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author | Nicholas D Steeves <nsteeves@gmail.com> | 2016-04-23 00:41:30 +0100 |
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committer | Nicholas D Steeves <nsteeves@gmail.com> | 2016-04-23 00:41:30 +0100 |
commit | cec572daccafa1e912cbed363df6f84687778c6f (patch) | |
tree | 7d99ab9f73d25c1ed8eaf6393f6374edf5316b03 /Documentation/mkfs.btrfs.asciidoc |
btrfs-progs (4.4.1-1.1) unstable; urgency=medium
* Non-maintainer upload.
* New upstream release.
* Rename package to btrfs-progs (Closes: #780081)
* Update standards version to 3.9.7 (no changes needed).
* debian/control: Add "Breaks" per Gianfranco Costamagna's suggestion
* Change lintian override to reflect package rename
* Switch from using postinst and postrm to using triggers
per Christian Seiler's recommendation.
# imported from the archive
Diffstat (limited to 'Documentation/mkfs.btrfs.asciidoc')
-rw-r--r-- | Documentation/mkfs.btrfs.asciidoc | 341 |
1 files changed, 341 insertions, 0 deletions
diff --git a/Documentation/mkfs.btrfs.asciidoc b/Documentation/mkfs.btrfs.asciidoc new file mode 100644 index 00000000..6a492658 --- /dev/null +++ b/Documentation/mkfs.btrfs.asciidoc @@ -0,0 +1,341 @@ +mkfs.btrfs(8) +============= + +NAME +---- +mkfs.btrfs - create a btrfs filesystem + +SYNOPSIS +-------- +*mkfs.btrfs* +$$[-A|--alloc-start <alloc-start>]$$ +$$[-b|--byte-count <byte-count>]$$ +$$[-d|--data <data-profile>]$$ +$$[-m|--metadata <metadata profile>]$$ +$$[-M|--mixed]$$ +$$[-l|--leafsize <leafsize>]$$ +$$[-n|--nodesize <nodesize>]$$ +$$[-s|--sectorsize <sectorsize>]$$ +$$[-L|--label <label>]$$ +$$[-K|--nodiscard]$$ +$$[-r|--rootdir <rootdir>]$$ +$$[-O|--features <feature1>[,<feature2>...]]$$ +$$[-U|--uuid <UUID>]$$ +$$[-f|--force]$$ +$$[-q|--quiet]$$ +$$[--help]$$ +$$[-V|--version]$$ +$$<device> [<device>...]$$ + +DESCRIPTION +----------- +*mkfs.btrfs* is used to create the btrfs filesystem on a single or multiple +devices. <device> is typically a block device but can be a file-backed image +as well. Multiple devices are grouped by UUID of the filesystem. + +Before mounting such filesystem, the kernel module must know all the devices +either via preceding execution of *btrfs device scan* or using the *device* +mount option. See section *MULTIPLE DEVICES* for more details. + +OPTIONS +------- +*-A|--alloc-start <offset>*:: +(An option to help debugging chunk allocator.) +Specify the (physical) offset from the start of the device at which allocations +start. The default value is zero. + +*-b|--byte-count <size>*:: +Specify the size of the filesystem. If this option is not used, +mkfs.btrfs uses the entire device space for the filesystem. + +*-d|--data <profile>*:: +Specify the profile for the data block groups. Valid values are 'raid0', +'raid1', 'raid5', 'raid6', 'raid10' or 'single' or dup (case does not matter). ++ +See 'DUP PROFILES ON A SINGLE DEVICE' for more. + +*-m|--metadata <profile>*:: +Specify the profile for the metadata block groups. +Valid values are 'raid0', 'raid1', 'raid5', 'raid6', 'raid10', 'single' or +'dup', (case does not matter). ++ +A single device filesystem will default to 'DUP', unless a SSD is detected. Then +it will default to 'single'. The detection is based on the value of +`/sys/block/DEV/queue/rotational`, where 'DEV' is the short name of the device. ++ +Note that the rotational status can be arbitrarily set by the underlying block +device driver and may not reflect the true status (network block device, memory-backed +SCSI devices etc). Use the options '--data/--metadata' to avoid confusion. ++ +See 'DUP PROFILES ON A SINGLE DEVICE' for more details. + +*-M|--mixed*:: +Normally the data and metadata block groups are isolated. The 'mixed' mode +will remove the isolation and store both types in the same block group type. +This helps to utilize the free space regardless of the purpose and is suitable +for small devices. The separate allocation of block groups leads to a situation +where the space is reserved for the other block group type, is not available for +allocation and can lead to ENOSPC state. ++ +The recommended size for the mixed mode is for filesystems less than 1GiB. The +soft recommendation is to use it for filesystems smaller than 5GiB. The mixed +mode may lead to degraded performance on larger filesystems, but is otherwise +usable, even on multiple devices. ++ +The 'nodesize' and 'sectorsize' must be equal, and the block group types must +match. ++ +NOTE: versions up to 4.2.x forced the mixed mode for devices