[Qemu-devel] [PATCH] block: posix: Always allocate the first block

[Nir Soffer]

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.

When using preallocation=off, we always allocate at least one filesystem
block:

    $ ./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

I did quick performance tests for these flows:
- Provisioning a VM with a new raw image.
- Copying disks with qemu-img convert to new raw target image

I installed Fedora 29 server on raw sparse image, measuring the time
from clicking "Begin installation" until the "Reboot" button appears:

Before(s)  After(s)     Diff(%)
-------------------------------
     356        389        +8.4

I ran this only once, so we cannot tell much from these results.

The second test was cloning the installation image with qemu-img
convert, doing 10 runs:

    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

Here we see very clear improvement in CPU usage.

Signed-off-by: Nir Soffer <address@hidden>
---
 block/file-posix.c         | 25 +++++++++++++++++++++++++
 tests/qemu-iotests/150.out |  1 +
 tests/qemu-iotests/160     |  4 ++++
 tests/qemu-iotests/175     | 19 +++++++++++++------
 tests/qemu-iotests/175.out |  8 ++++----
 tests/qemu-iotests/221.out | 12 ++++++++----
 tests/qemu-iotests/253.out | 12 ++++++++----
 7 files changed, 63 insertions(+), 18 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index b9c33c8f6c..3964dd2021 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1755,6 +1755,27 @@ static int handle_aiocb_discard(void *opaque)
     return ret;
 }
 
+/*
+ * Help alignment detection by allocating the first block.
+ *
+ * When reading with direct I/O from unallocated area on Gluster backed by XFS,
+ * reading succeeds regardless of request length. In this case we fallback to
+ * safe aligment which is not optimal. Allocating the first block avoids this
+ * fallback.
+ *
+ * Returns: 0 on success, -errno on failure.
+ */
+static int allocate_first_block(int fd)
+{
+    ssize_t n;
+
+    do {
+        n = pwrite(fd, "\0", 1, 0);
+    } while (n == -1 && errno == EINTR);
+
+    return (n == -1) ? -errno : 0;
+}
+
 static int handle_aiocb_truncate(void *opaque)
 {
     RawPosixAIOData *aiocb = opaque;
@@ -1794,6 +1815,8 @@ static int handle_aiocb_truncate(void *opaque)
                 /* posix_fallocate() doesn't set errno. */
                 error_setg_errno(errp, -result,
                                  "Could not preallocate new data");
+            } else if (current_length == 0) {
+                allocate_first_block(fd);
             }
         } else {
             result = 0;
@@ -1855,6 +1878,8 @@ static int handle_aiocb_truncate(void *opaque)
         if (ftruncate(fd, offset) != 0) {
             result = -errno;
             error_setg_errno(errp, -result, "Could not resize file");
+        } else if (current_length == 0 && offset > current_length) {
+            allocate_first_block(fd);
         }
         return result;
     default:
diff --git a/tests/qemu-iotests/150.out b/tests/qemu-iotests/150.out
index 2a54e8dcfa..3cdc7727a5 100644
--- a/tests/qemu-iotests/150.out
+++ b/tests/qemu-iotests/150.out
@@ -3,6 +3,7 @@ QA output created by 150
 === Mapping sparse conversion ===
 
 Offset          Length          File
+0               0x1000          TEST_DIR/t.IMGFMT
 
 === Mapping non-sparse conversion ===
 
diff --git a/tests/qemu-iotests/160 b/tests/qemu-iotests/160
index df89d3864b..ad2d054a47 100755
--- a/tests/qemu-iotests/160
+++ b/tests/qemu-iotests/160
@@ -57,6 +57,10 @@ for skip in $TEST_SKIP_BLOCKS; do
     $QEMU_IMG dd if="$TEST_IMG" of="$TEST_IMG.out" skip="$skip" -O "$IMGFMT" \
         2> /dev/null
     TEST_IMG="$TEST_IMG.out" _check_test_img
+
+    # We always write the first byte of an image.
+    printf "\0" > "$TEST_IMG.out.dd"
+
     dd if="$TEST_IMG" of="$TEST_IMG.out.dd" skip="$skip" status=none
 
     echo
diff --git a/tests/qemu-iotests/175 b/tests/qemu-iotests/175
index 51e62c8276..c6a3a7bb1e 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=$((extra_blocks + min_blocks))\\(\$\\|[^0-9]\\)/min 
allocation/" \
+        -e "s/blocks=$((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 6d9a5ed84e..263e521262 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/221.out b/tests/qemu-iotests/221.out
index 9f9dd52bb0..dca024a0c3 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 607c0baa0b..3d08b305d7 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
-- 
2.20.1