Re: [RFC]swapfso and "ioctl" function for filesystems

[phcoder]

Robert Millan wrote:
> On Wed, Sep 03, 2008 at 11:42:44AM +0200, phcoder wrote:
>> Hello, all.
>> For some FS sometimes additional functions are needed. It could be some
>> type of control (e.g. in ZFS manage zpools) or preparation for OS
>> booting (e.g. in FAT put IO.SYS and MSDOS.SYS at the begining of the
>> root directory). While theese functions are quite specific to FS
>> sometimes are important to implement.
> 
> What would be the purpose of that?  Please describe a use case.
> 
With ZFS or ext3cow: Suppose you made a huge mistake and installed
unbootable kernel and have no backup in another file. But ZFS/ext3cow
has its own backup. So ZFS/ext3cow driver may provide a call something like
static grub_err_t zfs_timeback (int timeref);
And anounce it like:
add_funcs={
{"timeback", zfs_timeback},
{0, 0}
};
Then a module timeback.mod can suply a command like
timeback <device> <date>
which uses the function supplied by zfs and ext3cow.
With FAT: suppose you have let's say DR-DOS and windows on the same
partition. Great their boot files have different names so they shouldn't
 conflict. But in fact they do because both require their boot files
(io.sys/msdos.sys or ibmbio.com/ibmdos.com) to be first entries in root
directory. So fat module can provide a call like
fat_put_rootfile_to_slot (char *filename, int place);

Then a module "dosprep.mod" can do something like:
put_rootfile_to ("io.sys",0);
put_rootfile_to ("msdos.sys",0);

>> [...]. So I intend to implement a call "swapfso" (FSO=File System
>> Object) at least for fat and ext2.
> 
> Do you mean a filesystem write that swaps two file references?
> 
Exactly. It's easy to implement (just exchange inode numbers and
filetype fields in dirent). It's also another usage case for first part
of my email.
Vladimir 'phcoder' Serbinenko