On Sun, 5 Dec 2010 21:42:31 -0500, Ted Ts'o <tytso@xxxxxxx> 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 > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxxx For more info on Linux MM, > see: http://www.linux-mm.org/ . > Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
