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 <kw...@redhat.com>
---
qemu-img-cmds.hx | 4 +-
qemu-img.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
qemu-img.texi | 5 +++-
3 files changed, 65 insertions(+), 5 deletions(-)
diff --git a/qemu-img-cmds.hx b/qemu-img-cmds.hx
index 1299e83..4be00a5 100644
--- a/qemu-img-cmds.hx
+++ b/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,
diff --git a/qemu-img.c b/qemu-img.c
index 0561d77..306cd41 100644
--- a/qemu-img.c
+++ b/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"
@@ -571,6 +573,48 @@ static int is_allocated_sectors(const uint8_t *buf, int n,
int *pnum)
}
/*
+ * 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 += 512 * *pnum;
+ n -= *pnum;
+ num_checked = num_used;
+
+ while (n > 0) {
+ ret = is_allocated_sectors(buf, n, pnum);
+
+ buf += 512 * *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
diff --git a/qemu-img.texi b/qemu-img.texi
index 495a1b6..a0579e7 100644
--- a/qemu-img.texi
+++ b/qemu-img.texi
@@ -40,6 +40,9 @@ 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
@end table
Parameters to snapshot subcommand:
@@ -86,7 +89,7 @@ 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{sprase_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}
--
1.7.6
