagated to the destination at the cost of slowing down I/O
of the guest to the synchronous speed.

This patch adds the new API flag and implements virsh support.

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

docs/manpages/virsh.rst

@@ -1218,7 +1218,7 @@ blockcopy
       [--shallow] [--reuse-external] [bandwidth]
       [--wait [--async] [--verbose]] [{--pivot | --finish}]
       [--timeout seconds] [granularity] [buf-size] [--bytes]
-      [--transient-job]
+      [--transient-job] [--synchronous-writes]
 
 Copy a disk backing image chain to a destination.  Either *dest* as
 the destination file name, or *--xml* with the name of an XML file containing
@@ -1278,6 +1278,11 @@ be recovered if the VM crashes or is turned off before the job completes. This
 flag removes the restriction of copy jobs to transient domains if that
 restriction is applied by the hypervisor.
 
+If *--synchronous-writes* is specified the block job will wait for guest writes
+to be propagated both to the original image and to the destination of the copy
+so that it's guaranteed that the job converges if the destination storage is
+slower. This may impact performance of writes while the blockjob is running.
+
 
 blockjob
 --------

include/libvirt/libvirt-domain.h

@@ -2598,6 +2598,10 @@ typedef enum {
 
     /* Don't force usage of recoverable job for the copy operation */
     VIR_DOMAIN_BLOCK_COPY_TRANSIENT_JOB = 1 << 2,
+
+    /* Force the copy job to synchronously propagate guest writes into
+     * the destination image, so that the copy is guaranteed to converge */
+    VIR_DOMAIN_BLOCK_COPY_SYNCHRONOUS_WRITES = 1 << 3,
 } virDomainBlockCopyFlags;
 
 /**

src/libvirt-domain.c

@@ -10509,6 +10509,12 @@ virDomainBlockRebase(virDomainPtr dom, const char *disk,
  * remove the restriction of copy jobs to transient domains. Note that this flag
  * is automatically implied if the VM is transient at the time it's started.
  *
+ * If @flags contains VIR_DOMAIN_BLOCK_COPY_SYNCHRONOUS_WRITES the job will wait
+ * for guest writes to be propagated both to the original image and to the
+ * destination of the copy so that it's guaranteed that the job converges if
+ * the destination storage is slower. This may impact performance of writes
+ * while the blockjob is running.
+ *
  * The @disk parameter is either an unambiguous source name of the
  * block device (the <source file='...'/> sub-element, such as
  * "/path/to/image"), or the device target shorthand (the

tools/virsh-domain.c

@@ -2284,6 +2284,10 @@ static const vshCmdOptDef opts_blockcopy[] = {
      .type = VSH_OT_BOOL,
      .help = N_("the copy job is not persisted if VM is turned off")
     },
+    {.name = "synchronous-writes",
+     .type = VSH_OT_BOOL,
+     .help = N_("the copy job forces guest writes to be synchronously written to the destination")
+    },
     {.name = NULL}
 };
 
@@ -2306,6 +2310,7 @@ cmdBlockcopy(vshControl *ctl, const vshCmd *cmd)
     bool async = vshCommandOptBool(cmd, "async");
     bool bytes = vshCommandOptBool(cmd, "bytes");
     bool transientjob = vshCommandOptBool(cmd, "transient-job");
+    bool syncWrites = vshCommandOptBool(cmd, "synchronous-writes");
     int timeout = 0;
     const char *path = NULL;
     int abort_flags = 0;
@@ -2337,6 +2342,8 @@ cmdBlockcopy(vshControl *ctl, const vshCmd *cmd)
         flags |= VIR_DOMAIN_BLOCK_REBASE_REUSE_EXT;
     if (transientjob)
         flags |= VIR_DOMAIN_BLOCK_COPY_TRANSIENT_JOB;
+    if (syncWrites)
+        flags |= VIR_DOMAIN_BLOCK_COPY_SYNCHRONOUS_WRITES;
     if (vshCommandOptTimeoutToMs(ctl, cmd, &timeout) < 0)
         return false;
 
@@ -2386,7 +2393,7 @@ cmdBlockcopy(vshControl *ctl, const vshCmd *cmd)
     }
 
     if (granularity || buf_size || (format && STRNEQ(format, "raw")) || xml ||
-        transientjob) {
+        transientjob || syncWrites) {
         /* New API */
         if (bandwidth || granularity || buf_size) {
             params = g_new0(virTypedParameter, 3);