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. I did not check how the "journal guided RAID resync" patches deal with the same issue, but they store all modified data block numbers in descriptor blocks, which is a lot more than just modified inodes. >> 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. > > 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
