Re: getdents - ext4 vs btrfs performance

[Ted Ts'o <tytso@mit.edu>]

On Wed, Mar 14, 2012 at 10:17:37AM -0400, Zach Brown wrote:
> 
> >We could do this if we have two b-trees, one indexed by filename and
> >one indexed by inode number, which is what JFS (and I believe btrfs)
> >does.
> 
> Typically the inode number of the destination inode isn't used to index
> entries for a readdir tree because of (wait for it) hard links.  You end
> up right back where you started with multiple entries per key.

Well, if you are using 32-bit (or even 48-bit) inode numbers and a
64-bit telldir cookie, it's possible to make the right thing happen.
But yes, if you are using 32-bit inode numbers and a 32-bit telldir
cookie, dealing with what happens when you have multiple hard links to
the same inode in the same directory gets tricky.

> A painful solution is to have the key in the readdir tree allocated by
> the tree itself -- count key populations in subtrees per child pointer
> and use that to find free keys.

One thing that might work is to have a 16-bit extra field in the
directory entry that gives an signed offset to the inode number so
that such that inode+offset is a unique value within the btree sorted
by inode+offset number.  Since this tree is only used for returning
entries in an optimal (or as close to optimal as can be arranged)
order, we could get away with that.

				- Ted