[Qemu-devel] [RFC PATCH v0 3/3] gluster: Add support for creating zero-filled image

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

GlusterFS supports creation of zero-filled file on GlusterFS volume
by means of an API called glfs_zerofill(). Use this API from QEMU to
create an image that is filled with zeroes by using the preallocation
option of qemu-img.

qemu-img create gluster://server/volume/image -o preallocation=full 10G

The allowed values for preallocation are 'full' and 'off'. By default
preallocation is off and image is not zero-filled.

glfs_zerofill() offloads the writing of zeroes to the server and if
the storage supports SCSI WRITESAME, GlusterFS server can issue
BLKZEROOUT ioctl to achieve the zeroing.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
---
 block/gluster.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/block/gluster.c b/block/gluster.c
index 15f5dfb..2368997 100644
--- a/block/gluster.c
+++ b/block/gluster.c
@@ -381,6 +381,29 @@ out:
     qemu_aio_release(acb);
     return ret;
 }
+
+static inline int gluster_supports_zerofill(void)
+{
+    return 1;
+}
+
+static inline int qemu_gluster_zerofill(struct glfs_fd *fd, int64_t offset,
+        int64_t size)
+{
+    return glfs_zerofill(fd, offset, size);
+}
+
+#else
+static inline int gluster_supports_zerofill(void)
+{
+    return 0;
+}
+
+static inline int qemu_gluster_zerofill(struct glfs_fd *fd, int64_t offset,
+        int64_t size)
+{
+    return 0;
+}
 #endif
 
 static int qemu_gluster_create(const char *filename,
@@ -389,6 +412,7 @@ static int qemu_gluster_create(const char *filename,
     struct glfs *glfs;
     struct glfs_fd *fd;
     int ret = 0;
+    int prealloc = 0;
     int64_t total_size = 0;
     GlusterConf *gconf = g_malloc0(sizeof(GlusterConf));
 
@@ -401,6 +425,18 @@ static int qemu_gluster_create(const char *filename,
     while (options && options->name) {
         if (!strcmp(options->name, BLOCK_OPT_SIZE)) {
             total_size = options->value.n / BDRV_SECTOR_SIZE;
+        } else if (!strcmp(options->name, BLOCK_OPT_PREALLOC)) {
+            if (!options->value.s || !strcmp(options->value.s, "off")) {
+                prealloc = 0;
+            } else if (!strcmp(options->value.s, "full") &&
+                    gluster_supports_zerofill()) {
+                prealloc = 1;
+            } else {
+                error_setg(errp, "Invalid preallocation mode: '%s'"
+                    " or GlusterFS doesn't support zerofill API",
+                           options->value.s);
+                return -EINVAL;
+            }
         }
         options++;
     }
@@ -413,6 +449,10 @@ static int qemu_gluster_create(const char *filename,
         if (glfs_ftruncate(fd, total_size * BDRV_SECTOR_SIZE) != 0) {
             ret = -errno;
         }
+        if (prealloc && qemu_gluster_zerofill(fd, 0,
+                total_size * BDRV_SECTOR_SIZE)) {
+            ret = -errno;
+        }
         if (glfs_close(fd) != 0) {
             ret = -errno;
         }
@@ -595,6 +635,11 @@ static QEMUOptionParameter qemu_gluster_create_options[] = {
         .type = OPT_SIZE,
         .help = "Virtual disk size"
     },
+    {
+        .name = BLOCK_OPT_PREALLOC,
+        .type = OPT_STRING,
+        .help = "Preallocation mode (allowed values: off, on)"
+    },
     { NULL }
 };
 
-- 
1.7.11.7