[RFC PATCH 2/9] migration/snap-tool: Snapshot image create/open routines for qemu-snap tool

[Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>]

Implementation of routines for QCOW2 image creation and opening. Some
predefined parameters for image creation and opening are introduced that
provide reasonable tradeoff between performance, file size and usability.

Thus, it was chosen to disable preallocation and keep image file dense on
host file system, the apparent file size equals allocated in this case
which is anyways beneficial for the user experience.
Larger 1MB cluster size is adopted to reduce allocation overhead and
improve I/O performance while keeping internal fragmentation of snapshot
image reasonably small.

Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
---
 qemu-snap.c | 94 ++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 90 insertions(+), 4 deletions(-)

diff --git a/qemu-snap.c b/qemu-snap.c
index c7118927f7..c9f8d7166a 100644
--- a/qemu-snap.c
+++ b/qemu-snap.c
@@ -31,6 +31,16 @@
 #include "migration/qemu-file.h"
 #include "qemu-snap.h"
 
+/* QCOW2 image options */
+#define BLK_FORMAT_DRIVER       "qcow2"
+#define BLK_CREATE_OPT_STRING   "preallocation=off,lazy_refcounts=on,"  \
+                                        "extended_l2=off,compat=v3,cluster_size=1M,"    \
+                                        "refcount_bits=8"
+/* L2 cache size to cover 2TB of memory */
+#define BLK_L2_CACHE_SIZE       "16M"
+/* Single L2 cache entry for the whole L2 table */
+#define BLK_L2_CACHE_ENTRY_SIZE "1M"
+
 #define OPT_CACHE   256
 #define OPT_AIO     257
 
@@ -104,30 +114,106 @@ static void snap_load_destroy_state(void)
     /* TODO: implement */
 }
 
+static BlockBackend *snap_create(const char *filename, int64_t image_size,
+        int flags, bool writethrough)
+{
+    char *create_opt_string;
+    QDict *blk_opts;
+    BlockBackend *blk;
+    Error *local_err = NULL;
+
+    /* Create QCOW2 image with given parameters */
+    create_opt_string = g_strdup(BLK_CREATE_OPT_STRING);
+    bdrv_img_create(filename, BLK_FORMAT_DRIVER, NULL, NULL,
+            create_opt_string, image_size, flags, true, &local_err);
+    g_free(create_opt_string);
+
+    if (local_err) {
+        error_reportf_err(local_err, "Could not create '%s': ", filename);
+        goto fail;
+    }
+
+    /* Block backend open options */
+    blk_opts = qdict_new();
+    qdict_put_str(blk_opts, "driver", BLK_FORMAT_DRIVER);
+    qdict_put_str(blk_opts, "l2-cache-size", BLK_L2_CACHE_SIZE);
+    qdict_put_str(blk_opts, "l2-cache-entry-size", BLK_L2_CACHE_ENTRY_SIZE);
+
+    /* Open block backend instance for the created image */
+    blk = blk_new_open(filename, NULL, blk_opts, flags, &local_err);
+    if (!blk) {
+        error_reportf_err(local_err, "Could not open '%s': ", filename);
+        /* Delete image file */
+        qemu_unlink(filename);
+        goto fail;
+    }
+
+    blk_set_enable_write_cache(blk, !writethrough);
+    return blk;
+
+fail:
+    return NULL;
+}
+
+static BlockBackend *snap_open(const char *filename, int flags)
+{
+    QDict *blk_opts;
+    BlockBackend *blk;
+    Error *local_err = NULL;
+
+    /* Block backend open options */
+    blk_opts = qdict_new();
+    qdict_put_str(blk_opts, "driver", BLK_FORMAT_DRIVER);
+    qdict_put_str(blk_opts, "l2-cache-size", BLK_L2_CACHE_SIZE);
+    qdict_put_str(blk_opts, "l2-cache-entry-size", BLK_L2_CACHE_ENTRY_SIZE);
+
+    /* Open block backend instance */
+    blk = blk_new_open(filename, NULL, blk_opts, flags, &local_err);
+    if (!blk) {
+        error_reportf_err(local_err, "Could not open '%s': ", filename);
+        return NULL;
+    }
+
+    return blk;
+}
+
 static int snap_save(const SnapSaveParams *params)
 {
     SnapSaveState *sn;
+    int res = -1;
 
     snap_save_init_state();
     sn = snap_save_get_state();
-    (void) sn;
 
+    sn->blk = snap_create(params->filename, params->image_size,
+            params->bdrv_flags, params->writethrough);
+    if (!sn->blk) {
+        goto fail;
+    }
+
+fail:
     snap_save_destroy_state();
 
-    return 0;
+    return res;
 }
 
 static int snap_load(SnapLoadParams *params)
 {
     SnapLoadState *sn;
+    int res = -1;
 
     snap_load_init_state();
     sn = snap_load_get_state();
-    (void) sn;
 
+    sn->blk = snap_open(params->filename, params->bdrv_flags);
+    if (!sn->blk) {
+        goto fail;
+    }
+
+fail:
     snap_load_destroy_state();
 
-    return 0;
+    return res;
 }
 
 static int64_t cvtnum_full(const char *name, const char *value,
-- 
2.25.1