smaller than 1GiB. +This has been removed in 4.3+ as it caused some usability issues. + +*-l|--leafsize <size>*:: +Alias for --nodesize. Deprecated. + +*-n|--nodesize <size>*:: +Specify the nodesize, the tree block size in which btrfs stores metadata. The +default value is 16KiB (16384) or the page size, whichever is bigger. Must be a +multiple of the sectorsize, but not larger than 64KiB (65536). Leafsize always +equals nodesize and the options are aliases. ++ +Smaller node size increases fragmentation but lead to higher b-trees which in +turn leads to lower locking contention. Higher node sizes give better packing +and less fragmentation at the cost of more expensive memory operations while +updating the metadata blocks. ++ +NOTE: versions up to 3.11 set the nodesize to 4k. + +*-s|--sectorsize <size>*:: +Specify the sectorsize, the minimum data block allocation unit. ++ +The default value is the page size and is autodetected. If the sectorsize +differs from the page size, the created filesystem may not be mountable by the +kernel. Therefore it is not recommended to use this option unless you are going +to mount it on a system with the appropriate page size. + +*-L|--label <string>*:: +Specify a label for the filesystem. The 'string' should be less than 256 +bytes and must not contain newline characters. + +*-K|--nodiscard*:: +Do not perform whole device TRIM operation on devices that are capable of that. + +*-r|--rootdir <rootdir>*:: +Populate the toplevel subvolume with files from 'rootdir'. This does not +require root permissions and does not mount the filesystem. + +*-O|--features <feature1>[,<feature2>...]*:: +A list of filesystem features turned on at mkfs time. Not all features are +supported by old kernels. To disable a feature, prefix it with '^'. ++ +See section *FILESYSTEM FEATURES* for more details. To see all available +features that mkfs.btrfs supports run: ++ ++mkfs.btrfs -O list-all+ + +*-f|--force*:: +Forcibly overwrite the block devices when an existing filesystem is detected. +By default, mkfs.btrfs will utilize 'libblkid' to check for any known +filesystem on the devices. Alternatively you can use the `wipefs` utility +to clear the devices. + +*-q|--quiet*:: +Print only error or warning messages. Options --features or --help are unaffected. + +*-U|--uuid <UUID>*:: +Create the filesystem with the given 'UUID'. The UUID must not exist on any +filesystem currently present. + +*-V|--version*:: +Print the *mkfs.btrfs* version and exit. + +*--help*:: +Print help. + +SIZE UNITS +---------- +The default unit is 'byte'. All size parameters accept suffixes in the 1024 +base. The recognized suffixes are: 'k', 'm', 'g', 't', 'p', 'e', both uppercase +and lowercase. + +MULTIPLE DEVICES +---------------- + +Before mounting a multiple device filesystem, the kernel module must know the +association of the block devices that are attached to the filesystem UUID. + +There is typically no action needed from the user. On a system that utilizes a +udev-like daemon, any new block device is automatically registered. The rules +call *btrfs device scan*. + +The same command can be used to trigger the device scanning if the btrfs kernel +module is reloaded (naturally all previous information about the device +registration is lost). + +Another possibility is to use the mount options *device* to specify the list of +devices to scan at the time of mount. + + # mount -o device=/dev/sdb,device=/dev/sdc /dev/sda /mnt + +NOTE: that this means only scanning, if the devices do not exist in the system, +mount will fail anyway. This can happen on systems without initramfs/initrd and +root partition created with RAID1/10/5/6 profiles. The mount action can happen +before all block devices are discovered. The waiting is usually done on the +initramfs/initrd systems. + +FILESYSTEM FEATURES +------------------- + +*mixed-bg*:: +mixed data and metadata block groups, also set by option '--mixed' + +*extref*:: +(default since btrfs-progs 3.12, kernel support since 3.7) ++ +increased hardlink limit per file in a directory to 65536, older kernels +supported a varying number of hardlinks depending on the sum of all file name +sizes that can be stored into one metadata block + +*raid56*:: +extended format for RAID5/6, also enabled if raid5 or raid6 block groups +are selected + +*skinny-metadata*:: +(default since btrfs-progs 3.18, kernel support since 3.10) ++ +reduced-size metadata for extent references, saves a few percent of metadata + +*no-holes*:: +improved representation of file extents