a few questions about ext3 journal

[Jérôme Petazzoni <jp enix org>]

A few wild ideas/questions :

1) Is there a way to check the size of the journal of an ext3 filesystem ?
I mean - the actually used size ; not the total size of the journal.

2) Would it be difficult to implement "freeze" of ext3 filesystem - that 
is, blocking all I/O to the filesystem until it's "unfrozen" (XFS can do 
that), for two purposes :
A/ allowing "freezing" in a clean state, to allow clean snapshotting
B/ allowing "freezing" while moving a SCSI disk or a network-connected 
disk without umounting filesystem

A/ would require some work at the FS layer I guess, but B/ might be 
doable at the devicemapper layer or something like that.

3) Is it possible to allow data to stay in the journal for a very long 
time ?
Rationale : for laptops with a lot of memory and some solid-state 
memory, this would allow to shutdown the hard disk (if all read data is 
in the cache, and all written data goes to the log on the solid-state disk).