storagevol: add nocow to vol xml

Add 'nocow' to storage volume xml so that user can have an option
to set NOCOW flag to the newly created volume. It's useful on btrfs
file system to enhance performance.

Btrfs has low performance when hosting VM images, even more when the guest
in those VM are also using btrfs as file system. One way to mitigate this
bad performance is to turn off COW attributes on VM files. Generally, there
are two ways to turn off COW on btrfs: a) by mounting fs with nodatacow,
then all newly created files will be NOCOW. b) per file. Add the NOCOW file
attribute. It could only be done to empty or new files.

This patch tries the second way, according to 'nocow' option, it could set
NOCOW flag per file:
for raw file images, handle 'nocow' in libvirt code; for non-raw file images,
pass 'nocow=on' option to qemu-img, and let qemu-img to handle that (requires
qemu-img version >= 2.1).

Signed-off-by: Chunyan Liu <cyliu@suse.com>

docs/formatstorage.html.in

@@ -385,6 +385,7 @@
             <label>virt_image_t</label>
           </permissions>
           <compat>1.1</compat>
+          <nocow/>
           <features>
             <lazy_refcounts/>
           </features>
@@ -424,6 +425,12 @@
         1.1 is used. If omitted, qemu-img default is used.
         <span class="since">Since 1.1.0</span>
       </dd>
+      <dt><code>nocow</code></dt>
+      <dd>Turn off COW of the newly created volume. So far, this is only valid
+        for a file image in btrfs file system. It will improve performance when
+        the file image is used in VM. To create non-raw file images, it
+        requires QEMU version since 2.1. <span class="since">Since 1.2.7</span>
+      </dd>
       <dt><code>features</code></dt>
       <dd>Format-specific features. Only used for <code>qcow2</code> now.
         Valid sub-elements are:

docs/schemas/storagevol.rng

@@ -137,6 +137,11 @@
         <optional>
           <ref name='compat'/>
         </optional>
+        <optional>
+          <element name='nocow'>
+            <empty/>
+          </element>
+        </optional>
         <optional>
           <ref name='fileFormatFeatures'/>
         </optional>

src/conf/storage_conf.c

@@ -1299,6 +1299,9 @@ virStorageVolDefParseXML(virStoragePoolDefPtr pool,
         virStringFreeList(version);
     }
 
+    if (virXPathNode("./target/nocow", ctxt))
+        ret->target.nocow = true;
+
     if (options->featureFromString && virXPathNode("./target/features", ctxt)) {
         if ((n = virXPathNodeSet("./target/features/*", ctxt, &nodes)) < 0)
             goto error;

src/storage/storage_backend.c

@@ -37,6 +37,9 @@
 #ifdef __linux__
 # include <sys/ioctl.h>
 # include <linux/fs.h>
+# ifndef FS_NOCOW_FL
+#  define FS_NOCOW_FL                     0x00800000 /* Do not cow file */
+# endif
 #endif
 
 #if WITH_SELINUX
@@ -453,6 +456,21 @@ virStorageBackendCreateRaw(virConnectPtr conn ATTRIBUTE_UNUSED,
         goto cleanup;
     }
 
+    if (vol->target.nocow) {
+#ifdef __linux__
+        int attr;
+
+        /* Set NOCOW flag. This is an optimisation for btrfs.
+         * The FS_IOC_SETFLAGS ioctl return value will be ignored since any
+         * failure of this operation should not block the left work.
+         */
+        if (ioctl(fd, FS_IOC_GETFLAGS, &attr) == 0) {
+            attr |= FS_NOCOW_FL;
+            ioctl(fd, FS_IOC_SETFLAGS, &attr);
+        }
+#endif
+    }
+
     if ((ret = createRawFile(fd, vol, inputvol)) < 0)
         /* createRawFile already reported the exact error. */
         ret = -1;
@@ -718,6 +736,7 @@ virStorageBackendCreateQemuImgOpts(char **opts,
                                    bool preallocate,
                                    int format,
                                    const char *compat,
+                                   bool nocow,
                                    virBitmapPtr features)
 {
     virBuffer buf = VIR_BUFFER_INITIALIZER;
@@ -730,6 +749,8 @@ virStorageBackendCreateQemuImgOpts(char **opts,
         virBufferAddLit(&buf, "encryption=on,");
     if (preallocate)
         virBufferAddLit(&buf, "preallocation=metadata,");
+    if (nocow)
+        virBufferAddLit(&buf, "nocow=on,");
 
     if (compat)
         virBufferAsprintf(&buf, "compat=%s,", compat);
@@ -950,6 +971,7 @@ virStorageBackendCreateQemuImgCmd(virConnectPtr conn,
                                                do_encryption, preallocate,
                                                vol->target.format,
                                                compat,
+                                               vol->target.nocow,
                                                vol->target.features) < 0) {
             virCommandFree(cmd);
             return NULL;

src/util/virstoragefile.h

@@ -247,6 +247,7 @@ struct _virStorageSource {
                  * pool-specific enum for storage volumes */
     virBitmapPtr features;
     char *compat;
+    bool nocow;
 
     virStoragePermsPtr perms;
     virStorageTimestampsPtr timestamps;