a few questions about ext3 journal

Christian evilninja at gmx.net
Fri Mar 11 15:04:32 UTC 2005

Hash: SHA1

Jérôme Petazzoni wrote:
> 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.

perhaps "logdump -ac" (within debugfs) will help - i you can tell from its
output what parts are "used".

> 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

would "remount,ro" be sufficient?

> B/ allowing "freezing" while moving a SCSI disk or a network-connected
> disk without umounting filesystem

err, "unplug the cable without unmounting the filesystem"?? you'd have to
hold the entire fs in ram for the "move" or i don't understand what you mean.

> 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).

the only tuneable which comes to my mind right now is the "commit"
paramater for mount:

        Sync all data and metadata every nrsec seconds. The default value
        is 5 seconds. Zero means default.

and there are the laptop-mode-tools [1] using some kernel hacks to spin
down disks and continue working.


[1] http://www.xs4all.nl/~bsamwel/laptop_mode/
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