btrfs-progs: docs: enhance manual page for balance

Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/btrfs-balance.asciidoc

@@ -3,7 +3,7 @@ btrfs-balance(8)
 
 NAME
 ----
-btrfs-balance - balance btrfs filesystem
+btrfs-balance - balance block groups on a btrfs filesystem
 
 SYNOPSIS
 --------
@@ -11,10 +11,37 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-*btrfs balance* is used to balance chunks in a btrfs filesystem across
+The primary purpose of the balance feature is to spread block groups accross
-multiple or even single device.
+all devices so they match constraints defined by the respective profiles. See
+`mkfs.btrfs`(8) section 'PROFILES' for more details.
+The scope of the balancing process can be further tuned by use of filters that
+can select the block groups to process. Balance works only on a mounted
+filesystem.
 
-See `btrfs-device`(8) for more details about the effect on device management.
+The balance operation is cancellable by the user. The on-disk state of the
+filesystem is always consistent so an unexpected interruption (eg. system crash,
+reboot) does not corrupt the filesystem. The progress of the balance operation
+is temporarily stored and will be resumed upon mount, unless the mount option
+'skip_balance' is specified.
+
+WARNING: running balance without filters will take a lot of time as it basically
+rewrites the entire filesystem and needs to update all block pointers.
+
+The filters can be used to perform following actions:
+
+- convert block group profiles (filter 'convert')
+- make block group usage more compact  (filter 'usage')
+- perform actions only on a given device (filters 'devid', 'drange')
+
+The filters can be applied to a combination of block group types (data,
+metadata, system). Note that changing 'system' needs the force option.
+
+NOTE: the balance operation needs enough work space, ie. space that is
+completely unused in the filesystem, otherwise this may lead to ENOSPC reports.
+See the section 'ENOSPC' for more details.
+
+COMPATIBILITY
+-------------
 
 NOTE: The balance subcommand also exists under the *filesystem* namespace. This
 still works for backward compatibility but is deprecated and should not be
@@ -27,36 +54,31 @@ command instead.
 SUBCOMMAND
 ----------
 *cancel* <path>::
-Cancel running or paused balance.
+cancel running or paused balance
 
 *pause* <path>::
-Pause running balance.
+pause running balance operation, this will store the state of the balance
+progress and used filters to the filesystem
 
 *resume* <path>::
-Resume interrupted balance.
+resume interrupted balance
 
 *start* [options] <path>::
-Balance chunks across the devices *online*.
+start the balance operation according to the specified filters, no filters
-+
+will rewrite the entire filesystem. The process runs in the foreground.
-Balance and/or convert (change allocation profile of) chunks that
-passed all filters in a comma-separated list of filters for a
-particular chunk type.
-If filter list is not given balance all chunks of that type.
-In case none of the -d, -m or -s options is
-given balance all chunks in a filesystem.
 +
 `Options`
 +
 -d[<filters>]::::
-act on data chunks. See `FILTERS` section for details about <filters>.
+act on data block groups, see `FILTERS` section for details about 'filters'
 -m[<filters>]::::
-act on metadata chunks. See `FILTERS` section for details about <filters>.
+act on metadata chunks, see `FILTERS` section for details about 'filters'
 -s[<filters>]::::
-act on system chunks (only under -f). See `FILTERS` section for details about <filters>.
+act on system chunks (requires '-f'), see `FILTERS` section for details about 'filters'.
 -v::::
-be verbose
+be verbose and print balance filter arguments
 -f::::
-force reducing of metadata integrity
+force reducing of metadata integrity, eg. when going from 'raid1' to 'single'
 
 *status* [-v] <path>::
 Show status of running or paused balance.
@@ -67,61 +89,92 @@ FILTERS
 -------
 From kernel 3.3 onwards, btrfs balance can limit its action to a subset of the
 full filesystem, and can be used to change the replication configuration (e.g.
-moving data from single to RAID-1). This functionality is accessed through the
+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.
 
