btrfs-progs: docs: add balance filter examples

Add more examples and explanations how the filters can be used.

Pull-request: #486
Author: Eideen
Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/Balance.rst

@@ -7,3 +7,8 @@ Filters
 -------
 
 .. include:: ch-balance-filters.rst
+
+Examples
+--------
+
+.. include:: ch-balance-examples.rst

Documentation/ch-balance-examples.rst

@@ -0,0 +1,104 @@
+Adding new device
+^^^^^^^^^^^^^^^^^
+
+The unallocated space requirements depend on the selected storage
+profiles.  The requirements for the storage profile must be met for the
+selected for both data and metadata (e.g. if you have single data and
+RAID1 metadata, the stricter RAID1 requirements must be met or the
+filesystem may run out of metadata space and go read-only).
+
+Before adding a drive, make sure there is enough unallocated space on
+existing drives to create new metadata block groups (for filesystems
+over 50GB, this is `1GB * (number_of_devices + 2))`.
+
+If using a striped profile (`raid0`, `raid10`, `raid5`, or `raid6`), then do a
+full data balance of all data after adding a drive.  If adding multiple
+drives at once, do a full data balance after adding the last one.
+
+.. code-block:: bash
+
+        btrfs balance start -v --full-balance mnt/
+
+If the balance is interrupted, it can be restarted using the *stripes*
+filter (i.e. `-dstripes=1..N` where *N* is the previous size of the array
+before the new device was added) as long as all devices are the same size.
+If the device sizes are different, a specialized userspace balance tool
+is required.  The data balance must be completed before adding any new
+devices or increasing the size of existing ones.
+
+.. code-block:: bash
+
+        # For going from 4 disk to 5 disks, in Raid 5
+        btrfs balance start -v -dstripes=1..4 mnt/
+
+If you are not using a striped profile now, but intend to convert to a
+striped profile in the future, always perform a full data balance after
+adding drives or replacing existing drives with larger ones.  The stock
+*btrfs balance* tool cannot cope with special cases on filesystems with
+striped raid profiles, and will paint itself into a corner that will
+require custom userspace balancing tools to recover if you try.
+
+To watch one can use the following:
+
+.. code-block:: bash
+
+        watch "btrfs filesystem usage -T mnt/; btrfs balance status mnt/"
+
+Convert RAID1 after mkfs with defaults
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If you forgot to set the block group profile when creating the volume, run the
+following command:
+
+.. code-block:: bash
+
+        btrfs balance start -v convert=raid1,soft mnt/
+
+This will convert all remaining profiles that are not yet *raid1*.
+
+Convert data to RAID10 with RAID1C4 for metadata
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If you a have multi device setup, or you'd like to have different profiles on a
+single disk, e.g. *RAID10* for data and *RAID1C4* for metadata and system:
+
+.. code-block:: bash
+
+        btrfs balance start -v -mconvert=raid1C4,soft -dconvert=raid10,soft mnt/
+
+Compact under used chunks
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If the data chunks are not balanced and used only partially, the *usage* filter
+can be used to make them more compact:
+
+.. code-block:: bash
+
+        btrfs balance start -v -dusage=10 mnt/
+
+If the percent starts from a small number, like 5 or 10, the chunks will be
+processed relatively quickly and will make more space available. Increasing the
+percentage can then make more chunks compact by relocating the data.
+
+Chunks utilized up to 50% can be relocated to other chunks while still freeing
+the space. With utilization higher than 50% the chunks will be basically only
+moved on the devices. The actual chunk layout may help to coalesce the free
+space but this is a secondary effect.
+
+.. code-block:: bash
+
+        for USAGE in {10..50..10} do
+            btrfs balance start -v -dusage=$USAGE mnt/
+        done
+
+Fix incomplete balance
+^^^^^^^^^^^^^^^^^^^^^^
+
+If the balance is interrupted (due to reboot or cancelled) during conversion to
+RAID1. The following command will skip all RAID1 chunks that have been already
+converted and continue with what's left to convert. Note that an interrupted
+conversion may leave the last chunk under utilized.
+
+.. code-block:: bash
+
+        btrfs balance start convert=raid1,soft mnt/

Documentation/ch-balance-filters.rst

@@ -1,16 +1,49 @@
-From kernel 3.3 onwards, btrfs balance can limit its action to a subset of the
+From kernel 3.3 onwards, BTRFS balance can limit its action to a subset of the
 whole filesystem, and can be used to change the replication configuration (e.g.
-moving data from single to RAID1). This functionality is accessed through the
-*-d*, *-m* or *-s* options to btrfs balance start, which filter on data,
-metadata and system blocks respectively.
+convert data from ``single`` to ``RAID1``).
 
