qemu-img: Require larger zero areas for sparse handling

By default, require 4k of consecutive zero bytes for qemu-img to make the
output file sparse by not issuing a write request for the zeroed parts. Add an
-S option to allow users to tune this setting.

This helps to avoid situations where a lot of zero sectors and data sectors are
mixed and qemu-img tended to issue many tiny 512 byte writes.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-img-cmds.hx

@@ -28,9 +28,9 @@ STEXI
 ETEXI
 
 DEF("convert", img_convert,
-    "convert [-c] [-p] [-f fmt] [-t cache] [-O output_fmt] [-o options] [-s snapshot_name] filename [filename2 [...]] output_filename")
+    "convert [-c] [-p] [-f fmt] [-t cache] [-O output_fmt] [-o options] [-s snapshot_name] [-S sparse_size] filename [filename2 [...]] output_filename")
 STEXI
-@item convert [-c] [-p] [-f @var{fmt}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_name}] @var{filename} [@var{filename2} [...]] @var{output_filename}
+@item convert [-c] [-p] [-f @var{fmt}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_name}] [-S @var{sparse_size}] @var{filename} [@var{filename2} [...]] @var{output_filename}
 ETEXI
 
 DEF("info", img_info,

qemu-img.c

@@ -82,6 +82,8 @@ static void help(void)
            "       rebasing in this case (useful for renaming the backing file)\n"
            "  '-h' with or without a command shows this help and lists the supported formats\n"
            "  '-p' show progress of command (only certain commands)\n"
+           "  '-S' indicates the consecutive number of bytes that must contain only zeros\n"
+           "       for qemu-img to create a sparse image during conversion\n"
            "\n"
            "Parameters to snapshot subcommand:\n"
            "  'snapshot' is the name of the snapshot to create, apply or delete\n"
@@ -570,6 +572,48 @@ static int is_allocated_sectors(const uint8_t *buf, int n, int *pnum)
     return v;
 }
 
+/*
+ * Like is_allocated_sectors, but if the buffer starts with a used sector,
+ * up to 'min' consecutive sectors containing zeros are ignored. This avoids
+ * breaking up write requests for only small sparse areas.
+ */
+static int is_allocated_sectors_min(const uint8_t *buf, int n, int *pnum,
+    int min)
+{
+    int ret;
+    int num_checked, num_used;
+
+    if (n < min) {
+        min = n;
+    }
+
+    ret = is_allocated_sectors(buf, n, pnum);
+    if (!ret) {
+        return ret;
+    }
+
+    num_used = *pnum;
+    buf += BDRV_SECTOR_SIZE * *pnum;
+    n -= *pnum;
+    num_checked = num_used;
+
+    while (n > 0) {
+        ret = is_allocated_sectors(buf, n, pnum);
+
+        buf += BDRV_SECTOR_SIZE * *pnum;
+        n -= *pnum;
+        num_checked += *pnum;
+        if (ret) {
+            num_used = num_checked;
+        } else if (*pnum >= min) {
+            break;
+        }
+    }
+
+    *pnum = num_used;
+    return 1;
+}
+
 /*
  * Compares two buffers sector by sector. Returns 0 if the first sector of both
  * buffers matches, non-zero otherwise.
@@ -620,6 +664,7 @@ static int img_convert(int argc, char **argv)
     char *options = NULL;
     const char *snapshot_name = NULL;
     float local_progress;
+    int min_sparse = 8; /* Need at least 4k of zeros for sparse detection */
 
     fmt = NULL;
     out_fmt = "raw";
@@ -627,7 +672,7 @@ static int img_convert(int argc, char **argv)
     out_baseimg = NULL;
     compress = 0;
     for(;;) {
-        c = getopt(argc, argv, "f:O:B:s:hce6o:pt:");
+        c = getopt(argc, argv, "f:O:B:s:hce6o:pS:t:");
         if (c == -1) {
             break;
         }
@@ -662,6 +707,18 @@ static int img_convert(int argc, char **argv)
         case 's':
             snapshot_name = optarg;
             break;
+        case 'S':
+        {
+            int64_t sval;
+            sval = strtosz_suffix(optarg, NULL, STRTOSZ_DEFSUFFIX_B);
+            if (sval < 0) {
+                error_report("Invalid minimum zero buffer size for sparse output specified");
+                return 1;
+            }
+
+            min_sparse = sval / BDRV_SECTOR_SIZE;
+            break;
+        }
         case 'p':
             progress = 1;
             break;
@@ -970,7 +1027,7 @@ static int img_convert(int argc, char **argv)
                    sectors that are entirely 0, since whatever data was
                    already there is garbage, not 0s. */
                 if (!has_zero_init || out_baseimg ||
-                    is_allocated_sectors(buf1, n, &n1)) {
+                    is_allocated_sectors_min(buf1, n, &n1, min_sparse)) {
                     ret = bdrv_write(out_bs, sector_num, buf1, n1);
                     if (ret < 0) {
                         error_report("error while writing sector %" PRId64

qemu-img.texi

@@ -40,6 +40,11 @@ indicates that target image must be compressed (qcow format only)
 with or without a command shows help and lists the supported formats
 @item -p
 display progress bar (convert and rebase commands only)
+@item -S @var{size}
+indicates the consecutive number of bytes that must contain only zeros
+for qemu-img to create a sparse image during conversion. This value is rounded
+down to the nearest 512 bytes. You may use the common size suffixes like
+@code{k} for kilobytes.
 @end table
 
 Parameters to snapshot subcommand:
@@ -86,7 +91,7 @@ it doesn't need to be specified separately in this case.
 
 Commit the changes recorded in @var{filename} in its base image.
 
-@item convert [-c] [-p] [-f @var{fmt}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_name}] @var{filename} [@var{filename2} [...]] @var{output_filename}
+@item convert [-c] [-p] [-f @var{fmt}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_name}] [-S @var{sparse_size}] @var{filename} [@var{filename2} [...]] @var{output_filename}
 
 Convert the disk image @var{filename} or a snapshot @var{snapshot_name} to disk image @var{output_filename}
 using format @var{output_fmt}. It can be optionally compressed (@code{-c}