Re: [Qemu-devel] [PATCH 2/4] qcow2: Allow update_cluster_refcount() to update several clusters refcount.

[Kevin Wolf <kwolf@suse.de>]

Laurent Vivier schrieb:
> pièce jointe document texte brut
> (0002-Allow-update_cluster_refcount-to-update-several-cl.patch)
> To improve performance when the qcow2 file is empty, this patch
> allows update_cluster_refcount() to update refcount of
> several clusters.
> 
> Signed-off-by: Laurent Vivier <Laurent.Vivier@bull.net>
> ---
>  block-qcow2.c |  107 ++++++++++++++++++++++++++++++++++++----------------------
>  1 file changed, 68 insertions(+), 39 deletions(-)
> 
> Index: qemu/block-qcow2.c
> ===================================================================
> --- qemu.orig/block-qcow2.c	2008-11-06 16:40:44.000000000 +0100
> +++ qemu/block-qcow2.c	2008-11-06 16:40:45.000000000 +0100
> @@ -159,6 +159,7 @@ static void refcount_close(BlockDriverSt
>  static int get_refcount(BlockDriverState *bs, int64_t cluster_index);
>  static int update_cluster_refcount(BlockDriverState *bs,
>                                     int64_t cluster_index,
> +                                   int nb_clusters,
>                                     int addend);
>  static void update_refcount(BlockDriverState *bs,
>                              int64_t offset, int64_t length,
> @@ -1711,7 +1712,7 @@ static int update_snapshot_refcount(Bloc
>                          refcount = 2;
>                      } else {
>                          if (addend != 0) {
> -                            refcount = update_cluster_refcount(bs, offset >> s->cluster_bits, addend);
> +                            refcount = update_cluster_refcount(bs, offset >> s->cluster_bits, 1, addend);
>                          } else {
>                              refcount = get_refcount(bs, offset >> s->cluster_bits);
>                          }
> @@ -1733,7 +1734,7 @@ static int update_snapshot_refcount(Bloc
>              }
>  
>              if (addend != 0) {
> -                refcount = update_cluster_refcount(bs, l2_offset >> s->cluster_bits, addend);
> +                refcount = update_cluster_refcount(bs, l2_offset >> s->cluster_bits, 1, addend);
>              } else {
>                  refcount = get_refcount(bs, l2_offset >> s->cluster_bits);
>              }
> @@ -2292,14 +2293,14 @@ static int64_t alloc_bytes(BlockDriverSt
>          if (free_in_cluster == 0)
>              s->free_byte_offset = 0;
>          if ((offset & (s->cluster_size - 1)) != 0)
> -            update_cluster_refcount(bs, offset >> s->cluster_bits, 1);
> +            update_cluster_refcount(bs, offset >> s->cluster_bits, 1, 1);
>      } else {
>          offset = alloc_clusters(bs, s->cluster_size);
>          cluster_offset = s->free_byte_offset & ~(s->cluster_size - 1);
>          if ((cluster_offset + s->cluster_size) == offset) {
>              /* we are lucky: contiguous data */
>              offset = s->free_byte_offset;
> -            update_cluster_refcount(bs, offset >> s->cluster_bits, 1);
> +            update_cluster_refcount(bs, offset >> s->cluster_bits, 1, 1);
>              s->free_byte_offset += size;
>          } else {
>              s->free_byte_offset = offset;
> @@ -2389,46 +2390,77 @@ static int grow_refcount_table(BlockDriv
>  /* XXX: cache several refcount block clusters ? */
>  static int update_cluster_refcount(BlockDriverState *bs,
>                                     int64_t cluster_index,
> +                                   int nb_clusters,
>                                     int addend)
>  {
>      BDRVQcowState *s = bs->opaque;
>      int64_t refcount_block_offset;
> -    int ret, refcount_table_index, block_index, refcount;
> +    int ret, refcount_table_index, refcount_table_last_index, block_index, refcount;
> +    int nb_block_index;
> +    int refcount_cache_size;
>  
> -    refcount_table_index = cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);
> -    if (refcount_table_index >= s->refcount_table_size) {
> +    if (nb_clusters == 0)
> +        return 0;
> +
> +    refcount_table_last_index = (cluster_index + nb_clusters - 1) >>
> +                                (s->cluster_bits - REFCOUNT_SHIFT);
> +
> +    /* grow the refcount table if needed */
> +
> +    if (refcount_table_last_index >= s->refcount_table_size) {
>          if (addend < 0)
>              return -EINVAL;
> -        ret = grow_refcount_table(bs, refcount_table_index + 1);
> +        ret = grow_refcount_table(bs, refcount_table_last_index + 1);
>          if (ret < 0)
>              return ret;
>      }
> -    refcount_block_offset = s->refcount_table[refcount_table_index];
> -    if (!refcount_block_offset) {
> -        if (addend < 0)
> -            return -EINVAL;
> -        refcount_block_offset = alloc_refcount_block(bs, refcount_table_index);
> -        if (refcount_block_offset < 0)
> -            return -EINVAL;
> -    } else {
> -        if (load_refcount_block(bs, refcount_block_offset) < 0)
> +
> +    while (nb_clusters) {
> +        refcount_table_index = cluster_index >>
> +                               (s->cluster_bits - REFCOUNT_SHIFT);
> +        refcount_block_offset = s->refcount_table[refcount_table_index];

I guess (comment? ;-)) this outer loop is for handling requests which
span multiple refcount blocks? If so, am I missing something or is
refcount_block_offset the same for each iteration because cluster_index
never changes?

