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authorKevin Wolf <kwolf@redhat.com>2011-08-26 15:27:13 +0200
committerKevin Wolf <kwolf@redhat.com>2011-08-29 14:42:39 +0200
commita22f123ca3d3c09a77af4341ed1fbcc175b54f1d (patch)
treeb9992c37d4a4645e6282375c669ae1e771206a1c /qemu-img.c
parentc488c7f649106d09df76f697adccbe6e72520b26 (diff)
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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>
Diffstat (limited to 'qemu-img.c')
-rw-r--r--qemu-img.c61
1 files changed, 59 insertions, 2 deletions
diff --git a/qemu-img.c b/qemu-img.c
index 0561d77..6a39731 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 += 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