qemu: make 'xz' image compression viable by using -3

For managed save we can choose between various compression
methods. I randomly tested the 'xz' program on a 8 GB guest
and was surprised to have to wait > 50 minutes for it to
finish compressing, with 'xz' burning 100% cpu for the
entire time. Despite the impressive compression, this is
completely useless in the real world as it is far too long
to wait to save the VM.

The 'xz' binary defaults to '-6' optimization level which
aims for high compression, with moderate memory usage,
at the expense of speed.

This change switches it to use the '-3' optimization level
which is documented as being the one that optimizes speed
at expense of compression. Even with this, it will still
outperform all the other options in terms of compression
level. It is a little less than x4 faster than '-6' which
means it starts to be a viable choice to use 'xz' for
people who really want best compression.

The test results on a 1 GB, fairly freshly booted VM are
as follows

  format | save  | restore  size
  =======+=======+=============
  raw    |   05s |    1s  | 428 MB
  lzop   |   05s |    3s  | 160 MB
  gzip   |   29s |    5s  | 118 MB
  bz2    |   54s |   22s  | 114 MB
  xz     | 4m37s |   13s  |  86 MB
  xz -3  | 1m20s |   12s  |  95 MB

Based on this we can say

 * For moderate compression with no noticable loss in speed

       => use lzop

 * For high compression with moderate loss in speed

       => use gzip

 * For best compression with significant loss in speed

       => use xz
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

src/qemu/qemu_driver.c

@@ -3177,7 +3177,7 @@ qemuDomainSaveMemory(virQEMUDriverPtr driver,
                      virDomainObjPtr vm,
                      const char *path,
                      virQEMUSaveDataPtr data,
-                     const char *compressedpath,
+                     virCommandPtr compressor,
                      unsigned int flags,
                      qemuDomainAsyncJob asyncJob)
 {
@@ -3216,7 +3216,7 @@ qemuDomainSaveMemory(virQEMUDriverPtr driver,
         goto cleanup;
 
     /* Perform the migration */
-    if (qemuMigrationSrcToFile(driver, vm, fd, compressedpath, asyncJob) < 0)
+    if (qemuMigrationSrcToFile(driver, vm, fd, compressor, asyncJob) < 0)
         goto cleanup;
 
     /* Touch up file header to mark image complete. */
@@ -3259,7 +3259,7 @@ qemuDomainSaveMemory(virQEMUDriverPtr driver,
 static int
 qemuDomainSaveInternal(virQEMUDriverPtr driver,
                        virDomainObjPtr vm, const char *path,
-                       int compressed, const char *compressedpath,
+                       int compressed, virCommandPtr compressor,
                        const char *xmlin, unsigned int flags)
 {
     g_autofree char *xml = NULL;
@@ -3345,7 +3345,7 @@ qemuDomainSaveInternal(virQEMUDriverPtr driver,
         goto endjob;
     xml = NULL;
 
-    ret = qemuDomainSaveMemory(driver, vm, path, data, compressedpath,
+    ret = qemuDomainSaveMemory(driver, vm, path, data, compressor,
                                flags, QEMU_ASYNC_JOB_SAVE);
     if (ret < 0)
         goto endjob;
@@ -3406,13 +3406,14 @@ qemuDomainSaveInternal(virQEMUDriverPtr driver,
  */
 static int ATTRIBUTE_NONNULL(2)
 qemuGetCompressionProgram(const char *imageFormat,
-                          char **compresspath,
+                          virCommandPtr *compressor,
                           const char *styleFormat,
                           bool use_raw_on_fail)
 {
     int ret;
+    const char *prog;
 
-    *compresspath = NULL;
+    *compressor = NULL;
 
     if (!imageFormat)
         return QEMU_SAVE_FORMAT_RAW;
@@ -3423,9 +3424,14 @@ qemuGetCompressionProgram(const char *imageFormat,
     if (ret == QEMU_SAVE_FORMAT_RAW)
         return QEMU_SAVE_FORMAT_RAW;
 
-    if (!(*compresspath = virFindFileInPath(imageFormat)))
+    if (!(prog = virFindFileInPath(imageFormat)))
         goto error;
 
