From a221f5a8ed02690687e6709c49ae0e1e01c5f466 Mon Sep 17 00:00:00 2001 From: Vladimir Sementsov-Ogievskiy Date: Fri, 17 Dec 2021 17:46:54 +0100 Subject: [PATCH 09/12] qemu-img: make is_allocated_sectors() more efficient RH-Author: Kevin Wolf RH-MergeRequest: 70: qemu-img convert: Fix sparseness of output image RH-Commit: [2/2] cc05aa4ac506b57ff9b430c007618cdf1485a03f (kmwolf/centos-qemu-kvm) RH-Bugzilla: 1882917 RH-Acked-by: Eric Blake RH-Acked-by: Stefan Hajnoczi RH-Acked-by: Hanna Reitz Consider the case when the whole buffer is zero and end is unaligned. If i <= tail, we return 1 and do one unaligned WRITE, RMW happens. If i > tail, we do on aligned WRITE_ZERO (or skip if target is zeroed) and again one unaligned WRITE, RMW happens. Let's do better: don't fragment the whole-zero buffer and report it as ZERO: in case of zeroed target we just do nothing and avoid RMW. If target is not zeroes, one unaligned WRITE_ZERO should not be much worse than one unaligned WRITE. Signed-off-by: Vladimir Sementsov-Ogievskiy Message-Id: <20211217164654.1184218-3-vsementsov@virtuozzo.com> Tested-by: Peter Lieven Signed-off-by: Kevin Wolf (cherry picked from commit 96054c76ff2db74165385a69f234c57a6bbc941e) Signed-off-by: Kevin Wolf --- qemu-img.c | 23 +++++++++++++++++++---- tests/qemu-iotests/122.out | 8 ++------ 2 files changed, 21 insertions(+), 10 deletions(-) diff --git a/qemu-img.c b/qemu-img.c index f036a1d428..d7ddfcc528 100644 --- a/qemu-img.c +++ b/qemu-img.c @@ -1171,19 +1171,34 @@ static int is_allocated_sectors(const uint8_t *buf, int n, int *pnum, } } + if (i == n) { + /* + * The whole buf is the same. + * No reason to split it into chunks, so return now. + */ + *pnum = i; + return !is_zero; + } + tail = (sector_num + i) & (alignment - 1); if (tail) { if (is_zero && i <= tail) { - /* treat unallocated areas which only consist - * of a small tail as allocated. */ + /* + * For sure next sector after i is data, and it will rewrite this + * tail anyway due to RMW. So, let's just write data now. + */ is_zero = false; } if (!is_zero) { - /* align up end offset of allocated areas. */ + /* If possible, align up end offset of allocated areas. */ i += alignment - tail; i = MIN(i, n); } else { - /* align down end offset of zero areas. */ + /* + * For sure next sector after i is data, and it will rewrite this + * tail anyway due to RMW. Better is avoid RMW and write zeroes up + * to aligned bound. + */ i -= tail; } } diff --git a/tests/qemu-iotests/122.out b/tests/qemu-iotests/122.out index 69b8e8b803..e18766e167 100644 --- a/tests/qemu-iotests/122.out +++ b/tests/qemu-iotests/122.out @@ -201,9 +201,7 @@ convert -S 4k { "start": 8192, "length": 4096, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET}, { "start": 12288, "length": 4096, "depth": 0, "present": false, "zero": true, "data": false}, { "start": 16384, "length": 4096, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET}, -{ "start": 20480, "length": 46080, "depth": 0, "present": false, "zero": true, "data": false}, -{ "start": 66560, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET}, -{ "start": 67584, "length": 67041280, "depth": 0, "present": false, "zero": true, "data": false}] +{ "start": 20480, "length": 67088384, "depth": 0, "present": false, "zero": true, "data": false}] convert -c -S 4k [{ "start": 0, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true}, @@ -215,9 +213,7 @@ convert -c -S 4k convert -S 8k [{ "start": 0, "length": 24576, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET}, -{ "start": 24576, "length": 41984, "depth": 0, "present": false, "zero": true, "data": false}, -{ "start": 66560, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET}, -{ "start": 67584, "length": 67041280, "depth": 0, "present": false, "zero": true, "data": false}] +{ "start": 24576, "length": 67084288, "depth": 0, "present": false, "zero": true, "data": false}] convert -c -S 8k [{ "start": 0, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true}, -- 2.27.0