Re: [RFC PATCH] Btrfs: add ioctl to set compress or cow per file/dir

[Chris Mason <chris.mason@oracle.com>]

Excerpts from liubo's message of 2011-02-24 04:40:55 -0500:
> 
> Data compression and data cow are controlled across the entire FS by mount
> options right now.  ioctls are needed to set this on a per file or per
> directory basis.  This has been proposed previously, but VFS developers
> wanted us to use generic ioctls rather than btrfs-specific ones.
> 
> We need to fit these into the existing per-inode flags, and to use the generic
> FS_IOCTL_SETFLAGS ioctl.  For data compression, there are the existing
> compression flags of vfs inode, while for datacow, there is no flag to
> indicate it, which we need to add.
> So, what we will do is to add datacow flag in vfs inode flags and then to
> set or to unset btrfs compress/cow flag on the corresponding btrfs inode's flag
> per file or per directory.  Moreover, we also add a compression type ioctl to
> make this feature more flexible.
> 
> I really expect some advices and comments on the followings:
> 
> - In this patch, I made a special ioctl to set compress type, and to record
>   the compress_type per inode on disk, I've consumed some reserved space of
>   btrfs_inode_item, so is this acceptable?

I don't expect people to mix compression types on the disk.  There
really should just be one true compression method (probably LZO once it
has been established for a while).  So, I'd prefer that we store this in
the super, and just have flags in the inode for enabling or disabling
compression.

>   Meanwhile, I got another idea from my collegue, could we just owe the whole
>   compress type thing to new proper mount options, ie,
>   mount xxx xxx -o compress=a,inode_compress=b?
>   Seems that this makes mount more flexible.

It does make it more flexible, but I think sometimes extra flexibility
leads to more QA time and isn't often used by the actual users ;)

> 
> - When we are inclined to set inode's compression type, should it be a "force"
>   mode?
>   This is much like the difference between mount as compress and mount as
>   compress-force.

I'd store this as flags in the super too.

> 
> - For directory basis, after compress/cow ioctl on it, any files that are
>   created or renamed in it, or moved into it, will inherit the directory's
>   compress and datacow attribute.
>   Here comes to some disputes, is it right that renamed and moved files
>   also inherit the father directory's compress & datacow attribute?
>   And if what we are dealing with is directory, should this behaviour be
>   recursive or not?
>   I'm inclined to leave these recursive things to btrfs-progs if this is
>   necessary.

I'd say that if we rename a file into a directory it does inherit, but
not make it recursive.

-chris