qemu-img: add compression option to rebase subcommand

If we rebase an image whose backing file has compressed clusters, we
might end up wasting disk space since the copied clusters are now
uncompressed.  In order to have better control over this, let's add
"--compress" option to the "qemu-img rebase" command.

Note that this option affects only the clusters which are actually being
copied from the original backing file.  The clusters which were
uncompressed in the target image will remain so.

Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Message-ID: <20230919165804.439110-8-andrey.drobyshev@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 20 insertions, 6 deletions

diff --git a/qemu-img.c b/qemu-img.c
index 01de772..369c2e8 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3534,11 +3534,13 @@ static int img_rebase(int argc, char **argv)
     char *filename;
     const char *fmt, *cache, *src_cache, *out_basefmt, *out_baseimg;
     int c, flags, src_flags, ret;
+    BdrvRequestFlags write_flags = 0;
     bool writethrough, src_writethrough;
     int unsafe = 0;
     bool force_share = false;
     int progress = 0;
     bool quiet = false;
+    bool compress = false;
     Error *local_err = NULL;
     bool image_opts = false;
     int64_t write_align;
@@ -3555,9 +3557,10 @@ static int img_rebase(int argc, char **argv)
             {"object", required_argument, 0, OPTION_OBJECT},
             {"image-opts", no_argument, 0, OPTION_IMAGE_OPTS},
             {"force-share", no_argument, 0, 'U'},
+            {"compress", no_argument, 0, 'c'},
             {0, 0, 0, 0}
         };
-        c = getopt_long(argc, argv, ":hf:F:b:upt:T:qU",
+        c = getopt_long(argc, argv, ":hf:F:b:upt:T:qUc",
                         long_options, NULL);
         if (c == -1) {
             break;
@@ -3605,6 +3608,9 @@ static int img_rebase(int argc, char **argv)
         case 'U':
             force_share = true;
             break;
+        case 'c':
+            compress = true;
+            break;
         }
     }
 
@@ -3657,6 +3663,14 @@ static int img_rebase(int argc, char **argv)
 
     unfiltered_bs = bdrv_skip_filters(bs);
 
+    if (compress && !block_driver_can_compress(unfiltered_bs->drv)) {
+        error_report("Compression not supported for this file format");
+        ret = -1;
+        goto out;
+    } else if (compress) {
+        write_flags |= BDRV_REQ_WRITE_COMPRESSED;
+    }
+
     if (out_basefmt != NULL) {
         if (bdrv_find_format(out_basefmt) == NULL) {
             error_report("Invalid format name: '%s'", out_basefmt);
@@ -3666,18 +3680,18 @@ static int img_rebase(int argc, char **argv)
     }
 
     /*
-     * We need overlay subcluster size to make sure write requests are
-     * aligned.
+     * We need overlay subcluster size (or cluster size in case writes are
+     * compressed) to make sure write requests are aligned.
      */
     ret = bdrv_get_info(unfiltered_bs, &bdi);
     if (ret < 0) {
         error_report("could not get block driver info");
         goto out;
     } else if (bdi.subcluster_size == 0) {
-        bdi.subcluster_size = 1;
+        bdi.cluster_size = bdi.subcluster_size = 1;
     }
 
-    write_align = bdi.subcluster_size;
+    write_align = compress ? bdi.cluster_size : bdi.subcluster_size;
 
     /* For safe rebasing we need to compare old and new backing file */
     if (!unsafe) {
@@ -3930,7 +3944,7 @@ static int img_rebase(int argc, char **argv)
                     } else {
                         assert(written + pnum <= IO_BUF_SIZE);
                         ret = blk_pwrite(blk, offset + written, pnum,
-                                         buf_old + written, 0);
+                                         buf_old + written, write_flags);
                     }
                     if (ret < 0) {
                         error_report("Error while writing to COW image: %s",