block: posix: Always allocate the first block

When creating an image with preallocation "off" or "falloc", the first
block of the image is typically not allocated. When using Gluster
storage backed by XFS filesystem, reading this block using direct I/O
succeeds regardless of request length, fooling alignment detection.

In this case we fallback to a safe value (4096) instead of the optimal
value (512), which may lead to unneeded data copying when aligning
requests.  Allocating the first block avoids the fallback.

Since we allocate the first block even with preallocation=off, we no
longer create images with zero disk size:

    $ ./qemu-img create -f raw test.raw 1g
    Formatting 'test.raw', fmt=raw size=1073741824

    $ ls -lhs test.raw
    4.0K -rw-r--r--. 1 nsoffer nsoffer 1.0G Aug 16 23:48 test.raw

And converting the image requires additional cluster:

    $ ./qemu-img measure -f raw -O qcow2 test.raw
    required size: 458752
    fully allocated size: 1074135040

When using format like vmdk with multiple files per image, we allocate
one block per file:

    $ ./qemu-img create -f vmdk -o subformat=twoGbMaxExtentFlat test.vmdk 4g
    Formatting 'test.vmdk', fmt=vmdk size=4294967296 compat6=off hwversion=undefined subformat=twoGbMaxExtentFlat

    $ ls -lhs test*.vmdk
    4.0K -rw-r--r--. 1 nsoffer nsoffer 2.0G Aug 27 03:23 test-f001.vmdk
    4.0K -rw-r--r--. 1 nsoffer nsoffer 2.0G Aug 27 03:23 test-f002.vmdk
    4.0K -rw-r--r--. 1 nsoffer nsoffer  353 Aug 27 03:23 test.vmdk

I did quick performance test for copying disks with qemu-img convert to
new raw target image to Gluster storage with sector size of 512 bytes:

    for i in $(seq 10); do
        rm -f dst.raw
        sleep 10
        time ./qemu-img convert -f raw -O raw -t none -T none src.raw dst.raw
    done

Here is a table comparing the total time spent:

Type    Before(s)   After(s)    Diff(%)
---------------------------------------
real      530.028    469.123      -11.4
user       17.204     10.768      -37.4
sys        17.881      7.011      -60.7

We can see very clear improvement in CPU usage.

Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Message-id: 20190827010528.8818-2-nsoffer@redhat.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

8 files changed, 48 insertions, 21 deletions

diff --git a/tests/qemu-iotests/059.out b/tests/qemu-iotests/059.out
index 4fab42a..fe3f861 100644
--- a/tests/qemu-iotests/059.out
+++ b/tests/qemu-iotests/059.out
@@ -27,7 +27,7 @@ Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1073741824000 subformat=twoGbMax
 image: TEST_DIR/t.vmdk
 file format: vmdk
 virtual size: 0.977 TiB (1073741824000 bytes)
-disk size: 16 KiB
+disk size: 1.97 MiB
 Format specific information:
     cid: XXXXXXXX
     parent cid: XXXXXXXX
diff --git a/tests/qemu-iotests/150.out b/tests/qemu-iotests/150.out.qcow2
index 2a54e8d..2a54e8d 100644
--- a/tests/qemu-iotests/150.out
+++ b/tests/qemu-iotests/150.out.qcow2
diff --git a/tests/qemu-iotests/150.out.raw b/tests/qemu-iotests/150.out.raw
new file mode 100644
index 0000000..3cdc772
--- /dev/null
+++ b/tests/qemu-iotests/150.out.raw
@@ -0,0 +1,12 @@
+QA output created by 150
+
+=== Mapping sparse conversion ===
+
+Offset          Length          File
+0               0x1000          TEST_DIR/t.IMGFMT
+
+=== Mapping non-sparse conversion ===
+
+Offset          Length          File
+0               0x100000        TEST_DIR/t.IMGFMT
+*** done
diff --git a/tests/qemu-iotests/175 b/tests/qemu-iotests/175
index 51e62c8..7ba28b3 100755
--- a/tests/qemu-iotests/175
+++ b/tests/qemu-iotests/175
@@ -37,14 +37,16 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 # the file size.  This function hides the resulting difference in the
 # stat -c '%b' output.
 # Parameter 1: Number of blocks an empty file occupies
-# Parameter 2: Image size in bytes
+# Parameter 2: Minimal number of blocks in an image
+# Parameter 3: Image size in bytes
 _filter_blocks()
 {
     extra_blocks=$1
-    img_size=$2
+    min_blocks=$2
+    img_size=$3
 
-    sed -e "s/blocks=$extra_blocks\\(\$\\|[^0-9]\\)/nothing allocated/" \
-        -e "s/blocks=$((extra_blocks + img_size / 512))\\(\$\\|[^0-9]\\)/everything allocated/"
+    sed -e "s/blocks=$min_blocks\\(\$\\|[^0-9]\\)/min allocation/" \
+        -e "s/blocks=$((extra_blocks + img_size / 512))\\(\$\\|[^0-9]\\)/max allocation/"
 }
 
