[PATCH v3 2/2] zonefs: Add documentation

[Damien Le Moal]

Add the new file Documentation/filesystems/zonefs.txt to document zonefs
principles and user-space tool usage.

Signed-off-by: Damien Le Moal <damien.lemoal@xxxxxxx>
Reviewed-by: Hannes Reinecke <hare@xxxxxxxx>
Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
---
 Documentation/filesystems/zonefs.txt | 215 +++++++++++++++++++++++++++
 MAINTAINERS                          |   1 +
 2 files changed, 216 insertions(+)
 create mode 100644 Documentation/filesystems/zonefs.txt

diff --git a/Documentation/filesystems/zonefs.txt b/Documentation/filesystems/zonefs.txt
new file mode 100644
index 000000000000..e88a4743bc64
--- /dev/null
+++ b/Documentation/filesystems/zonefs.txt
@@ -0,0 +1,215 @@
+ZoneFS - Zone filesystem for Zoned block devices
+
+Overview
+========
+
+zonefs is a very simple file system exposing each zone of a zoned block device
+as a file. Unlike a regular file system with zoned block device support (e.g.
+f2fs), zonefs does not hide the sequential write constraint of zoned block
+devices to the user. Files representing sequential write zones of the device
+must be written sequentially starting from the end of the file (append only
+writes).
+
+As such, zonefs is in essence closer to a raw block device access interface
+than to a full featured POSIX file system. The goal of zonefs is to simplify
+the implementation of zoned block devices support in applications by replacing
+raw block device file accesses with a richer file API, avoiding relying on
+direct block device file ioctls which may be more obscure to developers. One
+example of this approach is the implementation of LSM (log-structured merge)
+tree structures (such as used in RocksDB and LevelDB) on zoned block devices
+by allowing SSTables to be stored in a zone file similarly to a regular file
+system rather than as a range of sectors of the entire disk. The introduction
+of the higher level construct "one file is one zone" can help reducing the
+amount of changes needed in the application as well as introducing support for
+different application programming languages.
+
+zonefs on-disk metadata
+-----------------------
+
+zonefs on-disk metadata is reduced to an immutable super block which
+persistently stores a magic number and optional features flags and values. On
+mount, zonefs uses blkdev_report_zones() to obtain the device zone configuration
+and populates the mount point with a static file tree solely based on this
+information. File sizes come from the device zone type and write pointer
+position managed by the device itself.
+
+The super block is always writen on disk at sector 0. The first zone of the
+device storing the super block is never exposed as a zone file by zonefs. If the
+zone containing the super block is a sequential zone, the mkzonefs format tool
+always "finishes" the zone, that is, transition the zone to a full state to make
+it readonly, preventing any data write.
+
+Zone type sub-directories
+-------------------------
+
+Files representing zones of the same type are grouped together under the same
+sub-directory automatically created on mount.
+
+For conventional zones, the sub-directory "cnv" is used. This directory is
+however created only and only if the device has useable conventional zones. If
+the device only has a single conventional zone at sector 0, the zone will not
+be exposed as a file as it will be used to store zonefs super block. For such
+devices, the "cnv" sub-directory will not be created.
+
+For sequential write zones, the sub-directory "seq" is used.
+
+These two directories are the only directories that exist in zonefs. Users
+cannot create other directories and cannot rename nor delete the "cnv" and
+"seq" sub-directories.
+
+The size of the directories indicated by the st_size field of struct stat,
+obtained with the stat() or fstat() system calls, indicate the number of files
+existing under the directory.
+
+Zone files
+----------
+
+Zone files are named using the number of the zone they represent within the set
+of zones of a particular type. That is, both the "cnv" and "seq" directories
+contain files named "0", "1", "2", ... The file numbers also represent
+increasing zone start sector on the device.
+
+All read and write operations to zone files are not allowed beyond the file
+maximum size, that is, beyond the zone size. Any access exceeding the zone
+size is failed with the -EFBIG error.
+
+Creating, deleting, renaming or modifying any attribute of files and
+sub-directories is not allowed.
+
+The number of blocks of a file as reported by stat() and fstat() indicates the
+size of the file zone, or in other words, the maximum file size.
+
+Conventional zone files
+-----------------------
+
+The size of conventional zone files is fixed to the size of the zone they
+represent. Conventional zone files cannot be truncated.
+
+These files can be randomly read and written, using any form of IO operation:
+buffered IOs, direct IOs, memory mapped IOs (mmap) etc. There are no IO
+constraint for these files beyond the file size limit mentioned above.
+
+Sequential zone files
