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[Qemu-devel] [PATCH v4 15/23] qemu-img: Add find_nonzero()
From: |
Eric Blake |
Subject: |
[Qemu-devel] [PATCH v4 15/23] qemu-img: Add find_nonzero() |
Date: |
Wed, 13 Sep 2017 11:03:25 -0500 |
During 'qemu-img compare', when we are checking that an allocated
portion of one file is all zeros, we don't need to waste time
computing how many additional sectors after the first non-zero
byte are also non-zero. Create a new helper find_nonzero() to do
the check for a first non-zero sector, and rebase
check_empty_sectors() to use it.
The new interface intentionally uses bytes in its interface, even
though it still crawls the buffer a sector at a time; it is robust
to a partial sector at the end of the buffer.
Signed-off-by: Eric Blake <address@hidden>
---
v3: new patch
---
qemu-img.c | 32 ++++++++++++++++++++++++++++----
1 file changed, 28 insertions(+), 4 deletions(-)
diff --git a/qemu-img.c b/qemu-img.c
index f5ab29d176..dfccebe6bc 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1064,6 +1064,28 @@ done:
}
/*
+ * Returns -1 if 'buf' contains only zeroes, otherwise the byte index
+ * of the first sector boundary within buf where the sector contains a
+ * non-zero byte. This function is robust to a buffer that is not
+ * sector-aligned.
+ */
+static int64_t find_nonzero(const uint8_t *buf, int64_t n)
+{
+ int64_t i;
+ int64_t end = QEMU_ALIGN_DOWN(n, BDRV_SECTOR_SIZE);
+
+ for (i = 0; i < end; i += BDRV_SECTOR_SIZE) {
+ if (!buffer_is_zero(buf + i, BDRV_SECTOR_SIZE)) {
+ return i;
+ }
+ }
+ if (i < n && !buffer_is_zero(buf + i, n - end)) {
+ return i;
+ }
+ return -1;
+}
+
+/*
* Returns true iff the first sector pointed to by 'buf' contains at least
* a non-NUL byte.
*
@@ -1188,7 +1210,9 @@ static int check_empty_sectors(BlockBackend *blk, int64_t
sect_num,
int sect_count, const char *filename,
uint8_t *buffer, bool quiet)
{
- int pnum, ret = 0;
+ int ret = 0;
+ int64_t idx;
+
ret = blk_pread(blk, sect_num << BDRV_SECTOR_BITS, buffer,
sect_count << BDRV_SECTOR_BITS);
if (ret < 0) {
@@ -1196,10 +1220,10 @@ static int check_empty_sectors(BlockBackend *blk,
int64_t sect_num,
sectors_to_bytes(sect_num), filename, strerror(-ret));
return ret;
}
- ret = is_allocated_sectors(buffer, sect_count, &pnum);
- if (ret || pnum != sect_count) {
+ idx = find_nonzero(buffer, sect_count * BDRV_SECTOR_SIZE);
+ if (idx >= 0) {
qprintf(quiet, "Content mismatch at offset %" PRId64 "!\n",
- sectors_to_bytes(ret ? sect_num : sect_num + pnum));
+ sectors_to_bytes(sect_num) + idx);
return 1;
}
--
2.13.5
- [Qemu-devel] [PATCH v4 08/23] block: Switch bdrv_co_get_block_status() to byte-based, (continued)
- [Qemu-devel] [PATCH v4 08/23] block: Switch bdrv_co_get_block_status() to byte-based, Eric Blake, 2017/09/13
- [Qemu-devel] [PATCH v4 11/23] block: Switch bdrv_co_get_block_status_above() to byte-based, Eric Blake, 2017/09/13
- [Qemu-devel] [PATCH v4 10/23] block: Switch bdrv_common_block_status_above() to byte-based, Eric Blake, 2017/09/13
- [Qemu-devel] [PATCH v4 09/23] block: Switch BdrvCoGetBlockStatusData to byte-based, Eric Blake, 2017/09/13
- [Qemu-devel] [PATCH v4 14/23] qemu-img: Speed up compare on pre-allocated larger file, Eric Blake, 2017/09/13
- [Qemu-devel] [PATCH v4 15/23] qemu-img: Add find_nonzero(),
Eric Blake <=
- [Qemu-devel] [PATCH v4 16/23] qemu-img: Drop redundant error message in compare, Eric Blake, 2017/09/13
- [Qemu-devel] [PATCH v4 13/23] qemu-img: Simplify logic in img_compare(), Eric Blake, 2017/09/13
- [Qemu-devel] [PATCH v4 12/23] block: Convert bdrv_get_block_status_above() to bytes, Eric Blake, 2017/09/13