Btrees B 树
Btrees Introduction Btrees 介绍
Btrfs uses a single set of btree manipulation code for all metadata in
the filesystem. For performance or organizational purposes, the trees
are broken up into a few different types, and each type of tree will
hold a few different types of keys. The super block holds pointers to
the tree roots of the tree of tree roots and the chunk tree.
Btrfs 使用一组单一的 btree 操作代码来处理文件系统中的所有元数据。出于性能或组织目的，树被分成几种不同类型，并且每种类型的树将包含几种不同类型的键。超级块保存指向树根的指针，包括树根树和块树。
Tree of Tree roots
树的树根
This tree is used for indexing and finding the root of most of the other
trees in the filesystem. It attaches names to subvolumes and snapshots,
and stores the location of the extent allocation tree root. It also
stores pointers to all of the subvolumes or snapshots that are being
deleted by the transaction code. This allows the deletion to pick up
where it left off after a crash.
这棵树用于索引和查找文件系统中大多数其他树的根。它将名称附加到子卷和快照，并存储范围分配树根的位置。它还存储由事务代码删除的所有子卷或快照的指针。这使得删除可以在崩溃后从上次中断的地方继续进行。
Chunk Tree 块树
The chunk tree does all of the logical to physical block address mapping
for the filesystem, and it stores information about all of the devices
in the FS. In order to bootstrap lookup in the chunk tree, the super
block also duplicates the chunk items needed to resolve blocks in the
chunk tree. Over time, the chunk tree will be split into multiple roots
to allow access of larger storage pools.
块树为文件系统执行逻辑到物理块地址映射，并存储有关文件系统中所有设备的信息。为了在块树中引导查找，超级块还复制了解决块树中块的所需块项。随着时间的推移，块树将被分成多个根，以允许访问更大的存储池。
There are back references from the chunk items to the extent tree that
allocated them. Only a single extent tree can allocate extents out of a
given chunk.
分配它们的范围树中的块项存在反向引用。只有单个范围树可以从给定的块中分配范围。
Two types of key are stored in the chunk tree:
块树中存储两种类型的键：
DEV_ITEM (where the offset field is the internal devid), which contain information on all of the underlying block devices in the filesystem
DEV_ITEM（其中偏移字段是内部 devid），其中包含文件系统中所有底层块设备的信息CHUNK_ITEM (where the offset field is the start of the chunk as a virtual address), which maps a section of the virtual address space (a chunk) into physical storage.
CHUNK_ITEM（其中偏移字段是块的起始地址作为虚拟地址），将虚拟地址空间的一部分（块）映射到物理存储器。
Device Allocation Tree 设备分配树
The device allocation tree records which parts of each physical device
have been allocated into chunks. This is a relatively small tree that is
only updated as new chunks are allocated. It stores back references to
the chunk tree that allocated each physical extent on the device.
设备分配树记录了每个物理设备的哪些部分已经分配到块中。这是一个相对较小的树，只有在分配新块时才会更新。它存储了分配到设备上每个物理范围的块树的反向引用。
Extent Allocation Tree 范围分配树
The extent allocation tree records byte ranges that are in use,
maintains reference counts on each extent and records back references to
the tree or file that is using each extent. Logical block groups are
created inside the extent allocation tree, and these reference large
logical extents from the chunk tree.
范围分配树记录正在使用的字节范围，维护每个范围的引用计数，并记录对使用每个范围的树或文件的反向引用。逻辑块组被创建在范围分配树内部，并引用来自块树的大逻辑范围。
Each block group can only store a specific type of extent. This might
include metadata, or mirrored metadata, or striped data blocks etc.
每个块组只能存储特定类型的范围。这可能包括元数据，或镜像元数据，或条带数据块等。
Currently there is only one extent allocation tree shared by all the
other trees. This will change in order to scale better under load.
目前，所有其他树共享一个范围分配树。 为了更好地在负载下扩展，这将发生变化。
Keys for the extent tree use the start of the extent as the objectid. A BLOCK_GROUP_ITEM key will be followed by the EXTENT_ITEM keys for extents within that block group.
范围树的键使用范围的起始作为对象标识符。 BLOCK_GROUP_ITEM 键后面将跟随该块组内范围的 EXTENT_ITEM 键。
FS Trees 文件系统树
These store files and directories, and all of the normal metadata you
would expect to find in a filesystem. There is one root for each
subvolume or snapshot, but snapshots will share blocks between roots.
这些存储文件和目录，以及您在文件系统中期望找到的所有常规元数据。每个子卷或快照都有一个根目录，但快照将在根目录之间共享块。
Keys in FS trees always use the inode number of the filesystem object as the objectid.
FS 树中的键始终使用文件系统对象的 inode 号作为 objectid。
Each object will have one or more of:
每个对象将具有一个或多个：
Inode. I 节点。
Inode ref, indicating what name this object is known as, and in which directory.
I 节点引用，指示此对象被称为什么名称，以及在哪个目录中。For files, a set of extent information, indicating where on the filesystem this file’s data is.
对于文件，一组范围信息，指示文件数据在文件系统上的位置。For directories, two sequences of dir_items, one indexed by a hash of the object name, and one indexed by a unique sequential index number.
对于目录，有两个 dir_items 序列，一个按对象名称的哈希索引，另一个按唯一的顺序索引号。
Checksum Tree 校验和树
The checksum tree stores block checksums. Every 4k block of data stored
on disk has a checksum associated with it. The “offset” part of the keys
in the checksum tree indicates the start of the checksummed data on
disk. The value stored with the key is a sequence of (currently 4-byte)
checksums, for the 4k blocks starting at the offset.
校验和树存储块校验和。磁盘上存储的每个 4k 数据块都有一个与之关联的校验和。校验和树中键的“偏移”部分表示磁盘上校验数据的起始位置。与键一起存储的值是一个（目前为 4 字节）校验和序列，用于从偏移开始的 4k 块。