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btrfs-progs: docs: update Gloassary

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Bring up to date, reformat, remove some wiki references.

Signed-off-by: David Sterba <dsterba@suse.com>
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David Sterba 2022-05-31 00:14:15 +02:00
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Documentation/Glossary.rst
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@ -14,7 +14,7 @@ allocator
balance
An operation that can be done to a btrfs filesystem, for example
through <code>btrfs fi balance /path</code> (see *btrfs-progs*). A
through ``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
@ -43,22 +43,21 @@ B-tree
*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"
categories of B-tree. The name *btrfs*
refers to its use of B-trees.
btrfsck
Tool in *btrfs-progs* that checks a filesystem *offline* (ie.
unmounted), and reports on any errors in the filesystem structures it
finds. Does not ([[FAQ#When_will_Btrfs_have_a_fsck_like_tool.3F|yet]])
fix errors by default. Recently it got support to fix certain types of
corruption. See also *scrub*.
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://git.kernel.org/?p=linux/kernel/git/mason/btrfs-progs.git;a=summary|btrfs-progs
gitweb]. The main frontend to btrfs features is the
<code>[[Manpage/btrfs|btrfs]]</code> program, although other tools such
as *mkfs.btrfs* and *btrfsck* are also part of btrfs-progs.
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)
@ -66,7 +65,7 @@ chunk
chunk tree
A layer that keeps information about mapping between physical and
logical block addresses. It's stored within the *System* group.
logical block addresses. It's stored within the *system* group.
cleaner
Usually referred to in context of deleted subvolumes. It's a background
@ -84,7 +83,7 @@ copy-on-write
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
'always' consistent, provided the underlying hardware supports
*barriers*.
COW
@ -92,8 +91,7 @@ COW
default subvolume
The *subvolume* in a btrfs filesystem which is mounted when mounting
the filesystem without using the <code>subvol=</code> [[Mount
options|mount option]].
the filesystem without using the ``subvol=`` mount option.
device
A Linux block device, e.g. a whole disk, partition, LVM logical volume,
@ -103,8 +101,8 @@ device
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
<code>[[Manpage/btrfs|btrfs]]</code> command from *btrfs-progs* has
an implementation of df which shows space available in more detail. See
*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.
@ -113,10 +111,10 @@ 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*'
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.
*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
@ -126,7 +124,7 @@ ENOSPC
*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 <code>[[Mount options|enospc_debug]]</code> may give a report
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.
@ -151,25 +149,23 @@ 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 (but
[http://thread.gmane.org/gmane.comp.file-systems.ocfs2.devel/8894/focus=8902
not really with compressed files]). It uses the *FIEMAP* ioctl.
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 compressed representation of what is free. It is updated on
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 <code>fsync()</code> system call causes all buffered file
the ``lfsync()`` 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 <code>fsync()</code> causes any in-memory changes to be written
calling ``fsync()`` causes any in-memory changes to be written
to disk.
generation
@ -178,23 +174,21 @@ generation
generation is stored in the block, so that blocks which are too new
(and hence possibly inconsistent) can be identified.
genid
See *generation*.
Key
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. For
reference, see [[Btree Keys]].
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
item
A variable sized structure stored in B-tree leaves. Items hold
different types of data depending on key type. For reference, see
[[Btree Items]].
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
@ -203,8 +197,7 @@ metadata
*extents*. All btrfs metadata is stored in *B-trees*.
mkfs.btrfs
The tool (from *btrfs-progs*) to create a btrfs filesystem, see
[[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.
@ -215,7 +208,8 @@ online
work on online filesystems. Compare *offline*.
orphan
(file)
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
@ -224,8 +218,7 @@ RAID
*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 [[SysadminGuide#Data_usage_and_allocation|Sysadmin's
Guide]] for the details.
groups*.
RAID-0
A form of *RAID* which provides no form of error recovery, but
@ -241,14 +234,12 @@ RAID-1
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. Not yet
implemented in btrfs.
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. Not yet implemented
in btrfs.
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
@ -256,22 +247,17 @@ RAID-10
performance.
reflink
Parameter to <code>cp</code>, allowing it to take advantage of the
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: <code>cp --reflink file1 file2</code>
than a *subvolume*. Example: ``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.
restriper
A development name for the rewritten *balance* code implemented in the
v3.3 kernel. Allows to change RAID profiles of the filesystem,
*online*.
scrub
An *online* filesystem checking tool. Reads all the data and metadata
on the filesystem, and uses *checksums* and the duplicate copies from
@ -284,8 +270,7 @@ seed device
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.
systems can be built from the same seed.
single
A "*RAID*" level in btrfs, storing a single copy of each piece of data.
@ -340,7 +325,7 @@ transaction
filesystem at any one time.
transid
An alternative term for *genid*. See *generation*.
An alternative term for *generation*.
writeback
*Writeback* in the context of the Linux kernel can be defined as the