+---------------------
+
+The size of sequential zone files present in the "seq" sub-directory represent
+the file's zone write pointer position relative to the zone start sector.
+
+Sequential zone files can only be written sequentially, starting from the file
+end, that is, write operations can only be append writes. Zonefs makes no
+attempt at accepting random writes and will fail any write request that has a
+start offset not corresponding to the end of the last issued write.
+
+In order to give guarantees regarding write ordering, zonefs also prevents
+buffered writes and mmap writes for sequential files. Only direct IO writes are
+accepted. There are no restrictions on read operations nor on the type of IO
+used to request reads (buffered IOs, direct IOs and mmap reads are all
+accepted).
+
+Truncating sequential zone files is allowed only down to 0, in wich case, the
+zone is reset to rewind the file zone write pointer position to the start of
+the zone, or up to the zone size, in which case the file's zone is transitioned
+to the FULL state (finish zone operation).
+
+zonefs format options
+---------------------
+
+Several optional features of zonefs can be enabled at format time.
+* Conventional zone aggregation: ranges of contiguous conventional zones can be
+  agregated into a single larger file instead of the default one file per zone.
+* File ownership: The owner UID and GID of zone files is by default 0 (root)
+  but can be changed to any valid UID/GID.
+* File access permissions: the default 640 access permissions can be changed.
+
+User Space Tools
+----------------
+
+The mkzonefs tool is used to format zoned block devices for use with zonefs.
+This tool is available on Github at:
+
+git@xxxxxxxxxx:damien-lemoal/zonefs-tools.git.
+
+zonefs-tools also includes a test suite which can be run against any zoned
+block device, including null_blk block device created with zoned mode.
+
+Examples
+--------
+
+The following formats a 15TB host-managed SMR HDD with 256 MB zones
+with the conventional zones aggregation feature enabled.
+
+# mkzonefs -o aggr_cnv /dev/sdX
+# mount -t zonefs /dev/sdX /mnt
+# ls -l /mnt/
+total 0
+dr-xr-xr-x 2 root root     1 Nov 25 13:23 cnv
+dr-xr-xr-x 2 root root 55356 Nov 25 13:23 seq
+
+The size of the zone files sub-directories indicate the number of files
+existing for each type of zones. In this example, there is only one
+conventional zone file (all conventional zones are agreggated under a single
+file).
+
+# ls -l /mnt/cnv
+total 137101312
+-rw-r----- 1 root root 140391743488 Nov 25 13:23 0
+
+This aggregated conventional zone file can be used as a regular file.
+
+# mkfs.ext4 /mnt/cnv/0
+# mount -o loop /mnt/cnv/0 /data
+
+The "seq" sub-directory grouping files for sequential write zones has in this
+example 55356 zones.
+
+# ls -lv /mnt/seq
+total 14511243264
+-rw-r----- 1 root root 0 Nov 25 13:23 0
+-rw-r----- 1 root root 0 Nov 25 13:23 1
+-rw-r----- 1 root root 0 Nov 25 13:23 2
+...
+-rw-r----- 1 root root 0 Nov 25 13:23 55354
+-rw-r----- 1 root root 0 Nov 25 13:23 55355
+
+For sequential write zone files, the file size changes as data is appended at
+the end of the file, similarly to any regular file system.
+
+# dd if=/dev/zero of=/mnt/seq/0 bs=4096 count=1 conv=notrunc oflag=direct
+1+0 records in
+1+0 records out
+4096 bytes (4.1 kB, 4.0 KiB) copied, 1.05112 s, 3.9 kB/s
+
+# ls -l /mnt/seq/0
+-rw-r----- 1 root root 4096 Nov 25 13:23 /mnt/sdh/seq/0
+
+The written file can be truncated to the zone size, prventing any further write
+operation.
+
+# truncate -s 268435456 /mnt/seq/0
+# ls -l /mnt/seq/0
+-rw-r----- 1 root root 268435456 Nov 25 13:49 /mnt/seq/0
+
+Truncation to 0 size allows freeing the file zone storage space and restart
+append-writes to the file.
+
+# truncate -s 0 /mnt/seq/0
+# ls -l /mnt/seq/0
+-rw-r----- 1 root root 0 Nov 25 13:49 /mnt/seq/0
+
+Since files are statically mapped to zones on the disk, the number of blocks of
+a file as reported by stat() and fstat() indicates the size of the file zone.
+
+# stat /mnt/seq/0
+  File: /mnt/seq/0
+  Size: 0         	Blocks: 524288     IO Block: 4096   regular empty file
+Device: 870h/2160d	Inode: 50431       Links: 1
+Access: (0640/-rw-r-----)  Uid: (    0/    root)   Gid: (    0/    root)
+Access: 2019-11-25 13:23:57.048971997 +0900
+Modify: 2019-11-25 13:52:25.553805765 +0900
+Change: 2019-11-25 13:52:25.553805765 +0900
+ Birth: -
+
+The number of blocks of the file ("Blocks") in units of 512B blocks gives the
+maximum file size of 524288 * 512 B = 256 MB, corresponding to the device zone
+size in this example. Of note is that the "IO block" field always indicates the
+minimum IO size for writes and corresponds to the device physical sector size.
diff --git a/MAINTAINERS b/MAINTAINERS
index 8eb6f02a1efa..66f348fa90df 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -18292,6 +18292,7 @@ L:	linux-fsdevel@xxxxxxxxxxxxxxx
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/dlemoal/zonefs.git
 S:	Maintained
 F:	fs/zonefs/
+F:	Documentation/filesystems/zonefs.txt
 
 ZPOOL COMPRESSED PAGE STORAGE API
 M:	Dan Streetman <ddstreet@xxxxxxxx>
-- 
2.24.1