diff --git a/Documentation/Glossary.rst b/Documentation/Glossary.rst index a29537a3..3d897109 100644 --- a/Documentation/Glossary.rst +++ b/Documentation/Glossary.rst @@ -14,7 +14,7 @@ allocator balance An operation that can be done to a btrfs filesystem, for example - through btrfs fi balance /path (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 - [[Manpage/btrfs|btrfs]] 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 subvol= [[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 - [[Manpage/btrfs|btrfs]] 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 [[Mount options|enospc_debug]] 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 fsync() 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 fsync() 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 cp, 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: cp --reflink file1 file2 + 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