-A filter has the following stucture: ::
+A filter has the following stucture: 'type'[='params'][,'type'=...]
-'type'[='params'][,'type'=...]
 
 The available types are:
 
-*profiles*::
+*profiles=<profiles>*::
-Balances only block groups with the given replication profiles. Parameters
+Balances only block groups with the given profiles. Parameters
-are a list of profile names separated by |.
+are a list of profile names separated by "'|'" (pipe).
 
-*usage*::
+*usage=<percent>*::
 Balances only block groups with usage under the given percentage. The
 value of 0 is allowed and will clean up completely unused block groups, this
-should not require any new space allocated. You may want to use usage=0 in
+should not require any new space allocated. You may want to use 'usage=0' in
 case balance is returnin ENOSPC and your filesystem is not too full.
 
-*devid*::
+*devid=<id>*::
 Balances only block groups which have at least one chunk on the given
-device (by btrfs device ID -- use btrfs fi show to list device IDs)
+device. To list devices with ids use 'btrfs fi show'.
 
-*drange*::
+*drange=<range>*::
 Balances 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
+device.(Use in conjunction with 'devid' to filter on a specific device. The
-parameter is a range specified as <start..end>.
+parameter is a range specified as 'start..end'.
 
-*vrange*::
+*vrange=<range>*::
 Balances 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*::
+*convert=<profile>*::
 Convert each selected block group to the given profile name identified by
 parameters.
 
-*limit*::
+*limit=<number>*::
-Process only given number of chunks, after all filters apply. This can be used
+Process only given number of chunks, after all filters are applied. This can be
-to specifically target a chunk in connection with other filters (drange,
+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.
 
 *soft*::
 Takes no parameters. Only has meaning when converting between profiles.
 When doing convert from one profile to another and soft mode is on,
-restriper won't touch chunks that already have the target profile. This is
+chunks that alread yave the target profile are left untouched
-useful if e.g. half of the FS was converted earlier.
+ This is useful if e.g. half of the filesystem was converted earlier.
 +
-The soft mode switch is (like every other filter) per-type. This means
+The soft mode switch is (like every other filter) per-type.
-that we can convert for example meta chunks the "hard" way while converting
+For example, this means that we can convert metadata chunks the "hard" way
-data chunks selectively with soft switch.
+while converting data chunks selectively with soft switch.
 
 Profile names, used in profiles and convert are one of: 'raid0', 'raid1',
-'raid10', 'raid5', 'raid6', 'dup', 'single'.
+'raid10', 'raid5', 'raid6', 'dup', 'single'. The mixed data/metadata profiles
+can be converted in the same bay, but it's conversion between mixed and non-mixed
+is not implemented.
+
+ENOSPC
+------
+
+The way balance operates, it usually needs to temporarily create a new block
+group and move the old data there. For that it needs work space, otherwise
+it fails for ENOSPC reasons.
+This is not the same ENOSPC as if the free space is exhausted. This refers to
+the space on the level of block groups.
+
+The free work space can be calculated from the output of the 'btrfs filesystem show'
+command:
+
+   Label: 'BTRFS'  uuid: 8a9d72cd-ead3-469d-b371-9c7203276265
+	   Total devices 2 FS bytes used 77.03GiB
+	   devid    1 size 53.90GiB used 51.90GiB path /dev/sdc2
+	   devid    2 size 53.90GiB used 51.90GiB path /dev/sde1
+
+'size' - 'used' = 'free work space' +
+'53.90GiB' - '51.90GiB' = '2.00GiB'
+
+An example of a filter that does not require workspace is 'usage=0'. This will
+scan through all unused block groups of a given type and will reclaim the
+space. Ater that it might be possible to run other filters.
+
+**CONVERSIONS ON MULTIPLE DEVICES**
+
+Conversion to profiles based on striping (RAID0, RAID5/6) require the work
+space on each device. An interrupted balance may leave partially filled block
+groups that might consume the work space.
 
 EXIT STATUS
 -----------