Re: [PATCH RFC] fs: New zonefs file system

[Johannes Thumshirn <jthumshirn@suse.de>]

On Fri, Jul 12, 2019 at 08:31:32AM +0000, Damien Le Moal wrote:
[...]
> > I know I've been advocating for having on-disk metadata, but do we really
> > sacrifice a whole zone per default? I thought we'll have on-disk metadata
> > optional (I might be completely off the track here and need more coffee to
> > wake up though).
> 
> Yes, indeed we do not really need the super block for now. But it is still super
> useful to have so that:
> 1) libblkid and other such userland tools can probe the disk to see its format,
> and preserve the usual "use -force option if you really want to overwrite"
> behavior of all format tools.
> 2) Still related to previous point, the super block allows commands like:
> mount /dev/sdX /mnt
> and
> mount -t zonefs /dev/sdX /mnt
> to have the same result. That is, without the super block, if the drive was
> previously formatted for btrfs or f2fs, the first command will mount that old
> format, while the second will mount zonefs without necessarily erasing the old
> FS super block.
> 3) Having the super block with a version number will allow in the future to add
> more metadata (e.g. file names as decided by the application) while allowing
> backward compatibility of the code.
> 
> >> +	end = zones + sbi->s_nr_zones[ZONEFS_ZTYPE_ALL];
> >> +	for (zone = &zones[1]; zone < end; zone = next) {
> > 
> > [...]
> > 
> >> +
> >> +	/* Set defaults */
> >> +	sbi->s_uid = GLOBAL_ROOT_UID;
> >> +	sbi->s_gid = GLOBAL_ROOT_GID;
> >> +	sbi->s_perm = S_IRUSR | S_IWUSR | S_IRGRP; /* 0640 */
> >> +
> >> +
> >> +	ret = zonefs_read_super(sb);
> >> +	if (ret)
> >> +		return ret;
> > 
> > That would be cool to be controllable via a mount option and have it:
> > 	sbi->s_uid = opt.uid;
> > 	sbi->s_gid = opt.gid;
> > 	sbi->s_perm = opt.mode;
> > 
> > or pass these mount options to zonefs_read_super() and they can be set after
> > the feature validation.
> 
> Yes, I thought of that and even had that implemented in a previous version. I
> switched to the static format time definition only so that the resulting
> operation of the FS is a little more like a normal file system, namely, mounting
> the device does not change file attributes and so can be mounted and seen with
> the same attribute no matter where it is mounted, regardless of the mount options.

[...]

> > I'd rather not write the uid, gid, permissions and startsect name to the
> > superblock but have it controllable via a mount option. Just write the feature
> > to the superblock so we know we _can_ control this per mount.
> 
> This is another view. See my thinking above. Thoughts ?

Hm, both a valid views and I'm not sure which is better for the production use
cases either.

With the approach I had in mind one could pre-format dozens of drives and
deploy them in the field. The admins then can decide what
UID/GID/Permission/etc.. the application layer needs for a particular drive
and supply these parameters on mount time.

With the approach you implemented here we don't have the surprises if someone
accidentally (or maliciously) passed the wrong parameters.

A combined approach is also not 100% discussion free, as what has preference,
on-disk or mount time.

I'll be thinking about it and come back once I have an idea.

Byte,
	Johannes
-- 
Johannes Thumshirn                            SUSE Labs Filesystems
jthumshirn@suse.de                                +49 911 74053 689
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