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Terms in italics also appear in this glossary.

Usually allocator means the block allocator, ie. the logic inside filesystem which decides where to place newly allocated blocks in order to maintain several constraints (like data locality, low fragmentation).
In btrfs, allocator may also refer to chunk allocator, ie. the logic behind placing chunks on devices.
An operation that can be done to a btrfs filesystem, for example through btrfs fi balance /path (see btrfs-progs). A balance passes all data in the filesystem through the allocator again. It is primarily intended to rebalance the data in the filesystem across the devices when a device is added or removed. A balance will regenerate missing copies for the redundant RAID levels, if a device has failed. As of linux kernel 3.3, a balance operation can be made selective about which parts of the filesystem are rewritten.
An instruction to the disk hardware to ensure that everything before the barrier is physically written to permanent storage before anything after it. Used in btrfs's copy on write approach to ensure filesystem consistency.
A single physically and logically contiguous piece of storage on a device, of size eg. 4K.
block group
The unit of allocation of space in btrfs. A block group is laid out on the disk by the btrfs allocator, and will consist of one or more chunks, each stored on a different device. The number of chunks used in a block group will depend on its RAID level.
The fundamental storage data structure used in btrfs. Except for the superblocks, all of btrfs metadata is stored in one of several B-trees on disk. B-trees store key/item pairs. While the same code is used to implement all of the B-trees, there are a few different categories of B-tree. For reference, see Btrees. The name "btrfs" refers to its use of B-trees.
Tool in btrfs-progs that checks a filesystem offline (ie. unmounted), and reports on any errors in the filesystem structures it finds. Does not (yet) fix errors by default. Recently it got support to fix certain types of corruption. See also scrub.
User mode tools to manage btrfs-specific features. Maintained at gitweb. The main frontend to btrfs features is the btrfs program, although other tools such as mkfs.btrfs and btrfsck are also part of btrfs-progs.
A part of a block group. Chunks are either 1 GiB in size (for data) or 256 MiB (for metadata).
chunk tree
A layer that keeps information about mapping between physical and logical block addresses. It's stored within the System group.
Usually referred to in context of deleted subvolumes. It's a background process that removes the actual data once a subvolume has been deleted. Cleaning can involve lots of IO and CPU activity depending on the fragmentation and amount of shared data with other subvolumes.
Also known as CoW. The method that btrfs uses for modifying data. Instead of directly overwriting data in place, btrfs takes a copy of the data, alters it, and then writes the modified data back to a different (free) location on the disk. It then updates the metadata to reflect the new location of the data. In order to update the metadata, the affected metadata blocks are also treated in the same way. In CoW filesystems, files tend to fragment as they are modified. Copy-on-write is also used in the implementation of snapshots and reflink copies. A copy-on-write filesystem is, in theory, always consistent, provided the underlying hardware supports barriers.
See copy-on-write.
default subvolume
The subvolume in a btrfs filesystem which is mounted when mounting the filesystem without using the subvol= mount option.
A Linux block device, e.g. a whole disk, partition, LVM logical volume, loopback device, or network block device. A btrfs filesystem can reside on one or more devices.
A standard Unix tool for reporting the amount of space used and free in a filesystem. The standard tool does not give accurate results, but the btrfs command from btrfs-progs has an implementation of df which shows space available in more detail. See the FAQ for a more detailed explanation of btrfs free space accounting.
A form of "RAID" which stores two copies of each piece of data on the same device. This is similar to RAID-1, and protects against block-level errors on the device, but does not provide any guarantees if the device fails. DUP is btrfs default RAID level for metadata on a single device. Regular data cannot be assigned DUP level.
Error code returned by the OS to a user program when the filesystem cannot allocate enough data to fulfill the user requested. In most filesystems, it indicates there is no free space available in the filesystem. Due to the additional space requirements from btrfs's CoW behaviour, btrfs can sometimes return ENOSPC when there is apparently (in terms of df) a large amount of space free. This is effectively a bug in btrfs, and (if it is repeatable), using the mount option enospc_debug may give a report that will help the btrfs developers. See the FAQ entry on free space.
Contiguous sequence of bytes on disk that holds file data.
A file stored on disk with 3 extents means that it consists of three fragments of contiguous bytes. See filefrag. A file in one extent would mean it is not fragmented.
Extent buffer
An abstraction to allow access to B-tree blocks larger than a page size.
Command line tool in util-linux, and a syscall, that reserves space in the filesystem for a file, without actually writing any file data to the filesystem. First data write will turn the preallocated extents into regular ones. See man 1 fallocate and man 2 fallocate for more details.
A tool to show the number of extents in a file, and hence the amount of fragmentation in the file. It is usually part of the e2fsprogs package on most Linux distributions. While initially developed for the ext2 filesystem, it works on Btrfs as well (but not really with compressed files). It uses the FIEMAP ioctl.
free space cache
Btrfs doesn't track free space, it only tracks allocated space. Free space is by definition any holes in the allocated space, but finding these holes is actually fairly I/O intensive. The free space cache stores a compressed representation of what is free. It is updated on every transaction commit.
On Unix and Unix-like operating systems (of which Linux is the latter), the fsync() system call causes all buffered file descriptor related data changes to be flushed to the underlying block device. When a file is modified on a modern operating system the changes are generally not written to the disk immediately but rather those changes are buffered in memory for reasons of performance, calling fsync() causes any in-memory changes to be written to disk.
