Re: [PATCH] btrfs: start deprecation of mount option inode_cache

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2020/6/24 上午2:50, David Sterba wrote:
> Estimated time of removal of the functionality is 5.11, the option will
> be still parsed but not doing anything.

Great that we don't need to test for a mount option that no one really uses.

> 
> Reasons for deprecation and removal:
> 
> - very poor naming choice of the mount option, it's supposed to cache
>   and reuse the inode _numbers_, but it sounds a some generic cache for
>   inodes
> 
> - the only known usecase where this option would make sense is on a
>   32bit architecture where inode numbers in one subvolume would be
>   exhausted due to 32bit inode::i_ino
> 
> - the cache is stored on disk, consumes space, needs to be loaded and
>   written back
> 
> - new inode number allocation is slower due to lookups into the cache
>   (compared to a simple increment which is the default)
> 
> - uses the free-space-cache code that is going to be deprecated as well
>   in the future
> 
> Known problems:
> 
> - since 2011, returning EEXIST when there's not enough space in a page
>   to store all checksums, see commit 4b9465cb9e38 ("Btrfs: add mount -o
>   inode_cache")
> 
> Remaining issues:
> 
> - if the option was enabled, new inodes created, the option disabled
>   again, the cache is still stored on the devices and there's currently
>   no way to remove it

What about "btrfs rescue remove-deprecated-feature inode_cache"?
I really don't want kernel to do the hassle.

Thanks,
Qu

> 
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>  fs/btrfs/super.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 98fe2a634c70..3f1abbeef66c 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -827,6 +827,8 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
>  			}
>  			break;
>  		case Opt_inode_cache:
> +			btrfs_warn(info,
> +	"the 'inode_cache' option is deprecated and will be stop working in 5.11");
>  			btrfs_set_pending_and_info(info, INODE_MAP_CACHE,
>  					   "enabling inode map caching");
>  			break;
> 


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