117 lines
4.5 KiB
ReStructuredText
117 lines
4.5 KiB
ReStructuredText
BTRFS filesystem can be created on top of single or multiple block devices.
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Devices can be then added, removed or replaced on demand. Data and metadata are
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organized in allocation profiles with various redundancy policies. There's some
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similarity with traditional RAID levels, but this could be confusing to users
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familiar with the traditional meaning. Due to the similarity, the RAID
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terminology is widely used in the documentation. See :doc:`mkfs.btrfs(8)<mkfs.btrfs>` for more
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details and the exact profile capabilities and constraints.
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The device management works on a mounted filesystem. Devices can be added,
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removed or replaced, by commands provided by :command:`btrfs device` and :command:`btrfs replace`.
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The profiles can be also changed, provided there's enough workspace to do the
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conversion, using the :command:`btrfs balance` command and namely the filter *convert*.
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Type
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The block group profile type is the main distinction of the information stored
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on the block device. User data are called *Data*, the internal data structures
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managed by filesystem are *Metadata* and *System*.
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Profile
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A profile describes an allocation policy based on the redundancy/replication
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constraints in connection with the number of devices. The profile applies to
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data and metadata block groups separately. E.g. *single*, *RAID1*.
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RAID level
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Where applicable, the level refers to a profile that matches constraints of the
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standard RAID levels. At the moment the supported ones are: RAID0, RAID1,
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RAID10, RAID5 and RAID6.
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Typical use cases
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-----------------
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Starting with a single-device filesystem
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Assume we've created a filesystem on a block device */dev/sda* with profile
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*single/single* (data/metadata), the device size is 50GiB and we've used the
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whole device for the filesystem. The mount point is */mnt*.
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The amount of data stored is 16GiB, metadata have allocated 2GiB.
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Add new device
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""""""""""""""
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We want to increase the total size of the filesystem and keep the profiles. The
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size of the new device */dev/sdb* is 100GiB.
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.. code-block:: bash
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$ btrfs device add /dev/sdb /mnt
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The amount of free data space increases by less than 100GiB, some space is
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allocated for metadata.
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Convert to RAID1
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""""""""""""""""
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Now we want to increase the redundancy level of both data and metadata, but
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we'll do that in steps. Note, that the device sizes are not equal and we'll use
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that to show the capabilities of split data/metadata and independent profiles.
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The constraint for RAID1 gives us at most 50GiB of usable space and exactly 2
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copies will be stored on the devices.
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First we'll convert the metadata. As the metadata occupy less than 50GiB and
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there's enough workspace for the conversion process, we can do:
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.. code-block:: bash
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$ btrfs balance start -mconvert=raid1 /mnt
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This operation can take a while, because all metadata have to be moved and all
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block pointers updated. Depending on the physical locations of the old and new
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blocks, the disk seeking is the key factor affecting performance.
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You'll note that the system block group has been also converted to RAID1, this
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normally happens as the system block group also holds metadata (the physical to
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logical mappings).
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What changed:
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* available data space decreased by 3GiB, usable roughly (50 - 3) + (100 - 3) = 144 GiB
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* metadata redundancy increased
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IOW, the unequal device sizes allow for combined space for data yet improved
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redundancy for metadata. If we decide to increase redundancy of data as well,
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we're going to lose 50GiB of the second device for obvious reasons.
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.. code-block:: bash
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$ btrfs balance start -dconvert=raid1 /mnt
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The balance process needs some workspace (i.e. a free device space without any
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data or metadata block groups) so the command could fail if there's too much
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data or the block groups occupy the whole first device.
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The device size of */dev/sdb* as seen by the filesystem remains unchanged, but
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the logical space from 50-100GiB will be unused.
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Remove device
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"""""""""""""
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Device removal must satisfy the profile constraints, otherwise the command
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fails. For example:
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.. code-block:: bash
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$ btrfs device remove /dev/sda /mnt
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ERROR: error removing device '/dev/sda': unable to go below two devices on raid1
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In order to remove a device, you need to convert the profile in this case:
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.. code-block:: bash
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$ btrfs balance start -mconvert=dup -dconvert=single /mnt
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$ btrfs device remove /dev/sda /mnt
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