Re: [RFC] ext3 freeze feature

Eric Sandeen
2008-02-15T14:24:37Z

Takashi Sato wrote:
> Hi,
>
> Christoph Hellwig wrote:
>> On Fri, Feb 08, 2008 at 08:26:57AM -0500, Andreas Dilger wrote:
>>> You may as well make the common ioctl the same as the XFS version,
>>> both by number and parameters, so that applications which already
>>> understand the XFS ioctl will work on other filesystems.
>> Yes. In facy you should be able to lift the implementations of
>> XFS_IOC_FREEZE and XFS_IOC_THAW to generic code, there's nothing
>> XFS-specific in there.
>
> According to Documentation/ioctl-number.txt,
> XFS_IOC_XXXs (_IOWR('X', aa, bb)) are defined for XFS like below.
> From Documentation/ioctl-number.txt:
> ----------------------------------------------------------------------------
> Code Seq# Include File Comments
> ========================================================
> : :
> 'X' all linux/xfs_fs.h
> ----------------------------------------------------------------------------

It also says:

'f' 00-1F linux/ext2_fs.h

and yet include/linux.h has:

#define FS_IOC_GETFLAGS _IOR('f', 1, long)
#define FS_IOC_SETFLAGS _IOW('f', 2, long)

as generic vfs ioctls. These ioctls started out as
EXT2_IOC_SETFLAGS/EXT2_IOC_GETFLAGS but they were generically useful,
other filesystems picked them up, and they were "elevated" to the vfs.

> So XFS_IOC_FREEZE and XFS_IOC_THAW cannot be lifted to generic code simply.

It would be a simple matter of changing the documentation, I think.

> I think we should create new generic numbers for freeze and thaw
> like FIBMAP as followings.
> linux/fs.h:
> #define FIFREEZE _IO(0x00,3)
> #define FITHAW _IO(0x00,4)
>
> And xfs_freeze calls XFS_IOC_FREEZE with a magic number 1, but what is 1?

Looks like it's called "level" but it's probably a holdover, it doesn't
look like it's used.

> Instead, I'd like the sec to timeout on freeze API in order to thaw
> the filesystem automatically. It can prevent a filesystem from staying
> frozen forever.
> (Because a freezer may cause a deadlock by accessing the frozen filesystem.)

I'm still not very comfortable with the timeout; if you un-freeze on a
timer, how do you know that the work for which you needed the fileystem
frozen is complete? How would you know if your snapshot was good if
there's a possibility that the fs unfroze while it was being taken?

Thanks,
-Eric

> Any comments are very welcome.
>
> Cheers, Takashi
>