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Re: when does journal flush with data=journal

Theodore Tso wrote:
> On Fri, Feb 08, 2002 at 12:59:50PM -0800, Andrew Morton wrote:
> > The other (complimentary) option is to simply hack mke2fs so that it permits
> > larger journals.  In e2fsprogs's misc/util.c:figure_journal_size():
> Note though that interactivity may suffer with larger journals; when
> it finally *does* come time to flush the journal, the system may
> become very non-responsive until the journal is actually completely
> flushed.

Yes.  Once we get into forcing checkpoints, new writers can get
*seriously* gummed up, because the filesystem is actively writing
things to the main disk (ie: very seek-intensive), which is much
slower than journal writes.

And increasing the journal size will increase the amount of data which
is checkpointed, when a checkpoint force happens.

Neil Brown found that his NFS server was getting bitten by this -
pauses of a few seconds every thirty seconds, I think.  His workaround
was to drastically shorten the kupdate interval so that kupdate
performs the nice smooth checkpointing for us.  This is discussed
in Daniel Robbins' second article, at 


ext3's checkpointing code is too drastic at present - too much, too
late, too infrequent.  We do need a native solution, rather than
borrowing kupdate functionality, I guess.


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