xfs_freeze same as umount? How is that helpful?

[Dave Chinner]

On Thu, Oct 04, 2012 at 03:39:33PM -0700, Linda Walsh wrote:
> Greg Freemyer wrote:
> >Conceptually it is typically:
> >- quiesce system
> ----
> 	Um... it seems that this is equivalent to being
> able to umount the disk?

NO, it's not. freeze intentionally leaves the log dirty, whereas
unmount leaves it clean.

> When I tried xfs_freeze / fs_freeze got fs-busy -- same as I would
> if I tried to umount it.

Of course - it's got to write all the dirty data andmetadata in
memory to disk. Freeze is about providing a stable, consistent disk
image of the filesystem, so it must flush dirty objects from memory
to disk to provide that.

> I thought the point of xfs_freeze was to allow it to be brought to
> a consistent state without unmounting it?

Exactly.

> Coincidentally, after trying a few freezes, the system froze.

Entirely possible if you froze the root filesystem and something you
rely on tried to write to the filesystem.

Anyway, a one-line "it froze" report doesn't tell us anything about
the problem you saw. So:

http://xfs.org/index.php/XFS_FAQ#Q:_What_information_should_I_include_when_reporting_a_problem.3F

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com