Re: [RFC][PATCH] btrfs-progs: mkfs: add -O option to specify fs features

[Eric Sandeen <sandeen@redhat.com>]

On 2/19/13 5:16 AM, David Sterba wrote:
> Extend mkfs options to specify optional or potentially backwards
> incompatible features. Use it for extended references.
> 
> Signed-off-by: David Sterba <dsterba@suse.cz>
> ---
> 
> The RFC is for the option name, as there does not seem to be a common name
> amongst other mkfses.

What did you find when looking?

xfsprogs doesn't really have a feature flag option per se; there are just block,
inode, size, log, data etc. options which may trigger some feature set.

e2fsprogs has the rather -O feature -E extended-options set, and darned if I could
tell you the difference ;)

but I think -O is the right option, unless you found other common mkfs's that
use something different.  When in doubt, e2fsprogs conventions probably win.  :)

-Eric

> 
>  man/mkfs.btrfs.8.in |  7 +++++++
>  mkfs.c              | 14 ++++++++++++--
>  2 files changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/man/mkfs.btrfs.8.in b/man/mkfs.btrfs.8.in
> index 41163e0..5ff4e2c 100644
> --- a/man/mkfs.btrfs.8.in
> +++ b/man/mkfs.btrfs.8.in
> @@ -15,6 +15,7 @@ mkfs.btrfs \- create a btrfs filesystem
>  [ \fB\-s\fP\fI sectorsize\fP ]
>  [ \fB\-r\fP\fI rootdir\fP ]
>  [ \fB\-K\fP ]
> +[ \fB\-O\fP\fI feature1,feature2,...\fP ]
>  [ \fB\-h\fP ]
>  [ \fB\-V\fP ]
>  \fI device\fP [ \fIdevice ...\fP ]
> @@ -76,6 +77,12 @@ Specify a directory to copy into the newly created fs.
>  \fB\-K\fR, \fB\-\-nodiscard \fR
>  Do not perform whole device TRIM operation by default.
>  .TP
> +\fB\-O\fR, \fB\-\-features \fIfeature1,feature2,...\fR
> +A list of filesystem features turned on at mkfs time. Not all features are
> +supported by old kernels.
> +
> +extref - allow creating more hardlinks per file (the upper limit is 65535)
> +.TP
>  \fB\-V\fR, \fB\-\-version\fR
>  Print the \fBmkfs.btrfs\fP version and exit.
>  .SH UNIT
> diff --git a/mkfs.c b/mkfs.c
> index 5ece186..dd1e317 100644
> --- a/mkfs.c
> +++ b/mkfs.c
> @@ -335,6 +335,7 @@ static void print_usage(void)
>  	fprintf(stderr, "\t -s --sectorsize min block allocation\n");
>  	fprintf(stderr, "\t -r --rootdir the source directory\n");
>  	fprintf(stderr, "\t -K --nodiscard do not perform whole device TRIM\n");
> +	fprintf(stderr, "\t -O --features coma separated list of filesystem features\n");
>  	fprintf(stderr, "\t -V --version print the mkfs.btrfs version and exit\n");
>  	fprintf(stderr, "%s\n", BTRFS_BUILD_VERSION);
>  	exit(1);
> @@ -395,6 +396,7 @@ static struct option long_options[] = {
>  	{ "version", 0, NULL, 'V' },
>  	{ "rootdir", 1, NULL, 'r' },
>  	{ "nodiscard", 0, NULL, 'K' },
> +	{ "features", 0, NULL, 'O' },
>  	{ 0, 0, 0, 0}
>  };
>  
> @@ -1376,10 +1378,11 @@ int main(int ac, char **av)
>  	char *pretty_buf;
>  	struct btrfs_super_block *super;
>  	u64 flags;
> +	char *features = strdup("");
>  
>  	while(1) {
>  		int c;
> -		c = getopt_long(ac, av, "A:b:fl:n:s:m:d:L:r:VMK", long_options,
> +		c = getopt_long(ac, av, "A:b:fl:n:s:m:d:L:O:r:VMK", long_options,
>  				&option_index);
>  		if (c < 0)
>  			break;
> @@ -1409,6 +1412,10 @@ int main(int ac, char **av)
>  			case 'M':
>  				mixed = 1;
>  				break;
> +			case 'O':
> +				free(features);
> +				features = strdup(optarg);
> +				break;
>  			case 's':
>  				sectorsize = parse_size(optarg);
>  				break;
> @@ -1652,7 +1659,10 @@ raid_groups:
>  
>  	super = &root->fs_info->super_copy;
>  	flags = btrfs_super_incompat_flags(super);
> -	flags |= BTRFS_FEATURE_INCOMPAT_EXTENDED_IREF;
> +	if (strstr(features, "extref") == 0) {
> +		fprintf(stderr, "Turning ON incompat feature: extref (increased hardlink limit per file)\n");
> +		flags |= BTRFS_FEATURE_INCOMPAT_EXTENDED_IREF;
> +	}
>  
>  	if (mixed)
>  		flags |= BTRFS_FEATURE_INCOMPAT_MIXED_GROUPS;
>