Am 05.07.2018 um 12:52 hat Peter Lieven geschrieben:
> We currently don't enforce that the sparse segments we detect during convert
> are
> aligned. This leads to unnecessary and costly read-modify-write cycles either
> internally in Qemu or in the background on the storage device as nearly all
> modern filesystems or hardware have a 4k alignment internally.
>
> The number of RMW cycles when converting an example image [1] to a raw device
> that
> has 4k sector size is about 4600 4k read requests to perform a total of about
> 15000
> write requests. With this path the additional 4600 read requests are
> eliminated.
>
> [1]
> https://cloud-images.ubuntu.com/releases/16.04/release/ubuntu-16.04-server-cloudimg-amd64-disk1.vmdk
>
> Signed-off-by: Peter Lieven <p...@kamp.de>
> ---
> V2->V3: - ensure that s.alignment is a power of 2
> - correctly handle n < alignment in is_allocated_sectors if
> sector_num % alignment > 0.
> V1->V2: - take the current sector offset into account [Max]
> - try to figure out the target alignment [Max]
>
> qemu-img.c | 46 ++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 36 insertions(+), 10 deletions(-)
>
> diff --git a/qemu-img.c b/qemu-img.c
> index e1a506f..db91b9e 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -1105,8 +1105,11 @@ static int64_t find_nonzero(const uint8_t *buf,
> int64_t n)
> *
> * 'pnum' is set to the number of sectors (including and immediately
> following
> * the first one) that are known to be in the same allocated/unallocated
> state.
> + * The function will try to align 'pnum' to the number of sectors specified
> + * in 'alignment' to avoid unnecassary RMW cycles on modern hardware.
> */
> -static int is_allocated_sectors(const uint8_t *buf, int n, int *pnum)
> +static int is_allocated_sectors(const uint8_t *buf, int n, int *pnum,
> + int64_t sector_num, int alignment)
> {
> bool is_zero;
> int i;
> @@ -1115,14 +1118,26 @@ static int is_allocated_sectors(const uint8_t *buf,
> int n, int *pnum)
> *pnum = 0;
> return 0;
> }
> - is_zero = buffer_is_zero(buf, 512);
> - for(i = 1; i < n; i++) {
> - buf += 512;
> - if (is_zero != buffer_is_zero(buf, 512)) {
> +
> + if (n % alignment) {
> + alignment = 1;
> + }
So if n is unaligned, we keep the result unaligned, too. Makes sense,
because otherwise we'd just split the request in two, but still get the
same result.
Worth mentioning in the function comment, though?
> +
> + if (sector_num % alignment) {
> + n = ROUND_UP(sector_num, alignment) - sector_num;
> + alignment = 1;
> + }
So if the start is unaligned, only check until the next alignment
boundary.
This one isn't obvious to me. Doesn't it result in the same scenario
where a request is needlessly split in two? Wouldn't it be better to
first check the unaligned head and then continue with the rest of n if
that results in an aligned end offset?
Actually, should the order of both checks be reversed, because an
unaligned n with an unaligned sector_num could actually result in an
aligned end offset?
> + n /= alignment;
> +
> + is_zero = buffer_is_zero(buf, BDRV_SECTOR_SIZE * alignment);
> + for (i = 1; i < n; i++) {
> + buf += BDRV_SECTOR_SIZE * alignment;
> + if (is_zero != buffer_is_zero(buf, BDRV_SECTOR_SIZE * alignment)) {
> break;
> }
> }
> - *pnum = i;
> + *pnum = i * alignment;
> return !is_zero;
> }
Kevin
