On Sat, Dec 25, 2010 at 4:39 PM, Ted Ts'o <tytso@xxxxxxx> wrote: > On Sat, Dec 25, 2010 at 09:15:28PM +0800, Yongqiang Yang wrote: >> >> I found that if a block'allocation is delayed, and is not allocated >> when journal flushes it, then journal just redirties it and return in >> journal_submit_data_buffers. >> >> If I understand right, how to guarantee that the journal mode is ordered? > > The primary goal of ordered mode is to make sure that stale data is > not exposed after a crash. To the extent that delayed allocation also > achieves this goal, it's fine. The fact that ext3 forced data blocks > out as part of its jbd commit function was always an implementation > detail. Hi Ted, FYI, Yongqiang is researching move-on-write of extent mapped files data and opportunistic de-fragmentation on rewrite and this is the context of his question. As I mentioned to you on Plumbers the ordering requirements for move-on-write are a bit stronger than "not exposing un-initialized data". In the process of move-on-write, the extent must always map written data blocks and traditional 'ordered' mode can be used to guaranty that. Yongqiang, I believe in the case of delalloc move-on-rewrite, on write_begin(), you only need to reserve blocks (in-memory) and on writepages(), you do the actual move-on-write, modifying the extent from one valid data blocks range to another. > > In the long run we'll be getting rid of ordered mode even more so, by > writing the data block first, and only then updating the file system > metadata. At that point there will be no ordered flushing at all, and > in fact ordered mode will go away as a journal mode supported by ext4. > And that can simplify move-on-write implementation as well. But until that happens, snapshots will require 'ordered' mode. Amir. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
