On Wed, May 5, 2010 at 12:42 AM, <tytso@xxxxxxx> wrote: > On Mon, May 03, 2010 at 12:47:58PM +0300, Amir G. wrote: >> >> Next3 project was introduced 2 weeks ago. I hope you've had a chance >> to visit the wiki page. > > I took a quick look, but to be honest, I've been swamped lately, and > with the merge window close at hand, it was something I was going to > put off for another 2-3 weeks. I didn't realize you were in need of > some immediate comments so that you could finalize the on-disk format. > Indeed, on-disk format changes, that is, the first patch in each of the e2fsprogs and Next3 patch series, is the most urgent review for the purpose of finalizing Next3 on-disk format. > So the high bit on that front is that it looks like at least some of > the fields used, bit positions grabbed, etc., overlap with those used > by ext4. I would like to explain a few things regarding on-disk format changes: 1. Whenever it was possible, I tried to grab fields from the end of the structs (i.e., super_block, s_features, i_flags) to stay as far away as possible from ongoing changes in Ext4. If you find it better, I could move these fields to a different location you assign them to. 2. I plea guilt as changed in grabbing i_flags & 0x00F00000 for snapshot file non-persistent status flags, which overlaps with recent Ext4 flags. However, I do not, store these flags on-disk, they are only used by lsattr -X, to display in-memory snapshot status along with the on-disk snapshot status stored in i_flags & 0x1F000000. 3. I plea guilt as changed in grabbing l_i_reserved1 for snapshot on-disk list (i_next_snapshot), which overlaps with Ext4 on-disk i_version. However, since i_version can take any arbitrary value, this doesn't "break" the Ext4 on-disk format. The following wiki section lists the on disk format changes in detail: http://sourceforge.net/apps/mediawiki/next3/index.php?title=Code_documentation#Reserved_fields_and_bits_in_on-disk_structures > Ext4 is where new development takes place in the ext2/3/4 > series. So enhancements such as Next3 will probably not be received > with great welcome into ext3. Yes, of course, I realize that. This is the reason I chose to introduce Next3 as a new f/s, which was branched from Ext3 and not as a new feature to Ext3. Unfortunately, merging Next3 snapshots feature into Ext4 is not an easy task, because extent mapped files break the design concepts of Next3 snapshots. > And as far as e2fsprogs (which handles > the ext2, ext3, and ext4 file systems) is concerned, I don't want to > deal with the complexity of certain fields that mean one thing for > ext4, and something else for Next3. > I was kind of expecting you to say that and I understand why that can be a problem for you. Let's try to address this issue in the code review and find alternative solutions. > I'll try to carve out time to look at the Next3 patches in greater > detail this week. > That would be great. Thanks, 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
