Re: [PATCH 3/8] btrfs: don't use global rsv for chunk allocation

[Nikolay Borisov <nborisov@suse.com>]

On 3.12.18 г. 17:24 ч., Josef Bacik wrote:
> The should_alloc_chunk code has math in it to decide if we're getting
> short on space and if we should go ahead and pre-emptively allocate a
> new chunk.  Previously when we did not have the delayed_refs_rsv, we had
> to assume that the global block rsv was essentially used space and could
> be allocated completely at any time, so we counted this space as "used"
> when determining if we had enough slack space in our current space_info.
> But on any slightly used file system (10gib or more) you can have a
> global reserve of 512mib.  With our default chunk size being 1gib that
> means we just assume half of the block group is used, which could result
> in us allocating more metadata chunks than is actually required.
> 
> With the delayed refs rsv we can flush delayed refs as the space becomes
> tight, and if we actually need more block groups then they will be
> allocated based on space pressure.  We no longer require assuming the
> global reserve is used space in our calculations.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Reviewed-by: Nikolay Borisov <nborisov@suse.com>


> ---
>  fs/btrfs/extent-tree.c | 9 ---------
>  1 file changed, 9 deletions(-)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 204b35434056..667b992d322d 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -4398,21 +4398,12 @@ static inline u64 calc_global_rsv_need_space(struct btrfs_block_rsv *global)
>  static int should_alloc_chunk(struct btrfs_fs_info *fs_info,
>  			      struct btrfs_space_info *sinfo, int force)
>  {
> -	struct btrfs_block_rsv *global_rsv = &fs_info->global_block_rsv;
>  	u64 bytes_used = btrfs_space_info_used(sinfo, false);
>  	u64 thresh;
>  
>  	if (force == CHUNK_ALLOC_FORCE)
>  		return 1;
>  
> -	/*
> -	 * We need to take into account the global rsv because for all intents
> -	 * and purposes it's used space.  Don't worry about locking the
> -	 * global_rsv, it doesn't change except when the transaction commits.
> -	 */
> -	if (sinfo->flags & BTRFS_BLOCK_GROUP_METADATA)
> -		bytes_used += calc_global_rsv_need_space(global_rsv);
> -
>  	/*
>  	 * in limited mode, we want to have some free space up to
>  	 * about 1% of the FS size.
>