a few questions about ext3 journal

Andreas Dilger adilger at clusterfs.com
Fri Mar 11 06:08:23 UTC 2005

On Mar 10, 2005  12:10 +0100, Jérôme Petazzoni wrote:
> 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.

There is no current statistics on any journal usage (though it would be
nice to have this).  Knowing how much space there currently is in the
journal, some sort of average of the free journal space (e.g. abs(head-tail)
as each new handle started), how often the journal was totally full and
had to be flushed, etc.  This would go a long way to telling a user and
the ext3 developers how large a journal is needed under their workload.

Currently Lustre just creates very large (400MB) journals on all of the
filesystems because we know that a large journal improves the performance
dramatically, but we have never done the trial+error approach of finding
the "optimal" size.

> 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

This is already done, and is used by the LVM/device mapper subsystem
to do snapshots of the filesystem.  However, I'm not sure if there is
a user-space API to access this.

> 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).

Yes, this can be done (I think) by tuning the journal flush time and 
having a large enough journal to avoid filling it up.  However, I
don't think this would be practical because the only common way to do
this would be e.g. flash memory and the heavy usage of the journal
would quickly wear out such devices, and it would also be slow.  

Cheers, Andreas
Andreas Dilger
http://members.shaw.ca/adilger/             http://members.shaw.ca/golinux/

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