+    *compressor = virCommandNew(prog);
+    virCommandAddArg(*compressor, "-c");
+    if (ret == QEMU_SAVE_FORMAT_XZ)
+        virCommandAddArg(*compressor, "-3");
+
     return ret;
 
  error:
@@ -3466,7 +3472,7 @@ qemuDomainSaveFlags(virDomainPtr dom, const char *path, const char *dxml,
 {
     virQEMUDriverPtr driver = dom->conn->privateData;
     int compressed;
-    g_autofree char *compressedpath = NULL;
+    g_autoptr(virCommand) compressor = NULL;
     int ret = -1;
     virDomainObjPtr vm = NULL;
     g_autoptr(virQEMUDriverConfig) cfg = NULL;
@@ -3477,7 +3483,7 @@ qemuDomainSaveFlags(virDomainPtr dom, const char *path, const char *dxml,
 
     cfg = virQEMUDriverGetConfig(driver);
     if ((compressed = qemuGetCompressionProgram(cfg->saveImageFormat,
-                                                &compressedpath,
+                                                &compressor,
                                                 "save", false)) < 0)
         goto cleanup;
 
@@ -3491,7 +3497,7 @@ qemuDomainSaveFlags(virDomainPtr dom, const char *path, const char *dxml,
         goto cleanup;
 
     ret = qemuDomainSaveInternal(driver, vm, path, compressed,
-                                 compressedpath, dxml, flags);
+                                 compressor, dxml, flags);
 
  cleanup:
     virDomainObjEndAPI(&vm);
@@ -3522,7 +3528,7 @@ qemuDomainManagedSave(virDomainPtr dom, unsigned int flags)
     virQEMUDriverPtr driver = dom->conn->privateData;
     g_autoptr(virQEMUDriverConfig) cfg = NULL;
     int compressed;
-    g_autofree char *compressedpath = NULL;
+    g_autoptr(virCommand) compressor = NULL;
     virDomainObjPtr vm;
     g_autofree char *name = NULL;
     int ret = -1;
@@ -3548,7 +3554,7 @@ qemuDomainManagedSave(virDomainPtr dom, unsigned int flags)
 
     cfg = virQEMUDriverGetConfig(driver);
     if ((compressed = qemuGetCompressionProgram(cfg->saveImageFormat,
-                                                &compressedpath,
+                                                &compressor,
                                                 "save", false)) < 0)
         goto cleanup;
 
@@ -3558,7 +3564,7 @@ qemuDomainManagedSave(virDomainPtr dom, unsigned int flags)
     VIR_INFO("Saving state of domain '%s' to '%s'", vm->def->name, name);
 
     ret = qemuDomainSaveInternal(driver, vm, name, compressed,
-                                 compressedpath, NULL, flags);
+                                 compressor, NULL, flags);
     if (ret == 0)
         vm->hasManagedSave = true;
 
@@ -3754,14 +3760,14 @@ doCoreDump(virQEMUDriverPtr driver,
     unsigned int flags = VIR_FILE_WRAPPER_NON_BLOCKING;
     const char *memory_dump_format = NULL;
     g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(driver);
-    g_autofree char *compressedpath = NULL;
+    g_autoptr(virCommand) compressor = NULL;
 
     /* We reuse "save" flag for "dump" here. Then, we can support the same
      * format in "save" and "dump". This path doesn't need the compression
      * program to exist and can ignore the return value - it only cares to
-     * get the compressedpath */
+     * get the compressor */
     ignore_value(qemuGetCompressionProgram(cfg->dumpImageFormat,
-                                           &compressedpath,
+                                           &compressor,
                                            "dump", true));
 
     /* Create an empty file with appropriate ownership.  */
@@ -3809,7 +3815,7 @@ doCoreDump(virQEMUDriverPtr driver,
         if (!qemuMigrationSrcIsAllowed(driver, vm, false, 0))
             goto cleanup;
 
-        ret = qemuMigrationSrcToFile(driver, vm, fd, compressedpath,
+        ret = qemuMigrationSrcToFile(driver, vm, fd, compressor,
                                      QEMU_ASYNC_JOB_DUMP);
     }
 
@@ -15586,7 +15592,7 @@ qemuDomainSnapshotCreateActiveExternal(virQEMUDriverPtr driver,
     int thaw = 0; /* 1 if freeze succeeded, -1 if freeze failed */
     bool pmsuspended = false;
     int compressed;
-    g_autofree char *compressedpath = NULL;
+    g_autoptr(virCommand) compressor = NULL;
     virQEMUSaveDataPtr data = NULL;
 
     /* If quiesce was requested, then issue a freeze command, and a
@@ -15656,7 +15662,7 @@ qemuDomainSnapshotCreateActiveExternal(virQEMUDriverPtr driver,
                                           JOB_MASK(QEMU_JOB_MIGRATION_OP)));
 
         if ((compressed = qemuGetCompressionProgram(cfg->snapshotImageFormat,
-                                                    &compressedpath,
+                                                    &compressor,
                                                     "snapshot", false)) < 0)
             goto cleanup;
 
