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

[John Snow <jsnow@redhat.com>]

On 8/16/19 5:21 PM, Nir Soffer wrote:
> 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.
> 

Where does this detection/fallback happen? (Can it be improved?)

> 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.
> 

That seems like a pretty big difference for just having pre-allocated a
single block. What was the actual command line / block graph for that test?

Was this over a network that could explain the variance?

> 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.
> 

Hard to argue much with that. I feel a little strange trying to force
the allocation of the first block, but I suppose in practice "almost no
preallocation" is indistinguishable from "exactly no preallocation" if
you squint.

> Signed-off-by: Nir Soffer <nsoffer@redhat.com>
> ---
>  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
>