On Thu 14-04-11 10:12:26, Amir Goldstein wrote: > On Thu, Apr 14, 2011 at 12:39 AM, Jan Kara <jack@xxxxxxx> wrote: > > On Wed 13-04-11 21:16:40, Amir Goldstein wrote: > >> On Tue, Apr 12, 2011 at 5:48 PM, Jan Kara <jack@xxxxxxx> wrote: > >> > modification stamps have possibly larger race windows but I haven't really > >> > tried how much (I just know that even mtime races are not that hard to > >> > trigger if you try). So it really depends on how big reliability do you > >> > expect and I personally don't find much value in just rescanning and > >> > checking for mtime after a crash. Reading all the data and doing checksum > >> > certainly has more value but at a high cost. > >> > > >> > >> What do you thing about the approach to store recursively modified dir inodes in > >> a journal "modified inode descriptor block" and update the recursive mtime of > >> those dirs on journal recovery? > > The trouble is you don't know the number of directories that may need > > to have timestamp updated - you find that out only as you travel upwards. > > So it's hard to reserve any fixed space for this. > > > > True, but you can save *so* many inode numbers in just one descriptor > block and in case of an overflow, we can just pass a hint to the top > level application to do a full directory scan, so I hardly see that as a > big problem. Well, about 1000 but you can still have about 8000 inodes modified in a transaction for a standard 128 MB journal. You can notify the userspace when an overflow happens but the interface gets kind of ugly... Also it would be only specific to ext3/4 while I'd prefer to get a wider fs support. > >> I would also consider to use a mount option rec_mtime and then just > >> store recursive > >> mtime in the directory's inode mtime instead of an extended attribute. > >> That doesn't break any contract with user space, it's just a re-interpretation > >> of the dir modification notion. > > It breaks POSIX specification - POSIX pretty much specifies when mtime is > > supposed to be changed - so I'm not sure we really want to do that... > > I disagree, POSIX doesn't forbid a user space daemon from touching directory > inodes and updating their mtime. The rec_mtime feature should be treated as > a little kernel "daemon" which propagates information to user space by touching > recursively modified directories. OK, if you look at it this way it makes some sense. You loose the distinction whether something has been created / deleted in the directory or whether only something happened in its subdirectory or file but that does not seem too important for any use case I can think of. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- 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
