On Wed, Mar 12, 2014 at 11:52:48PM -0400, Theodore Ts'o wrote: > On Mon, Mar 10, 2014 at 11:54:49PM -0700, Darrick J. Wong wrote: > > If a directory's contents are stored entirely inside the inode, > > there's no index to rebuild and no dirblock checksum to recompute. > > As far as I know these are the only two reasons to call dir rehash. > > Well, actually, there is a third reason to rehash directories, and > that is to reorganize a directory to optimize out deleted entries that > are scattered in the middle of the directory. Ooh, I forgot about that. :/ > That being said, it's more critical for inline directories, since we > very much want to keep them from spilling over to an external block, > this process of compressing out deleted space is something that should > be done in real time as we operate on the directory, by the kernel, > and not just at fsck time. > > The only reason why we don't do this today is because if the directory > is open for scanning using opendir/readdir, if we reorganize a > directory block, it could end up corrupting the readdir --- and for > non-inline directories, it's much less important. > > What I think would might make sense is to have the kernel track > whether the directory has been opened for reading, and if it hasn't, > then it would be safe to try compressing all of the directory entries > in the block so that the free space is in a single unused directory > entry at the end of the block. We could try doing this "dynamic > compression" of directory free space both at unlink(2) time, and also > when we try inserting a directory entry into the block and there is > apparently no space in the directory block. > > So I'm fine with skipping the rehashing of inline directories now, but > this is a future, relatively small, kernel project we might want to > think about for ext4. Probably we ought to fix up rehash.c to be able to compress directory entries too. The only reason I kicked them here was that somehow an inline data dir would end up on the rehash list, causing the block iteration to fail and e2fsck stops cold. --D > > Cheers, > > - Ted > -- > 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 -- 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
