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From 273237507842493f78cd492cd54137e828a986ef Mon Sep 17 00:00:00 2001
From: Thomas Huth <thuth@redhat.com>
Date: Fri, 30 Aug 2019 12:56:27 +0100
Subject: [PATCH 09/10] block: posix: Always allocate the first block

RH-Author: Thomas Huth <thuth@redhat.com>
Message-id: <20190830125628.23668-5-thuth@redhat.com>
Patchwork-id: 90210
O-Subject: [RHEL-8.1.0 qemu-kvm PATCH v2 4/5] block: posix: Always allocate the first block
Bugzilla: 1738839
RH-Acked-by: Cornelia Huck <cohuck@redhat.com>
RH-Acked-by: Max Reitz <mreitz@redhat.com>
RH-Acked-by: David Hildenbrand <david@redhat.com>

From: Nir Soffer <nirsof@gmail.com>

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>
(cherry picked from commit 3f900188502670a15f8915d5363533512ecd035f)
Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>

Conflicts:
	block/file-posix.c (simple contextual conflict)
	tests/qemu-iotests/059.out (Needed to adapt output a little bit)

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
---
 block/file-posix.c               | 51 ++++++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/059.out       |  2 +-
 tests/qemu-iotests/150.out       | 11 ---------
 tests/qemu-iotests/150.out.qcow2 | 11 +++++++++
 tests/qemu-iotests/150.out.raw   | 12 ++++++++++
 tests/qemu-iotests/175           | 19 ++++++++++-----
 tests/qemu-iotests/175.out       |  8 +++----
 tests/qemu-iotests/178.out.qcow2 |  4 ++--
 tests/qemu-iotests/221.out       | 12 ++++++----
 tests/qemu-iotests/253.out       | 12 ++++++----
 10 files changed, 110 insertions(+), 32 deletions(-)
 delete mode 100644 tests/qemu-iotests/150.out
 create mode 100644 tests/qemu-iotests/150.out.qcow2
 create mode 100644 tests/qemu-iotests/150.out.raw

diff --git a/block/file-posix.c b/block/file-posix.c
index 84c5a31..dfe0bca 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1605,6 +1605,43 @@ static ssize_t handle_aiocb_discard(RawPosixAIOData *aiocb)
     return ret;
 }
 
+/*
+ * Help alignment probing 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 alignment which is not optimal. Allocating the first block avoids this
+ * fallback.
+ *
+ * fd may be opened with O_DIRECT, but we don't know the buffer alignment or
+ * request alignment, so we use safe values.
+ *
+ * Returns: 0 on success, -errno on failure. Since this is an optimization,
+ * caller may ignore failures.
+ */
+static int allocate_first_block(int fd, size_t max_size)
+{
+    size_t write_size = (max_size < MAX_BLOCKSIZE)
+        ? BDRV_SECTOR_SIZE
+        : MAX_BLOCKSIZE;
+    size_t max_align = MAX(MAX_BLOCKSIZE, getpagesize());
+    void *buf;
+    ssize_t n;
+    int ret;
+
+    buf = qemu_memalign(max_align, write_size);
+    memset(buf, 0, write_size);
+
+    do {
+        n = pwrite(fd, buf, write_size, 0);
+    } while (n == -1 && errno == EINTR);
+
+    ret = (n == -1) ? -errno : 0;
+
+    qemu_vfree(buf);
+    return ret;
+}
+
 static int handle_aiocb_truncate(RawPosixAIOData *aiocb)
 {
     int result = 0;
@@ -1642,6 +1679,17 @@ static int handle_aiocb_truncate(RawPosixAIOData *aiocb)
                 /* posix_fallocate() doesn't set errno. */
                 error_setg_errno(errp, -result,
                                  "Could not preallocate new data");
+            } else if (current_length == 0) {
+                /*
+                 * posix_fallocate() uses fallocate() if the filesystem
+                 * supports it, or fallback to manually writing zeroes. If
+                 * fallocate() was used, unaligned reads from the fallocated
+                 * area in raw_probe_alignment() will succeed, hence we need to
+                 * allocate the first block.
+                 *
+                 * Optimize future alignment probing; ignore failures.
+                 */
+                allocate_first_block(fd, offset);
             }
         } else {
             result = 0;
@@ -1700,6 +1748,9 @@ static int handle_aiocb_truncate(RawPosixAIOData *aiocb)
         if (ftruncate(fd, offset) != 0) {
             result = -errno;
             error_setg_errno(errp, -result, "Could not resize file");
+        } else if (current_length == 0 && offset > current_length) {
+            /* Optimize future alignment probing; ignore failures. */
+            allocate_first_block(fd, offset);
         }
         return result;
     default:
diff --git a/tests/qemu-iotests/059.out b/tests/qemu-iotests/059.out
index f6dce79..19cd591 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: 1.0T (1073741824000 bytes)
-disk size: 16K
+disk size: 2.0M
 Format specific information:
     cid: XXXXXXXX
     parent cid: XXXXXXXX
diff --git a/tests/qemu-iotests/150.out b/tests/qemu-iotests/150.out
deleted file mode 100644
index 2a54e8d..0000000
--- a/tests/qemu-iotests/150.out
+++ /dev/null
@@ -1,11 +0,0 @@
-QA output created by 150
-
-=== Mapping sparse conversion ===
-
-Offset          Length          File
-
-=== Mapping non-sparse conversion ===
-
-Offset          Length          File
-0               0x100000        TEST_DIR/t.IMGFMT
-*** done
diff --git a/tests/qemu-iotests/150.out.qcow2 b/tests/qemu-iotests/150.out.qcow2
new file mode 100644
index 0000000..2a54e8d
--- /dev/null
+++ b/tests/qemu-iotests/150.out.qcow2
@@ -0,0 +1,11 @@
+QA output created by 150
+
+=== Mapping sparse conversion ===
+
+Offset          Length          File
+
+=== Mapping non-sparse conversion ===
+
+Offset          Length          File
+0               0x100000        TEST_DIR/t.IMGFMT
+*** done
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 2e37c9a..b3b7712 100755
--- a/tests/qemu-iotests/175
+++ b/tests/qemu-iotests/175
@@ -38,14 +38,16 @@ 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
@@ -61,16 +63,21 @@ 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 d42d4a4..12edc3d 100644
--- a/tests/qemu-iotests/178.out.qcow2
+++ b/tests/qemu-iotests/178.out.qcow2
@@ -96,7 +96,7 @@ 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)
@@ -240,7 +240,7 @@ 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
-- 
1.8.3.1