An internal counter which updates for each transaction. When a metadata block is written (using copy on write), current generation is stored in the block, so that blocks which are too new (and hence possibly inconsistent) can be identified.
See generation.
A fixed sized tuple used to identify and sort items in a B-tree. The key is broken up into 3 parts: objectid, type, and offset. The type field indicates how each of the other two fields should be used, and what to expect to find in the item. For reference, see Btree Keys.
A variable sized structure stored in B-tree leaves. Items hold different types of data depending on key type. For reference, see Btree Items.
log tree
Data about data. In btrfs, this includes all of the internal data structures of the filesystem, including directory structures, filenames, file permissions, checksums, and the location of each file's extents. All btrfs metadata is stored in B-trees.
The tool (from btrfs-progs) to create a btrfs filesystem, see mkfs.btrfs.
A filesystem which is not mounted is offline. Some tools (e.g. btrfsck) will only work on offline filesystems. Compare online.
A filesystem which is mounted is online. Most btrfs tools will only work on online filesystems. Compare offline.
A class of different methods for writing some additional redundant data across multiple devices so that if one device fails, the missing data can be reconstructed from the remaining ones. See RAID-0, RAID-1, RAID-5, RAID-6, RAID-10, DUP and single. Traditional RAID methods operate across multiple devices of equal size, whereas btrfs's RAID implementation works inside block groups. See the Sysadmin's Guide for the details.
A form of RAID which provides no form of error recovery, but stripes a single copy of data across multiple devices for performance purposes. The stripe size is fixed to 64KB for now.
A form of RAID which stores two complete copies of each piece of data. Each copy is stored on a different device. btrfs requires a minimum of two devices to use RAID-1. This is the default for btrfs's metadata on more than one device.
A form of RAID which stripes a single copy of data across multiple devices, including one device's worth of additional parity data. Can be used to recover from a single device failure. Not yet implemented in btrfs.
A form of RAID which stripes a single copy of data across multiple devices, including two device's worth of additional parity data. Can be used to recover from the failure of two devices. Not yet implemented in btrfs.
A form of RAID which stores two complete copies of each piece of data, and also stripes each copy across multiple devices for performance.
Parameter to cp, allowing it to take advantage of the capabilities of CoW-capable filesystems. Allows for files to be copied and modified, with only the modifications taking up additional storage space. May be considered as snapshots on a single file rather than a subvolume. Example: cp --reflink file1 file2
The process of moving block groups within the filesystem while maintaining full filesystem integrity and consistency. This functionality is underlying balance and device removing features.
A development name for the rewritten balance code implemented in the v3.3 kernel. Allows to change RAID profiles of the filesystem, online.
An online filesystem checking tool. Reads all the data and metadata on the filesystem, and uses checksums and the duplicate copies from RAID storage to identify and repair any corrupt data.
seed device
A readonly device can be used as a filesystem seed or template (e.g. a CD-ROM containing an OS image). Read/write devices can be added to store modifications (using copy on write), changes to the writable devices are persistent across reboots. The original device remains unchanged and can be removed at any time (after Btrfs has been instructed to copy over all missing blocks). Multiple read/write file systems can be built from the same seed. See Seed-device for an example.
A "RAID" level in btrfs, storing a single copy of each piece of data. The default for data (as opposed to metadata) in btrfs.
A subvolume which is a copy on write copy of another subvolume. The two subvolumes share all of their common (unmodified) data, which means that snapshots can be used to keep the historical state of a filesystem very cheaply. After the snapshot is made, the original subvolume and the snapshot are of equal status: the original does not "own" the snapshot, and either one can be deleted without affecting the other one.
A tree of files and directories inside a btrfs that can be mounted as if it were an independent filesystem. A subvolume is created by taking a reference on the root of another subvolume. Each btrfs filesystem has at least one subvolume, the top-level subvolume, which contains everything else in the filesystem. Additional subvolumes can be created and deleted with the btrfs tool. All subvolumes share the same pool of free space in the filesystem. See also default subvolume.
The block on the disk, at a fixed known location and of fixed size, which contains pointers to the disk blocks containing all the other filesystem metadata structures. btrfs stores multiple copies of the superblock on each device in the filesystem at offsets 64 KiB, 64 MiB, 256 GiB, 1 TiB and PiB.
system array
Cryptic name of superblock metadata describing how to assemble a filesystem from multiple device. Prior to mount, the command btrfs dev scan has to be called, or all the devices have to be specified via mount option device=/dev/ice.
top-level subvolume
The subvolume at the very top of the filesystem. This is the only subvolume present in a newly-created btrfs filesystem, and internally has ID 5, otherwise could be referenced as 0 (eg. within the set-default subcommand of btrfs).
A consistent set of changes. To avoid generating very large amounts of disk activity, btrfs caches changes in RAM for up to 30 seconds (sometimes more often if the filesystem is running short on space or doing a lot of fsyncs), and then writes (commits) these changes out to disk in one go (using copy on write behaviour). This period of caching is called a transaction. Only one transaction is active on the filesystem at any one time.
An alternative term for genid. See generation.
Writeback in the context of the Linux kernel can be defined as the process of writing "dirty" memory from the page cache to the disk, when certain conditions are met (timeout, number of dirty pages over a ratio).
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