[Qemu-devel] [PATCH v3 0/1] block: enforce minimal 4096 alignment in qemu_blockalign

["Denis V. Lunev" <den@openvz.org>]

The following sequence
    int fd = open(argv[1], O_RDWR | O_CREAT | O_DIRECT, 0644);
    for (i = 0; i < 100000; i++)
            write(fd, buf, 4096);
iperforms 5% better if buf is aligned to 4096 bytes rather then to
512 bytes.

I have used the following program to test
#define _GNU_SOURCE

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <malloc.h>
#include <string.h>

int main(int argc, char *argv[])
{
    int fd = open(argv[1], O_RDWR | O_CREAT | O_DIRECT, 0644);
    void *buf;
    int i = 0, align = atoi(argv[2]);

    do {
        buf = memalign(align, 4096);
        if (align >= 4096)
            break;
        if ((unsigned long)buf & 4095)
            break;
        i++;
    } while (1);
    printf("%d %p\n", i, buf);

    memset(buf, 0x11, 4096);

    for (i = 0; i < 100000; i++) {
        lseek(fd, SEEK_CUR, 4096);
        write(fd, buf, 4096);
    }

    close(fd);
    return 0;
}
for in in `seq 1 30` ; do a.out aa ; done

The file was placed into 8 GB partition on HDD below to avoid speed
change due to different offset on disk. Results are reliable:
- 189 vs 180 seconds on Linux 3.16

The following setups have been tested:
1) ext4 with block size equals to 1024 over 512/512 physical/logical
   sector size SSD disk
2) ext4 with block size equals to 4096 over 512/512 physical/logical
   sector size SSD disk
3) ext4 with block size equals to 4096 over 512/4096 physical/logical
   sector size rotational disk (WDC WD20EZRX)
4) xfs with block size equals to 4096 over 512/512 physical/logical
   sector size SSD disk

The difference is quite reliable and the same 5%.
  qemu-io -n -c 'write -P 0xaa 0 1G' 1.img
for image in qcow2 format is 1% faster.

Changes from v2:
- opt_mem_alignment is split to opt_mem_alignment for bounce buffering
  and min_mem_alignment to check buffers coming from guest.

Changes from v1:
- enforces 4096 alignment in qemu_(try_)blockalign, avoid touching of
  bdrv_qiov_is_aligned path not to enforce additional bounce buffering
  as suggested by Paolo
- reduces 10% to 5% in patch description to better fit 180 vs 189
  difference

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>