When looking for a clone source for an extent, we are iterating over all
the backreferences for an extent. This is often a waste of time, because
once we find a good clone source we could stop immediately instead of
continuing backref walking, which is expensive.
Basically what happens currently is this:
1) Call iterate_extent_inodes() to iterate over all the backreferences;
2) It calls btrfs_find_all_leafs() which in turn calls the main function
to walk over backrefs and collect them - find_parent_nodes();
3) Then we collect all the references for our target data extent from the
extent tree (and delayed refs if any), add them to the rb trees,
resolve all the indirect backreferences and search for all the file
extent items in fs trees, building a list of inodes for each one of
them (struct extent_inode_elem);
4) Then back at iterate_extent_inodes() we find all the roots associated
to each found leaf, and call the callback __iterate_backrefs defined
at send.c for each inode in the inode list associated to each leaf.
Some times one the first backreferences we find in a fs tree is optimal
to satisfy the clone operation that send wants to perform, and in that
case we could stop immediately and avoid resolving all the remaining
indirect backreferences (search fs trees for the respective file extent
items, etc). This possibly if when we find a fs tree leaf with a file
extent item we are able to know what are all the roots that can lead to
the leaf - this is now possible after the previous patch in the series
that adds a cache that maps leaves to a list of roots. So we can now
shortcircuit backref walking during send, by having the callback we
pass to iterate_extent_inodes() to be called when we find a file extent
item for an indirect backreference, and have it return a special value
when it found a suitable backreference and it does not need to look for
more backreferences. This change does that.
This change is part of a patchset comprised of the following patches:
01/17 btrfs: fix inode list leak during backref walking at resolve_indirect_refs()
02/17 btrfs: fix inode list leak during backref walking at find_parent_nodes()
03/17 btrfs: fix ulist leaks in error paths of qgroup self tests
04/17 btrfs: remove pointless and double ulist frees in error paths of qgroup tests
05/17 btrfs: send: avoid unnecessary path allocations when finding extent clone
06/17 btrfs: send: update comment at find_extent_clone()
07/17 btrfs: send: drop unnecessary backref context field initializations
08/17 btrfs: send: avoid unnecessary backref lookups when finding clone source
09/17 btrfs: send: optimize clone detection to increase extent sharing
10/17 btrfs: use a single argument for extent offset in backref walking functions
11/17 btrfs: use a structure to pass arguments to backref walking functions
12/17 btrfs: reuse roots ulist on each leaf iteration for iterate_extent_inodes()
13/17 btrfs: constify ulist parameter of ulist_next()
14/17 btrfs: send: cache leaf to roots mapping during backref walking
15/17 btrfs: send: skip unnecessary backref iterations
16/17 btrfs: send: avoid double extent tree search when finding clone source
17/17 btrfs: send: skip resolution of our own backref when finding clone source
Performance test results are in the changelog of patch 17/17.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
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