 # get standard environment, filters and checks
@@ -60,16 +62,21 @@ size=$((1 * 1024 * 1024))
 touch "$TEST_DIR/empty"
 extra_blocks=$(stat -c '%b' "$TEST_DIR/empty")
 
+# We always write the first byte; check how many blocks this filesystem
+# allocates to match empty image alloation.
+printf "\0" > "$TEST_DIR/empty"
+min_blocks=$(stat -c '%b' "$TEST_DIR/empty")
+
 echo
 echo "== creating image with default preallocation =="
 _make_test_img $size | _filter_imgfmt
-stat -c "size=%s, blocks=%b" $TEST_IMG | _filter_blocks $extra_blocks $size
+stat -c "size=%s, blocks=%b" $TEST_IMG | _filter_blocks $extra_blocks $min_blocks $size
 
 for mode in off full falloc; do
     echo
     echo "== creating image with preallocation $mode =="
     IMGOPTS=preallocation=$mode _make_test_img $size | _filter_imgfmt
-    stat -c "size=%s, blocks=%b" $TEST_IMG | _filter_blocks $extra_blocks $size
+    stat -c "size=%s, blocks=%b" $TEST_IMG | _filter_blocks $extra_blocks $min_blocks $size
 done
 
 # success, all done
diff --git a/tests/qemu-iotests/175.out b/tests/qemu-iotests/175.out
index 6d9a5ed..263e521 100644
--- a/tests/qemu-iotests/175.out
+++ b/tests/qemu-iotests/175.out
@@ -2,17 +2,17 @@ QA output created by 175
 
 == creating image with default preallocation ==
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576
-size=1048576, nothing allocated
+size=1048576, min allocation
 
 == creating image with preallocation off ==
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 preallocation=off
-size=1048576, nothing allocated
+size=1048576, min allocation
 
 == creating image with preallocation full ==
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 preallocation=full
-size=1048576, everything allocated
+size=1048576, max allocation
 
 == creating image with preallocation falloc ==
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 preallocation=falloc
-size=1048576, everything allocated
+size=1048576, max allocation
  *** done
diff --git a/tests/qemu-iotests/178.out.qcow2 b/tests/qemu-iotests/178.out.qcow2
index 55a8dc9..9e7d8c4 100644
--- a/tests/qemu-iotests/178.out.qcow2
+++ b/tests/qemu-iotests/178.out.qcow2
@@ -101,7 +101,7 @@ converted image file size in bytes: 196608
 == raw input image with data (human) ==
 
 Formatting 'TEST_DIR/t.qcow2', fmt=IMGFMT size=1073741824
-required size: 393216
+required size: 458752
 fully allocated size: 1074135040
 wrote 512/512 bytes at offset 512
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
@@ -257,7 +257,7 @@ converted image file size in bytes: 196608
 
 Formatting 'TEST_DIR/t.qcow2', fmt=IMGFMT size=1073741824
 {
-    "required": 393216,
+    "required": 458752,
     "fully-allocated": 1074135040
 }
 wrote 512/512 bytes at offset 512
diff --git a/tests/qemu-iotests/221.out b/tests/qemu-iotests/221.out
index 9f9dd52..dca024a 100644
--- a/tests/qemu-iotests/221.out
+++ b/tests/qemu-iotests/221.out
@@ -3,14 +3,18 @@ QA output created by 221
 === Check mapping of unaligned raw image ===
 
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=65537
-[{ "start": 0, "length": 66048, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
-[{ "start": 0, "length": 66048, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
+[{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 4096, "length": 61952, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
+[{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 4096, "length": 61952, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
 wrote 1/1 bytes at offset 65536
 1 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-[{ "start": 0, "length": 65536, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
+[{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 4096, "length": 61440, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
 { "start": 65536, "length": 1, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 65537, "length": 511, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
-[{ "start": 0, "length": 65536, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
+[{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 4096, "length": 61440, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
 { "start": 65536, "length": 1, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 65537, "length": 511, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
 *** done
diff --git a/tests/qemu-iotests/253.out b/tests/qemu-iotests/253.out
index 607c0ba..3d08b30 100644
--- a/tests/qemu-iotests/253.out
+++ b/tests/qemu-iotests/253.out
@@ -3,12 +3,16 @@ QA output created by 253
 === Check mapping of unaligned raw image ===
 
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048575
-[{ "start": 0, "length": 1048576, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
-[{ "start": 0, "length": 1048576, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
+[{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 4096, "length": 1044480, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
+[{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 4096, "length": 1044480, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
 wrote 65535/65535 bytes at offset 983040
 63.999 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-[{ "start": 0, "length": 983040, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
+[{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 4096, "length": 978944, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
 { "start": 983040, "length": 65536, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
-[{ "start": 0, "length": 983040, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
+[{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 4096, "length": 978944, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
 { "start": 983040, "length": 65536, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
 *** done