@@ -15673,7 +15679,7 @@ qemuDomainSnapshotCreateActiveExternal(virQEMUDriverPtr driver,
         xml = NULL;
 
         if ((ret = qemuDomainSaveMemory(driver, vm, snapdef->file, data,
-                                        compressedpath, 0,
+                                        compressor, 0,
                                         QEMU_ASYNC_JOB_SNAPSHOT)) < 0)
             goto cleanup;
 

src/qemu/qemu_migration.c

@@ -5170,13 +5170,12 @@ qemuMigrationDstFinish(virQEMUDriverPtr driver,
 int
 qemuMigrationSrcToFile(virQEMUDriverPtr driver, virDomainObjPtr vm,
                        int fd,
-                       const char *compressor,
+                       virCommandPtr compressor,
                        qemuDomainAsyncJob asyncJob)
 {
     qemuDomainObjPrivatePtr priv = vm->privateData;
     int rc;
     int ret = -1;
-    virCommandPtr cmd = NULL;
     int pipeFD[2] = { -1, -1 };
     unsigned long saveMigBandwidth = priv->migMaxBandwidth;
     char *errbuf = NULL;
@@ -5221,25 +5220,17 @@ qemuMigrationSrcToFile(virQEMUDriverPtr driver, virDomainObjPtr vm,
                                     QEMU_MONITOR_MIGRATE_BACKGROUND,
                                     fd);
     } else {
-        const char *prog = compressor;
-        const char *args[] = {
-            prog,
-            "-c",
-            NULL
-        };
-
-        cmd = virCommandNewArgs(args);
-        virCommandSetInputFD(cmd, pipeFD[0]);
-        virCommandSetOutputFD(cmd, &fd);
-        virCommandSetErrorBuffer(cmd, &errbuf);
-        virCommandDoAsyncIO(cmd);
+        virCommandSetInputFD(compressor, pipeFD[0]);
+        virCommandSetOutputFD(compressor, &fd);
+        virCommandSetErrorBuffer(compressor, &errbuf);
+        virCommandDoAsyncIO(compressor);
         if (virSetCloseExec(pipeFD[1]) < 0) {
             virReportSystemError(errno, "%s",
                                  _("Unable to set cloexec flag"));
             ignore_value(qemuDomainObjExitMonitor(driver, vm));
             goto cleanup;
         }
-        if (virCommandRunAsync(cmd, NULL) < 0) {
+        if (virCommandRunAsync(compressor, NULL) < 0) {
             ignore_value(qemuDomainObjExitMonitor(driver, vm));
             goto cleanup;
         }
@@ -5260,7 +5251,7 @@ qemuMigrationSrcToFile(virQEMUDriverPtr driver, virDomainObjPtr vm,
     if (rc < 0) {
         if (rc == -2) {
             virErrorPreserveLast(&orig_err);
-            virCommandAbort(cmd);
+            virCommandAbort(compressor);
             if (virDomainObjIsActive(vm) &&
                 qemuDomainObjEnterMonitorAsync(driver, vm, asyncJob) == 0) {
                 qemuMonitorMigrateCancel(priv->mon);
@@ -5270,7 +5261,7 @@ qemuMigrationSrcToFile(virQEMUDriverPtr driver, virDomainObjPtr vm,
         goto cleanup;
     }
 
-    if (cmd && virCommandWait(cmd, NULL) < 0)
+    if (compressor && virCommandWait(compressor, NULL) < 0)
         goto cleanup;
 
     qemuDomainEventEmitJobCompleted(driver, vm);
@@ -5290,10 +5281,9 @@ qemuMigrationSrcToFile(virQEMUDriverPtr driver, virDomainObjPtr vm,
 
     VIR_FORCE_CLOSE(pipeFD[0]);
     VIR_FORCE_CLOSE(pipeFD[1]);
-    if (cmd) {
+    if (errbuf) {
         VIR_DEBUG("Compression binary stderr: %s", NULLSTR(errbuf));
         VIR_FREE(errbuf);
-        virCommandFree(cmd);
     }
 
     virErrorRestore(&orig_err);

src/qemu/qemu_migration.h

@@ -204,7 +204,7 @@ int
 qemuMigrationSrcToFile(virQEMUDriverPtr driver,
                        virDomainObjPtr vm,
                        int fd,
-                       const char *compressor,
+                       virCommandPtr compressor,
                        qemuDomainAsyncJob asyncJob)
     ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2) G_GNUC_WARN_UNUSED_RESULT;