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btrfs-tree: introduce BtrfsDataExtentTreeFetcher to read data extents without metadata

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Binary searches can be extremely slow if the target bytenr is near a
metadata block group, because metadata items are not visible to the
binary search algorithm.  In a non-mixed-bg filesystem, there can be
hundreds of thousands of metadata items between data extent items, and
since the binary search algorithm can't see them, it will run searches
that iterate over hundreds of thousands of objects about a dozen times.

This is less of a problem for mixed-bg filesystems because the data and
metadata blocks are not isolated from each other.  The binary search
algorithm still can't see the metadata items, but there are usually
some data items close by to prevent the linear item filter from running
too long.

Introduce a new fetcher class (all the good names were taken) that tracks
where the end of the current block group is.  When the end of the current
block group is reached in the linear search, skip ahead to a block group
that can contain data items.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This commit is contained in:
Zygo Blaxell 2025-01-27 19:54:07 -05:00
parent c4ba6ec269
commit f9a697518d
2 changed files with 64 additions and 0 deletions
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10
include/crucible/btrfs-tree.h
View File

@ -203,6 +203,16 @@ namespace crucible {
BtrfsTreeItem root(uint64_t subvol); BtrfsTreeItem root(uint64_t subvol);
}; };
/// Fetch data extent items from extent tree, skipping metadata-only block groups
class BtrfsDataExtentTreeFetcher : public BtrfsExtentItemFetcher {
BtrfsTreeItem m_current_bg;
BtrfsTreeOffsetFetcher m_chunk_tree;
protected:
virtual void next_sk(BtrfsIoctlSearchKey &key, const BtrfsIoctlSearchHeader &hdr) override;
public:
BtrfsDataExtentTreeFetcher(const Fd &fd);
};
} }
#endif #endif

54
lib/btrfs-tree.cc
View File

@ -707,4 +707,58 @@ namespace crucible {
} }
return item; return item;
} }
BtrfsDataExtentTreeFetcher::BtrfsDataExtentTreeFetcher(const Fd &fd) :
BtrfsExtentItemFetcher(fd),
m_chunk_tree(fd)
{
tree(BTRFS_EXTENT_TREE_OBJECTID);
type(BTRFS_EXTENT_ITEM_KEY);
m_chunk_tree.tree(BTRFS_CHUNK_TREE_OBJECTID);
m_chunk_tree.type(BTRFS_CHUNK_ITEM_KEY);
m_chunk_tree.objectid(BTRFS_FIRST_CHUNK_TREE_OBJECTID);
}
void
BtrfsDataExtentTreeFetcher::next_sk(BtrfsIoctlSearchKey &key, const BtrfsIoctlSearchHeader &hdr)
{
key.min_type = key.max_type = type();
key.max_objectid = key.max_offset = numeric_limits<uint64_t>::max();
key.min_offset = 0;
key.min_objectid = hdr.objectid;
const auto step = scale_size();
if (key.min_objectid < numeric_limits<uint64_t>::max() - step) {
key.min_objectid += step;
} else {
key.min_objectid = numeric_limits<uint64_t>::max();
}
// If we're still in our current block group, check here
if (!!m_current_bg) {
const auto bg_begin = m_current_bg.offset();
const auto bg_end = bg_begin + m_current_bg.chunk_length();
// If we are still in our current block group, return early
if (key.min_objectid >= bg_begin && key.min_objectid < bg_end) return;
}
// We don't have a current block group or we're out of range
// Find the chunk that this bytenr belongs to
m_current_bg = m_chunk_tree.rlower_bound(key.min_objectid);
// Make sure it's a data block group
while (!!m_current_bg) {
// Data block group, stop here
if (m_current_bg.chunk_type() & BTRFS_BLOCK_GROUP_DATA) break;
// Not a data block group, skip to end
key.min_objectid = m_current_bg.offset() + m_current_bg.chunk_length();
m_current_bg = m_chunk_tree.lower_bound(key.min_objectid);
}
if (!m_current_bg) {
// Ran out of data block groups, stop here
return;
}
// Check to see if bytenr is in the current data block group
const auto bg_begin = m_current_bg.offset();
if (key.min_objectid < bg_begin) {
// Move forward to start of data block group
key.min_objectid = bg_begin;
}
}
} }