Re: [Qemu-devel] [PATCH 1/3] COW: Speed up writes

[Paolo Bonzini <pbonzini@redhat.com>]

Il 06/11/2013 13:23, Charlie Shepherd ha scritto:
> Process a whole sector's worth of COW bits by reading a sector, setting the bits, then writing it
> out again. Make sure we only flush once, before writing metadata.
> 
> Signed-off-by: Charlie Shepherd <charlie@ctshepherd.com>
> ---
>  block/cow.c | 79 ++++++++++++++++++++++++++++++++-----------------------------
>  1 file changed, 41 insertions(+), 38 deletions(-)
> 
> diff --git a/block/cow.c b/block/cow.c
> index 909c3e7..66f1478 100644
> --- a/block/cow.c
> +++ b/block/cow.c
> @@ -103,40 +103,18 @@ static int cow_open(BlockDriverState *bs, QDict *options, int flags,
>      return ret;
>  }
>  
> -/*
> - * XXX(hch): right now these functions are extremely inefficient.
> - * We should just read the whole bitmap we'll need in one go instead.
> - */
> -static inline int cow_set_bit(BlockDriverState *bs, int64_t bitnum, bool *first)
> +static inline void cow_set_bits(uint8_t *bitmap, int start, int64_t nb_sectors)
>  {
> -    uint64_t offset = sizeof(struct cow_header_v2) + bitnum / 8;
> -    uint8_t bitmap;
> -    int ret;
> -
> -    ret = bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap));
> -    if (ret < 0) {
> -       return ret;
> -    }
> -
> -    if (bitmap & (1 << (bitnum % 8))) {
> -        return 0;
> -    }
> -
> -    if (*first) {
> -        ret = bdrv_flush(bs->file);
> -        if (ret < 0) {
> -            return ret;
> +    int64_t bitnum = start, last = start + nb_sectors;
> +    while (bitnum < last) {
> +        if ((bitnum & 7) == 0) {

I think this is missing " && bitnum + 8 <= last" in the condition.

Otherwise looks good.

Paolo

> +            bitmap[bitnum / 8] = 0xFF;
> +            bitnum += 8;
> +            continue;
>          }
> -        *first = false;
> -    }
> -
> -    bitmap |= (1 << (bitnum % 8));
> -
> -    ret = bdrv_pwrite(bs->file, offset, &bitmap, sizeof(bitmap));
> -    if (ret < 0) {
> -       return ret;
> +        bitmap[bitnum/8] |= (1 << (bitnum % 8));
> +        bitnum++;
>      }
> -    return 0;
>  }
>  
>  #define BITS_PER_BITMAP_SECTOR (512 * 8)
> @@ -204,18 +182,43 @@ static int64_t coroutine_fn cow_co_get_block_status(BlockDriverState *bs,
>  static int cow_update_bitmap(BlockDriverState *bs, int64_t sector_num,
>          int nb_sectors)
>  {
> -    int error = 0;
> -    int i;
> +    int64_t bitnum = sector_num + sizeof(struct cow_header_v2) * 8;
> +    uint64_t offset = (bitnum / 8) & -BDRV_SECTOR_SIZE;
>      bool first = true;
>  
> -    for (i = 0; i < nb_sectors; i++) {
> -        error = cow_set_bit(bs, sector_num + i, &first);
> -        if (error) {
> -            break;
> +    while (nb_sectors) {
> +        int ret;
> +        uint8_t bitmap[BDRV_SECTOR_SIZE];
> +
> +        bitnum &= BITS_PER_BITMAP_SECTOR - 1;
> +        int sector_bits = MIN(nb_sectors, BITS_PER_BITMAP_SECTOR - bitnum);
> +
> +        ret = bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap));
> +        if (ret < 0) {
> +            return ret;
>          }
> +
> +        if (first) {
> +            ret = bdrv_flush(bs->file);
> +            if (ret < 0) {
> +                return ret;
> +            }
> +            first = false;
> +        }
> +
> +        cow_set_bits(bitmap, bitnum, sector_bits);
> +
> +        ret = bdrv_pwrite(bs->file, offset, &bitmap, sizeof(bitmap));
> +        if (ret < 0) {
> +            return ret;
> +        }
> +
> +        bitnum += sector_bits;
> +        nb_sectors -= sector_bits;
> +        offset += BDRV_SECTOR_SIZE;
>      }
>  
> -    return error;
> +    return 0;
>  }
>  
>  static int coroutine_fn cow_read(BlockDriverState *bs, int64_t sector_num,
>