Re: [RFC PATCH v0.8 02/14] btrfs-progs: Allow __btrfs_map_block_v2 to remove unrelated stripes

[Sanidhya Solanki <lkml.page@gmail.com>]

On Mon, 17 Oct 2016 09:27:31 +0800
Qu Wenruo <quwenruo@cn.fujitsu.com> wrote:

> For READ, caller normally hopes to get what they request, other than
> full stripe map.
> 
> In this case, we should remove unrelated stripe map, just like the
> following case:
>                32K               96K
>                |<-request range->|
>          0              64k           128K
> RAID0:   |    Data 1    |   Data 2    |
>               disk1         disk2
> Before this patch, we return the full stripe:
> Stripe 0: Logical 0, Physical X, Len 64K, Dev disk1
> Stripe 1: Logical 64k, Physical Y, Len 64K, Dev disk2
> 
> After this patch, we limit the stripe result to the request range:
> Stripe 0: Logical 32K, Physical X+32K, Len 32K, Dev disk1
> Stripe 1: Logical 64k, Physical Y, Len 32K, Dev disk2
> 
> And if it's a RAID5/6 stripe, we just handle it like RAID0, ignoring
> parities.
> 
> This should make caller easier to use.
> 
> Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
> ---
>  volumes.c | 103 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 102 insertions(+), 1 deletion(-)
> 
> diff --git a/volumes.c b/volumes.c
> index 94f3e42..ba16d19 100644
> --- a/volumes.c
> +++ b/volumes.c
> @@ -1682,6 +1682,107 @@ static int fill_full_map_block(struct map_lookup *map, u64 start, u64 length,
>  	return 0;
>  }
>  
> +static void del_one_stripe(struct btrfs_map_block *map_block, int i)
> +{
> +	int cur_nr = map_block->num_stripes;
> +	int size_left = (cur_nr - 1 - i) * sizeof(struct btrfs_map_stripe);
> +
> +	memmove(&map_block->stripes[i], &map_block->stripes[i + 1], size_left);
> +	map_block->num_stripes--;
> +}
> +
> +static void remove_unrelated_stripes(struct map_lookup *map,
> +				     int rw, u64 start, u64 length,
> +				     struct btrfs_map_block *map_block)
> +{
> +	int i = 0;
> +	/*
> +	 * RAID5/6 write must use full stripe.
> +	 * No need to do anything.
> +	 */
> +	if (map->type & (BTRFS_BLOCK_GROUP_RAID5 | BTRFS_BLOCK_GROUP_RAID6) &&
> +	    rw == WRITE)
> +		return;

I believe it should be an "or" operation in the loop condition, rather than a
single pipe.

Sanidhya

> +	/*
> +	 * For RAID0/1/10/DUP, whatever read/write, we can remove unrelated
> +	 * stripes without causing anything wrong.
> +	 * RAID5/6 READ is just like RAID0, we don't care parity unless we need
> +	 * to recovery.
> +	 * For recovery, rw should be set to WRITE.
> +	 */
> +	while (i < map_block->num_stripes) {
> +		struct btrfs_map_stripe *stripe;
> +		u64 orig_logical; /* Original stripe logical start */
> +		u64 orig_end; /* Original stripe logical end */
> +
> +		stripe = &map_block->stripes[i];
> +
> +		/*
> +		 * For READ, we don't really care parity
> +		 */
> +		if (stripe->logical == BTRFS_RAID5_P_STRIPE ||
> +		    stripe->logical == BTRFS_RAID6_Q_STRIPE) {
> +			del_one_stripe(map_block, i);
> +			continue;
> +		}
> +		/* Completely unrelated stripe */
> +		if (stripe->logical >= start + length ||
> +		    stripe->logical + stripe->length <= start) {
> +			del_one_stripe(map_block, i);
> +			continue;
> +		}
> +		/* Covered stripe, modify its logical and physical */
> +		orig_logical = stripe->logical;
> +		orig_end = stripe->logical + stripe->length;
> +		if (start + length <= orig_end) {
> +			/*
> +			 * |<--range-->|
> +			 *   |  stripe   |
> +			 * Or
> +			 *     |<range>|
> +			 *   |  stripe   |
> +			 */
> +			stripe->logical = max(orig_logical, start);
> +			stripe->length = start + length;
> +			stripe->physical += stripe->logical - orig_logical;
> +		} else if (start >= orig_logical) {
> +			/*
> +			 *     |<-range--->|
> +			 * |  stripe     |
> +			 * Or
> +			 *     |<range>|
> +			 * |  stripe     |
> +			 */
> +			stripe->logical = start;
> +			stripe->length = min(orig_end, start + length);
> +			stripe->physical += stripe->logical - orig_logical;
> +		}
> +		/*
> +		 * Remaining case:
> +		 * |<----range----->|
> +		 *   | stripe |
> +		 * No need to do any modification
> +		 */
> +		i++;
> +	}
> +
> +	/* Recaculate map_block size */
> +	map_block->start = 0;
> +	map_block->length = 0;
> +	for (i = 0; i < map_block->num_stripes; i++) {
> +		struct btrfs_map_stripe *stripe;
> +
> +		stripe = &map_block->stripes[i];
> +		if (stripe->logical > map_block->start)
> +			map_block->start = stripe->logical;
> +		if (stripe->logical + stripe->length >
> +		    map_block->start + map_block->length)
> +			map_block->length = stripe->logical + stripe->length -
> +					    map_block->start;
> +	}
> +}
> +
>  int __btrfs_map_block_v2(struct btrfs_fs_info *fs_info, int rw, u64 logical,
>  			 u64 length, struct btrfs_map_block **map_ret)
>  {
> @@ -1717,7 +1818,7 @@ int __btrfs_map_block_v2(struct btrfs_fs_info *fs_info, int rw, u64 logical,
>  		free(map_block);
>  		return ret;
>  	}
> -	/* TODO: Remove unrelated map_stripes for READ operation */
> +	remove_unrelated_stripes(map, rw, logical, length, map_block);
>  
>  	*map_ret = map_block;
>  	return 0;