where holes are not explicitly +stored as an extent, saves a few percent of metadata if sparse files are used + +BLOCK GROUPS, CHUNKS, RAID +-------------------------- + +The highlevel organizational units of a filesystem are block groups of three types: +data, metadata and system. + +*DATA*:: +store data blocks and nothing else + +*METADATA*:: +store internal metadata in b-trees, can store file data if they fit into the +inline limit + +*SYSTEM*:: +store structures that describe the mapping between the physical devices and the +linear logical space representing the filesystem + +Other terms commonly used: + +*block group*:: +*chunk*:: +a logical range of space of a given profile, stores data, metadata or both; +sometimes the terms are used interchangably ++ +A typical size of metadata block group is 256MiB (filesystem smaller than +50GiB) and 1GiB (larger than 50GiB), for data it's 1GiB. The system block group +size is a few megabytes. + +*RAID*:: +a block group profile type that utilizes RAID-like features on multiple +devices: striping, mirroring, parity + +*profile*:: +when used in connection with block groups refers to the allocation strategy +and constraints, see the section 'PROFILES' for more details + +PROFILES +-------- + +There are the following block group types available: + +[ cols="^,^,^,^,^",width="60%" ] +|============================================================= +.2+^.<h| Profile 3+^.^h| Redundancy .2+^.<h| Min/max devices + ^.^h| Copies ^.^h| Parity ^.<h| Striping +| single | 1 | | | 1/any +| DUP | 2 / 1 device | | | 1/1 ^(see note)^ +| RAID0 | | | 1 to N | 2/any +| RAID1 | 2 | | | 2/any +| RAID10 | 2 | | 1 to N | 4/any +| RAID5 | 1 | 1 | 2 to N - 1 | 2/any +| RAID6 | 1 | 2 | 3 to N - 2 | 3/any +|============================================================= + +'Note:' DUP may exist on more than 1 device if it starts on a single device and +another one is added, but *mkfs.btrfs* will not let you create DUP on multiple +devices. + +DUP PROFILES ON A SINGLE DEVICE +------------------------------- + +The mkfs utility will let the user create a filesystem with profiles that write +the logical blocks to 2 physical locations. Whether there are really 2 +physical copies highly depends on the underlying device type. + +For example, a SSD drive can remap the blocks internally to a single copy thus +deduplicating them. This negates the purpose of increased redunancy and just +wastes space. + +The duplicated data/metadata may still be useful to statistically improve the +chances on a device that might perform some internal optimizations. The actual +details are not usually disclosed by vendors. As another example, the widely +used USB flash or SD cards use a translation layer. The data lifetime may +be affected by frequent plugging. The memory cells could get damaged, hopefully +not destroying both copies of particular data. + +The traditional rotational hard drives usually fail at the sector level. + +In any case, a device that starts to misbehave and repairs from the DUP copy +should be replaced! *DUP is not backup*. + +KNOWN ISSUES +------------ + +**SMALL FILESYSTEMS AND LARGE NODESIZE** + +The combination of small filesystem size and large nodesize is not recommended +in general and can lead to various ENOSPC-related issues during mount time or runtime. + +Since mixed block group creation is optional, we allow small +filesystem instances with differing values for 'sectorsize' and 'nodesize' +to be created and could end up in the following situation: + + # mkfs.btrfs -f -n 65536 /dev/loop0 + btrfs-progs v3.19-rc2-405-g976307c + See http://btrfs.wiki.kernel.org for more information. + + Performing full device TRIM (512.00MiB) ... + Label: (null) + UUID: 49fab72e-0c8b-466b-a3ca-d1bfe56475f0 + Node size: 65536 + Sector size: 4096 + Filesystem size: 512.00MiB + Block group profiles: + Data: single 8.00MiB + Metadata: DUP 40.00MiB + System: DUP 12.00MiB + SSD detected: no + Incompat features: extref, skinny-metadata + Number of devices: 1 + Devices: + ID SIZE PATH + 1 512.00MiB /dev/loop0 + + # mount /dev/loop0 /mnt/ + mount: mount /dev/loop0 on /mnt failed: No space left on device + +The ENOSPC occurs during the creation of the UUID tree. This is caused +by large metadata blocks and space reservation strategy that allocates more +than can fit into the filesystem. + + +AVAILABILITY +------------ +*mkfs.btrfs* is part of btrfs-progs. +Please refer to the btrfs wiki http://btrfs.wiki.kernel.org for +further details. + +SEE ALSO +-------- +`btrfs`(8), `wipefs`(8) |