btrfs-progs: Update README and other docs

[platform/upstream/btrfs-progs.git] / Documentation / btrfs-filesystem.asciidoc

diff --git a/Documentation/btrfs-filesystem.asciidoc b/Documentation/btrfs-filesystem.asciidoc
index ff8ff23..9782af9 100644 (file)
--- a/Documentation/btrfs-filesystem.asciidoc
+++ b/Documentation/btrfs-filesystem.asciidoc
@@ -11,9 +11,10 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-*btrfs filesystem* is used to do the whole filesystem level tasks, including
-all the regular filesystem operations like resizing, space stats, label
-setting/getting, and defragmentation.
+*btrfs filesystem* is used to perform several whole filesystem level tasks,
+including all the regular filesystem operations like resizing, space stats,
+label setting/getting, and defragmentation. There are other whole filesystem
+taks like scrub or balance that are grouped in separate commands.
 
 SUBCOMMAND
 ----------
@@ -97,6 +98,14 @@ snapshots or de-duplicated data).
 This may cause considerable increase of space usage depending on the broken up
 ref-links.
 +
+NOTE: Directory arguments without '-r' do not defragment files recursively but will
+defragment certain internal trees (extent tree and the subvolume tree). This has been
+confusing and could be removed in the future.
++
+For 'start', 'len', 'size' it is possible to append
+units designator: \'K', \'M', \'G', \'T', \'P', or \'E', which represent
+KiB, MiB, GiB, TiB, PiB, or EiB, respectively (case does not matter).
++
 `Options`
 +
 -v::::
@@ -108,23 +117,23 @@ compression. See also section 'EXAMPLES'.
 -r::::
 defragment files recursively in given directories
 -f::::
-flush data for each file before going to the next file. This will limit the amount
-of dirty data to current file, otherwise the amount cumulates from several files
-and may increase system load.
+flush data for each file before going to the next file.
++
+This will limit the amount of dirty data to current file, otherwise the amount
+cumulates from several files and will increase system load. This can also lead
+to ENOSPC if there's too much dirty data to write and it's not possible to make
+the reservations for the new data (ie. how the COW design works).
++
 -s <start>[kKmMgGtTpPeE]::::
 defragmentation will start from the given offset, default is beginning of a file
 -l <len>[kKmMgGtTpPeE]::::
 defragment only up to 'len' bytes, default is the file size
 -t <size>[kKmMgGtTpPeE]::::
-target extent size, do not touch extents bigger than 'size'
-+
-For 'start', 'len', 'size' it is possible to append
-units designator: \'K', \'M', \'G', \'T', \'P', or \'E', which represent
-KiB, MiB, GiB, TiB, PiB, or EiB, respectively (case does not matter).
+target extent size, do not touch extents bigger than 'size', default: 32M
 +
-NOTE: Directory arguments without '-r' do not defragment files recursively but will
-defragment certain internal trees (extent tree and the subvolume tree). This has been
-confusing and could be removed in the future.
+The value is only advisory and the final size of the extents may differ,
+depending on the state of the free space and fragmentation or other internal
+logic. Reasonable values are from tens to hundreds of megabytes.
 
 *du* [options] <path> [<path>..]::
 Calculate disk usage of the target files using FIEMAP. For individual