> +
> +        if (!refcount_block_offset) {
> +            if (addend < 0)
> +                return -EINVAL;
> +            refcount_block_offset = alloc_refcount_block(bs, refcount_table_index);
> +            if (refcount_block_offset < 0)
> +		    return -EINVAL;
> +        } else {
> +            if (load_refcount_block(bs, refcount_block_offset) < 0)
> +                return -EIO;
> +        }
> +
> +        /* we can update the count and save it */
> +
> +        refcount_cache_size = 1 << (s->cluster_bits - REFCOUNT_SHIFT);
> +        nb_block_index = 0;

I have to admit that nb_block_index is a name where I can't image what
the variable might be for. Seems to be a counter for the changed
refcounts in the current refcount block, and block_index seems to be the
first index to be touched in this block. What about first_index and
cur_refcount or something like that? (I don't like these suggestions too
much, either. Maybe someone has better names.)

> +        block_index = cluster_index & (refcount_cache_size - 1);
> +        refcount = 0;
> +        while (nb_clusters &&
> +               block_index + nb_block_index < refcount_cache_size) {
> +
> +            refcount = be16_to_cpu(
> +                         s->refcount_block_cache[block_index + nb_block_index]);
> +            refcount += addend;
> +            if (refcount < 0 || refcount > 0xffff)
> +                return -EINVAL;

Here we possibly abort in the middle of the operation. If it fails
somewhere in the fifth refcount block, what happens with the four
already updated blocks?

> +            if (refcount == 0 &&
> +                cluster_index + nb_block_index < s->free_cluster_index) {
> +                s->free_cluster_index = cluster_index + nb_block_index;
> +            }
> +            s->refcount_block_cache[block_index + nb_block_index] =
> +                                                          cpu_to_be16(refcount);
> +            nb_block_index++;
> +            nb_clusters--;
> +        }
> +        if (bdrv_pwrite(s->hd,
> +                        refcount_block_offset + (block_index << REFCOUNT_SHIFT),
> +                        s->refcount_block_cache + block_index,
> +                        nb_block_index * sizeof(uint16_t)) !=
> +                        nb_block_index * sizeof(uint16_t))
>              return -EIO;

Same here.

>      }
> -    /* we can update the count and save it */
> -    block_index = cluster_index &
> -        ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);
> -    refcount = be16_to_cpu(s->refcount_block_cache[block_index]);
> -    refcount += addend;
> -    if (refcount < 0 || refcount > 0xffff)
> -        return -EINVAL;
> -    if (refcount == 0 && cluster_index < s->free_cluster_index) {
> -        s->free_cluster_index = cluster_index;
> -    }
> -    s->refcount_block_cache[block_index] = cpu_to_be16(refcount);
> -    if (bdrv_pwrite(s->hd,
> -                    refcount_block_offset + (block_index << REFCOUNT_SHIFT),
> -                    &s->refcount_block_cache[block_index], 2) != 2)
> -        return -EIO;
>      return refcount;
>  }
>  
> @@ -2437,7 +2469,7 @@ static void update_refcount(BlockDriverS
>                              int addend)
>  {
>      BDRVQcowState *s = bs->opaque;
> -    int64_t start, last, cluster_offset;
> +    int64_t start, last;
>  
>  #ifdef DEBUG_ALLOC2
>      printf("update_refcount: offset=%lld size=%lld addend=%d\n",
> @@ -2445,12 +2477,9 @@ static void update_refcount(BlockDriverS
>  #endif
>      if (length <= 0)
>          return;
> -    start = offset & ~(s->cluster_size - 1);
> -    last = (offset + length - 1) & ~(s->cluster_size - 1);
> -    for(cluster_offset = start; cluster_offset <= last;
> -        cluster_offset += s->cluster_size) {
> -        update_cluster_refcount(bs, cluster_offset >> s->cluster_bits, addend);
> -    }
> +    start = offset >> s->cluster_bits;
> +    last = (offset + length) >> s->cluster_bits;

Off by one for length % cluster_size == 0?

> +    update_cluster_refcount(bs, start, last - start + 1, addend);
>  }
>  
>  #ifdef DEBUG_ALLOC
> 

Kevin