Re: [PATCH 1/5] btrfs: change the minimum global reserve size

[Nikolay Borisov <nborisov@suse.com>]

On 16.08.19 г. 18:20 ч., Josef Bacik wrote:
> It made sense to have the global reserve set at 16M in the past, but
> since it is used less nowadays set the minimum size to the number of
> items we'll need to update the main trees we update during a transaction
> commit, plus some slop area so we can do unlinks if we need to.
> 
> In practice this doesn't affect normal file systems, but for xfstests
> where we do things like fill up a fs and then rm * it can fall over in
> weird ways.  This enables us for more sane behavior at extremely small
> file system sizes.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/block-rsv.c | 22 +++++++++++++++++++++-
>  1 file changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
> index c64b460a4301..657675eef443 100644
> --- a/fs/btrfs/block-rsv.c
> +++ b/fs/btrfs/block-rsv.c
> @@ -258,6 +258,7 @@ void btrfs_update_global_block_rsv(struct btrfs_fs_info *fs_info)
>  	struct btrfs_block_rsv *block_rsv = &fs_info->global_block_rsv;
>  	struct btrfs_space_info *sinfo = block_rsv->space_info;
>  	u64 num_bytes;
> +	unsigned min_items;
>  
>  	/*
>  	 * The global block rsv is based on the size of the extent tree, the
> @@ -267,7 +268,26 @@ void btrfs_update_global_block_rsv(struct btrfs_fs_info *fs_info)
>  	num_bytes = btrfs_root_used(&fs_info->extent_root->root_item) +
>  		btrfs_root_used(&fs_info->csum_root->root_item) +
>  		btrfs_root_used(&fs_info->tree_root->root_item);
> -	num_bytes = max_t(u64, num_bytes, SZ_16M);
> +
> +	/*
> +	 * We at a minimum are going to modify the csum root, the tree root, and
> +	 * the extent root.
> +	 */
> +	min_items = 3;
> +
> +	/*
> +	 * But we also want to reserve enough space so we can do the fallback
> +	 * global reserve for an unlink, which is an additional 5 items (see the
> +	 * comment in __unlink_start_trans for what we're modifying.)
> +	 *
> +	 * But we also need space for the delayed ref updates from the unlink,
> +	 * so its 10, 5 for the actual operation, and 5 for the delayed ref
> +	 * updates.
> +	 */
> +	min_items += 10;
> +
> +	num_bytes = max_t(u64, num_bytes,
> +			  btrfs_calc_insert_metadata_size(fs_info, min_items));

For ordinary, 16k nodesize filesystem, btrfs_calc_insert_metadata_size
will really return 3.4m -> 16 * 8 * 2 * 13 * 1024 = 3407872 bytes . In
those cases I expect that the code will always be doing num_bytes =
num_bytes.

>  
>  	spin_lock(&sinfo->lock);
>  	spin_lock(&block_rsv->lock);
>