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Re: [Qemu-block] [PATCH v2 1/2] block: posix: Always allocate the first
From: |
Max Reitz |
Subject: |
Re: [Qemu-block] [PATCH v2 1/2] block: posix: Always allocate the first block |
Date: |
Mon, 26 Aug 2019 14:31:53 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 |
On 26.08.19 00:03, 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.
>
> 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
>
> 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 <address@hidden>
> ---
> block/file-posix.c | 43 +++++++++++++++++++
> tests/qemu-iotests/{150.out => 150.out.qcow2} | 0
> 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 ++++--
> 8 files changed, 90 insertions(+), 20 deletions(-)
> rename tests/qemu-iotests/{150.out => 150.out.qcow2} (100%)
> create mode 100644 tests/qemu-iotests/150.out.raw
>
> diff --git a/block/file-posix.c b/block/file-posix.c
> index fbeb0068db..51688ae3fc 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -1749,6 +1749,39 @@ static int handle_aiocb_discard(void *opaque)
> 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 = MIN(MAX_BLOCKSIZE, max_size);
Hm, well, there was a reason why I proposed rounding this down to the
next power of two. If max_size is not a power of two but below
MAX_BLOCKSIZE, write_size will not be a power of two, and thus the write
below may fail even if write_size exceeds the physical block size.
You can see that in the test case you add by using e.g. 768 as the
destination size (provided your test filesystem has a block size of 512).
Now I would like to say that it’s stupid to resize an O_DIRECT file to a
size that is not a multiple of the block size; but I’ve had a bug
assigned to me before because that didn’t work.
But maybe it’s actually better if it doesn’t work. I don’t know.
> + size_t max_align = MAX(MAX_BLOCKSIZE, getpagesize());
> + void *buf;
> + ssize_t n;
> +
> + 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);
> +
> + qemu_vfree(buf);
> +
> + return (n == -1) ? -errno : 0;
> +}
> +
> static int handle_aiocb_truncate(void *opaque)
> {
> RawPosixAIOData *aiocb = opaque;
> @@ -1788,6 +1821,13 @@ 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) {
> + /*
> + * Needed only if posix_fallocate() used fallocate(), but we
> + * don't have a way to detect that.
This sounds a bit weird because fallocate() is what we call
posix_fallocate() for. I’d’ve liked something that states more
explicitly that unaligned reads from fallocated areas may succeed even
with O_DIRECT, hence the need for allocate_first_block().
> Optimize future alignment
> + * probing; ignore failures.
> + */
> + allocate_first_block(fd, offset);
> }
> } else {
> result = 0;
[...]
> diff --git a/tests/qemu-iotests/175 b/tests/qemu-iotests/175
> index 51e62c8276..d54cb43c39 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/" \
Superfluous parentheses ($(())), but not wrong.
So I think I can give a
Reviewed-by: Max Reitz <address@hidden>
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Re: [Qemu-block] [Qemu-devel] [PATCH v2 0/2] Optimize alignment probing, no-reply, 2019/08/25