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Re: when does journal flush with data=journal

Andreas Dilger wrote:
> On Feb 08, 2002  12:42 -0500, Bill McGonigle wrote:
> > I'm trying to eek every last bit of performance out of a sync NFS server
> > using ext3.
> >
> > I created a 400MB journal, for my data=journal disk.  When copying a
> > 200MB file to the machine, by the sound of the disk, it flushed data to
> > disk about 4 times, so, about every 50MB.
> I think there is also a 5 second flush interval. IIRC, there is a
> parameter which can be tuned, but I don't know the exact mechanism
> (compile time, mount option, /proc entry).
> Hmm, looking further, it seems it is a compile-time option in
> fs/jbd/journal.c:journal_init_common.c where it sets the
> journal_commit_interval.
> I thought Andrew at least had a patch to make the journal_commit_interval
> match the flush interval for bdflush.

Actually, I just set the commit interval to 100000000.  So commits
are initiated by kupdate instead of kjournald.  So you can then
tune the commit interval with /proc/sys/vm/bdflush (there's a userspace
app which sets bdflush parameters too, but I can't remember its name).

In Bill's testing, commits will most likely be forced by exhaustion
of journal space, rather than kjournald timeout.  This is determined
by journal->j_max_transaction_buffers, which is initialised in journal_reset():

        journal->j_max_transaction_buffers = journal->j_maxlen / 4;

probably this could be changed to journal->j_maxlen / 2 with no
ill effect.  I haven't tried it.

The other (complimentary) option is to simply hack mke2fs so that it permits
larger journals.  In e2fsprogs's misc/util.c:figure_journal_size():

-                if (j_blocks < 1024 || j_blocks > 102400) {
+                if (j_blocks < 1024 || j_blocks >= fs->super->s_free_blocks_count) {


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