Re: [PATCH 01/13] writeback: IO-less balance_dirty_pages()

[Dmitry <dmonakhov@openvz.org>]

On Sun, 5 Dec 2010 21:42:31 -0500, Ted Ts'o <tytso@mit.edu> wrote:
> On Mon, Dec 06, 2010 at 12:14:35AM +0800, Wu Fengguang wrote:
> > 
> > Ah I seem to find the root cause. See the attached graphs. Ext4 should
> > be calling redirty_page_for_writepage() to redirty ~300MB pages on
> > every ~10s. The redirties happen in big bursts, so not surprisingly
> > the dd task's dirty weight will suddenly drop to 0.
> > 
> > It should be the same ext4 issue discussed here:
> > 
> >         http://www.spinics.net/lists/linux-fsdevel/msg39555.html
> 
> Yeah, unfortunately the fix suggested isn't the right one.
> 
> The right fix is going to involve making much more radical changes to
> the ext4 write submission path, which is on my todo queue.  For now,
> if people don't like these nasty writeback dynamics, my suggestion for
> now is to mount the filesystem data=writeback.
> 
> This is basically the clean equivalent of the patch suggested by Feng
> Tang in his e-mail referenced above.  Given that ext4 uses delayed
> allocation, most of the time unwritten blocks are not allocated, and
> so stale data isn't exposed.
May be it is reasonable to introduce new mount option which control
dynamic delalloc on/off behavior for example like this:
0) -odelalloc=off : analog of nodelalloc
1) -odelalloc=normal : Default mode (disable delalloc if close to full fs)
2) -odelalloc=force  : delalloc mode always enabled, so we have to do
                     writeback more aggressive in case of ENOSPC.

So one can force delalloc and can safely use this writeback mode in 
multi-user environment. Openvz already has this. I'll prepare the patch
if you are interesting in that feature?
> 
> The case which you're seeing here is where both the jbd2 data=order
> forced writeback is colliding with the writeback thread, and
> unfortunately, the forced writeback in the jbd2 layer is done in an
> extremely inefficient manner.  So data=writeback is the workaround,
> and unlike ext3, it's not a serious security leak.  It is possible for
> some stale data to get exposed if you get unlucky when you crash,
> though, so there is a potential for some security exposure.
> 
> The long-term solution to this problem is to rework the ext4 writeback
> path so that we write the data blocks when they are newly allocated,
> and then only update fs metadata once they are written.  As I said,
> it's on my queue.  Until then, the only suggestion I can give folks is
> data=writeback.
> 
> 						- Ted
> 
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