btrfs-progs/Documentation/Glossary.rst

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Glossary
========
Terms in *italics* also appear in this glossary.
allocator
Usually *allocator* means the *block* allocator, i.e. 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, i.e. the
logic behind placing chunks on devices.
balance
An operation that can be done to a btrfs filesystem, for example
through :command:`btrfs fi balance /path`. 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.
barrier
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.
block
A single physically and logically contiguous piece of storage on a
device, of size e.g. 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.
B-tree
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. The name *btrfs*
refers to its use of B-trees.
btrfsck
Tool in *btrfs-progs* that checks a filesystem *offline* (i.e.
unmounted), and reports on any errors in the filesystem structures it
finds. By default the tool runs in read-only mode as fixing errors is
potentially dangerous. See also *scrub*.
btrfs-progs
User mode tools to manage btrfs-specific features. Maintained at
http://github.com/kdave/btrfs-progs.git . The main frontend to btrfs
features is the standalone tool *btrfs*, although
other tools such as *mkfs.btrfs* and *btrfstune* are also part of
btrfs-progs.
chunk
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.
cleaner
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.
copy-on-write
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*.
COW
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.
device
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.
df
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#Why_does_df_show_incorrect_free_space_for_my_RAID_volume.3F|FAQ]]
for a more detailed explanation of btrfs free space accounting.
DUP
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 entire device fails. By default, btrfs uses *DUP*
profile for metadata on filesystems with one rotational device,
*single* profile on filesystems with one non-rotational device, and
*RAID1* profile on filesystems with more than one device.
ENOSPC
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#if_your_device_is_large_.28.3E16GiB.29|FAQ entry]] on free space.
extent
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.
fallocate
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 *fallocate(1)* and *fallocate(2)* manual pages
for more details.
filefrag
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. 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 condensed representation of what is free. It is updated on
every *transaction* commit.
fsync
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.
generation
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.
key
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.
item
A variable sized structure stored in B-tree leaves. Items hold
different types of data depending on key type.
log tree
A b-tree that temporarily tracks ongoing metadata updates until a full
transaction commit is done. It's a performance optimization of
``fsync``. The log tracked in the tree are replayed if the filesystem
is not unmounted cleanly.
metadata
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*.
mkfs.btrfs
The tool (from *btrfs-progs*) to create a btrfs filesystem.
offline
A filesystem which is not mounted is offline. Some tools (e.g.
*btrfsck*) will only work on offline filesystems. Compare *online*.
online
A filesystem which is mounted is online. Most btrfs tools will only
work on online filesystems. Compare *offline*.
orphan
A file that's still in use (opened by a running process) but all
directory entries of that file have been removed.
RAID
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*.
RAID-0
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.
RAID-1
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.
RAID-5
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.
RAID-6
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.
RAID-10
A form of *RAID* which stores two complete copies of each piece of
data, and also stripes each copy across multiple devices for
performance.
reflink
Parameter to :command:`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: :command:`cp --reflink file1 file2`
relocation
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.
scrub
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.
single
A "*RAID*" level in btrfs, storing a single copy of each piece of data.
The default for data (as opposed to *metadata*) in btrfs. Single is
also default metadata profile for non-rotational (SSD, flash) devices.
snapshot
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.
subvolume
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*.
superblock
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 (e.g. within the *set-default* subcommand of
*btrfs*).
transaction
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 *fsync*s), 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.
transid
An alternative term for *generation*.
writeback
*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).