-A filter has the following structure: *type[=params][,type=...]*
+Balance can be limited to a block group profile with the following options:
 
-The available types are:
+* ``-d`` for data block groups
+* ``-m`` for metadata block groups (also implicitly applies to *-s*)
+* ``-s`` for system block groups
+
+The options have an optional parameter which means that the parameter must start
+right after the option without a space (this is mandatory getopt syntax), like
+``-dusage=10``. Options for all block group types can be specified in one command.
+
+A filter has the following structure: ``filter[=params][,filter=...]``
+
+To combine multiple filters use ``,``, without spaces. Example: ``-dconvert=raid1,soft``
+
+BTRFS can have different profiles on a single device or the same profile on
+multiple device.
+
+The main reason why you want to have different profiles for data and metadata
+is to provide additional protection of the filesystem's metadata when devices fail,
+since a single sector of unrecoverable metadata will break the filesystem,
+while a single sector of lost data can be trivially recovered by deleting the broken file.
+
+Before changing profiles, make sure there is enough unallocated space on
+existing drives to create new metadata block groups (for filesystems
+over 50GiB, this is ``1GB * (number_of_devices + 2))``.
+
+Default profiles on BTRFS are:
+
+* data: ``single``
+* metadata:
+        * single devices: ``dup``
+        * multiple devices: ``raid1``
+
+
+The available filter types are:
+
+Filter types
+^^^^^^^^^^^^
 
 profiles=<profiles>
         Balances only block groups with the given profiles. Parameters
-        are a list of profile names separated by "*|*" (pipe).
+        are a list of profile names separated by ``|`` (pipe).
 
 usage=<percent>, usage=<range>
         Balances only block groups with usage under the given percentage. The
@@ -18,8 +51,8 @@ usage=<percent>, usage=<range>
         should not require any new work space allocated. You may want to use *usage=0*
         in case balance is returning ENOSPC and your filesystem is not too full.
 
-        The argument may be a single value or a range. The single value *N* means *at
-        most N percent used*, equivalent to *..N* range syntax. Kernels prior to 4.4
+        The argument may be a single value or a range. The single value ``N`` means *at
+        most N percent used*, equivalent to ``..N`` range syntax. Kernels prior to 4.4
         accept only the single value format.
         The minimum range boundary is inclusive, maximum is exclusive.
 
@@ -29,44 +62,44 @@ devid=<id>
 
 drange=<range>
         Balance only block groups which overlap with the given byte range on any
-        device. Use in conjunction with *devid* to filter on a specific device. The
-        parameter is a range specified as *start..end*.
+        device. Use in conjunction with ``devid`` to filter on a specific device. The
+        parameter is a range specified as ``start..end``.
 
 vrange=<range>
         Balance only block groups which overlap with the given byte range in the
         filesystem's internal virtual address space. This is the address space that
         most reports from btrfs in the kernel log use. The parameter is a range
-        specified as *start..end*.
+        specified as ``start..end``.
 
 convert=<profile>
         Convert each selected block group to the given profile name identified by
         parameters.
 
         .. note::
-                Starting with kernel 4.5, the *data* chunks can be converted to/from the
-                *DUP* profile on a single device.
+                Starting with kernel 4.5, the ``data`` chunks can be converted to/from the
+                ``DUP`` profile on a single device.
 
         .. note::
-                Starting with kernel 4.6, all profiles can be converted to/from *DUP* on
+                Starting with kernel 4.6, all profiles can be converted to/from ``DUP`` on
                 multi-device filesystems.
 
 limit=<number>, limit=<range>
         Process only given number of chunks, after all filters are applied. This can be
-        used to specifically target a chunk in connection with other filters (*drange*,
-        *vrange*) or just simply limit the amount of work done by a single balance run.
+        used to specifically target a chunk in connection with other filters (``drange``,
+        ``vrange``) or just simply limit the amount of work done by a single balance run.
 
-        The argument may be a single value or a range. The single value *N* means *at
-        most N chunks*, equivalent to *..N* range syntax. Kernels prior to 4.4 accept
+        The argument may be a single value or a range. The single value ``N`` means *at
+        most N chunks*, equivalent to ``..N`` range syntax. Kernels prior to 4.4 accept
         only the single value format.  The range minimum and maximum are inclusive.
 
 stripes=<range>
         Balance only block groups which have the given number of stripes. The parameter
-        is a range specified as *start..end*. Makes sense for block group profiles that
+        is a range specified as ``start..end``. Makes sense for block group profiles that
         utilize striping, i.e. RAID0/10/5/6.  The range minimum and maximum are
         inclusive.
 
 soft
-        Takes no parameters. Only has meaning when converting between profiles.
+        Takes no parameters. Only has meaning when converting between profiles, or
         When doing convert from one profile to another and soft mode is on,
         chunks that already have the target profile are left untouched.
         This is useful e.g. when half of the filesystem was converted earlier but got
@@ -76,9 +109,19 @@ soft
         For example, this means that we can convert metadata chunks the "hard" way
         while converting data chunks selectively with soft switch.
 
-Profile names, used in *profiles* and *convert* are one of: *raid0*, *raid1*,
-*raid1c3*, *raid1c4*, *raid10*, *raid5*, *raid6*, *dup*, *single*.  The mixed
-data/metadata profiles can be converted in the same way, but it's conversion
+Profile names, used in ``profiles`` and ``convert`` are one of:
+
+* ``raid0``
+* ``raid1``
+* ``raid1c3``
+* ``raid1c4``
+* ``raid10``
+* ``raid5``
+* ``raid6``
+* ``dup``
+* ``single``
+
+The mixed data/metadata profiles can be converted in the same way, but conversion
 between mixed and non-mixed is not implemented. For the constraints of the
 profiles please refer to :doc:`mkfs.btrfs(8)<mkfs.btrfs>` section
 :ref:`PROFILES<man-mkfs-profiles>`.