btrfs-progs: mkfs: Only zero out the first 1M for rootdir

It's a waste of IO to fill the whole image before creating btrfs on it,
just wiping the first 1M, and then write 1 byte to the last position to
create a sparse file.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/mkfs/main.c b/mkfs/main.c
index 90fab59..d817ad8 100644 (file)
--- a/mkfs/main.c
+++ b/mkfs/main.c
@@ -1161,18 +1161,25 @@ static int zero_output_file(int out_fd, u64 size)
 {
        int loop_num;
        u64 location = 0;
-       char buf[4096];
+       char buf[SZ_4K];
        int ret = 0, i;
        ssize_t written;
 
-       memset(buf, 0, 4096);
-       loop_num = size / 4096;
+       memset(buf, 0, SZ_4K);
+
+       /* Only zero out the first 1M */
+       loop_num = SZ_1M / SZ_4K;
        for (i = 0; i < loop_num; i++) {
-               written = pwrite64(out_fd, buf, 4096, location);
-               if (written != 4096)
+               written = pwrite64(out_fd, buf, SZ_4K, location);
+               if (written != SZ_4K)
                        ret = -EIO;
-               location += 4096;
+               location += SZ_4K;
        }
+
+       /* Then enlarge the file to size */
+       written = pwrite64(out_fd, buf, 1, size - 1);
+       if (written < 1)
+               ret = -EIO;
        return ret;
 }