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Mapping UUIDs to subvolume IDs is an operation with a high effort today. Today, the algorithm even has quadratic effort (based on the number of existing subvolumes), which means, that it takes minutes to send/receive a single subvolume if 10,000 subvolumes exist. But even linear effort would be too much since it is a waste. And these data structures to allow mapping UUIDs to subvolume IDs are created every time a btrfs send/receive instance is started. It is much more efficient to maintain a searchable persistent data structure in the filesystem, one that is updated whenever a subvolume/snapshot is created and deleted, and when the received subvolume UUID is set by the btrfs-receive tool. Therefore kernel code is added with this commit that is able to maintain data structures in the filesystem that allow to quickly search for a given UUID and to retrieve data that is assigned to this UUID, like which subvolume ID is related to this UUID. This commit adds a new tree to hold UUID-to-data mapping items. The key of the items is the full UUID plus the key type BTRFS_UUID_KEY. Multiple data blocks can be stored for a given UUID, a type/length/ value scheme is used. Now follows the lengthy justification, why a new tree was added instead of using the existing root tree: The first approach was to not create another tree that holds UUID items. Instead, the items should just go into the top root tree. Unfortunately this confused the algorithm to assign the objectid of subvolumes and snapshots. The reason is that btrfs_find_free_objectid() calls btrfs_find_highest_objectid() for the first created subvol or snapshot after mounting a filesystem, and this function simply searches for the largest used objectid in the root tree keys to pick the next objectid to assign. Of course, the UUID keys have always been the ones with the highest offset value, and the next assigned subvol ID was wastefully huge. To use any other existing tree did not look proper. To apply a workaround such as setting the objectid to zero in the UUID item key and to implement collision handling would either add limitations (in case of a btrfs_extend_item() approach to handle the collisions) or a lot of complexity and source code (in case a key would be looked up that is free of collisions). Adding new code that introduces limitations is not good, and adding code that is complex and lengthy for no good reason is also not good. That's the justification why a completely new tree was introduced. Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de> Signed-off-by: Josef Bacik <jbacik@fusionio.com> Signed-off-by: Chris Mason <chris.mason@fusionio.com>
18 lines
746 B
Makefile
18 lines
746 B
Makefile
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obj-$(CONFIG_BTRFS_FS) := btrfs.o
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btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
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file-item.o inode-item.o inode-map.o disk-io.o \
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transaction.o inode.o file.o tree-defrag.o \
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extent_map.o sysfs.o struct-funcs.o xattr.o ordered-data.o \
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extent_io.o volumes.o async-thread.o ioctl.o locking.o orphan.o \
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export.o tree-log.o free-space-cache.o zlib.o lzo.o \
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compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
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reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
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uuid-tree.o
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btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
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btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o
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btrfs-$(CONFIG_BTRFS_FS_RUN_SANITY_TESTS) += tests/free-space-tests.o
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