qemu-img: Truncate before full preallocation

In a previous commit (qemu-img: Do not truncate before preallocation) we
moved truncate to the PREALLOC_MODE_OFF branch to avoid slowdown in
posix_fallocate().

However this change is not optimal when using PREALLOC_MODE_FULL, since
knowing the final size from the beginning could allow the file system
driver to do less allocations and possibly avoid fragmentation of the
file.

Now we truncate also before doing full preallocation.

Signed-off-by: Nir Soffer <nirsof@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/block/file-posix.c b/block/file-posix.c
index 442f080eaa..d24e34be36 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1604,6 +1604,17 @@ static int raw_create(const char *filename, QemuOpts *opts, Error **errp)
 #endif
     case PREALLOC_MODE_FULL:
     {
+        /*
+         * Knowing the final size from the beginning could allow the file
+         * system driver to do less allocations and possibly avoid
+         * fragmentation of the file.
+         */
+        if (ftruncate(fd, total_size) != 0) {
+            result = -errno;
+            error_setg_errno(errp, -result, "Could not resize file");
+            goto out_close;
+        }
+
         int64_t num = 0, left = total_size;
         buf = g_malloc0(65536);
 
@@ -1642,6 +1653,7 @@ static int raw_create(const char *filename, QemuOpts *opts, Error **errp)
         break;
     }
 
+out_close:
     if (qemu_close(fd) != 0 && result == 0) {
         result = -errno;
         error_setg_errno(errp, -result, "Could not close the new file");