zonefs: Add documentation

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@wdc.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

Documentation/filesystems/zonefs.txt

+ZoneFS - Zone filesystem for Zoned block devices
+
+Introduction
+============
+
+zonefs is a very simple file system exposing each zone of a zoned block device
+as a file. Unlike a regular POSIX-compliant file system with native 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 device 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.
+
+Zoned block devices
+-------------------
+
+Zoned storage devices belong to a class of storage devices with an address
+space that is divided into zones. A zone is a group of consecutive LBAs and all
+zones are contiguous (there are no LBA gaps). Zones may have different types.
+* Conventional zones: there are no access constraints to LBAs belonging to
+  conventional zones. Any read or write access can be executed, similarly to a
+  regular block device.
+* Sequential zones: these zones accept random reads but must be written
+  sequentially. Each sequential zone has a write pointer maintained by the
+  device that keeps track of the mandatory start LBA position of the next write
+  to the device. As a result of this write constraint, LBAs in a sequential zone
+  cannot be overwritten. Sequential zones must first be erased using a special
+  command (zone reset) before rewriting.
+
+Zoned storage devices can be implemented using various recording and media
+technologies. The most common form of zoned storage today uses the SCSI Zoned
+Block Commands (ZBC) and Zoned ATA Commands (ZAC) interfaces on Shingled
+Magnetic Recording (SMR) HDDs.
+
+Solid State Disks (SSD) storage devices can also implement a zoned interface
+to, for instance, reduce internal write amplification due to garbage collection.
+The NVMe Zoned NameSpace (ZNS) is a technical proposal of the NVMe standard
+committee aiming at adding a zoned storage interface to the NVMe protocol.
+
+Zonefs Overview
+===============
+
+Zonefs exposes the zones of a zoned block device as files. The files
+representing zones are grouped by zone type, which are themselves represented
+by sub-directories. This file structure is built entirely using zone information
+provided by the device and so does not require any complex on-disk metadata
+structure.
+
+On-disk metadata
+----------------
+
+zonefs on-disk metadata is reduced to an immutable super block which
+persistently stores a magic number and optional feature 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 written 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, it transitions the zone to a full
+state to make it read-only, 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 if and only if the device has usable 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 the 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, indicates 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 type of I/O operation:
+buffered I/Os, direct I/Os, memory mapped I/Os (mmap), etc. There are no I/O
+constraint for these files beyond the file size limit mentioned above.
+
+Sequential zone files
+---------------------
+
+The size of sequential zone files grouped in the "seq" sub-directory represents
+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 file, or to the end of the last
+write issued and still in-flight (for asynchrnous I/O operations).
+
+Since dirty page writeback by the page cache does not guarantee a sequential
+write pattern, zonefs prevents buffered writes and writeable shared mappings
+on sequential files. Only direct I/O writes are accepted for these files.
+zonefs relies on the sequential delivery of write I/O requests to the device
+implemented by the block layer elevator. An elevator implementing the sequential
+write feature for zoned block device (ELEVATOR_F_ZBD_SEQ_WRITE elevator feature)
+must be used. This type of elevator (e.g. mq-deadline) is the set by default
+for zoned block devices on device initialization.
+
+There are no restrictions on the type of I/O used for read operations in
+sequential zone files. Buffered I/Os, direct I/Os and shared read mappings are
+all accepted.
+
+Truncating sequential zone files is allowed only down to 0, in which 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).
+
+Format options
+--------------
+
+Several optional features of zonefs can be enabled at format time.
+* Conventional zone aggregation: ranges of contiguous conventional zones can be
+  aggregated 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.
+
+IO error handling
+-----------------
+
+Zoned block devices may fail I/O requests for reasons similar to regular block
+devices, e.g. due to bad sectors. However, in addition to such known I/O
+failure pattern, the standards governing zoned block devices behavior define
+additional conditions that result in I/O errors.
+
+* A zone may transition to the read-only condition (BLK_ZONE_COND_READONLY):
+  While the data already written in the zone is still readable, the zone can
+  no longer be written. No user action on the zone (zone management command or
+  read/write access) can change the zone condition back to a normal read/write
+  state. While the reasons for the device to transition a zone to read-only
+  state are not defined by the standards, a typical cause for such transition
+  would be a defective write head on an HDD (all zones under this head are
+  changed to read-only).
+
+* A zone may transition to the offline condition (BLK_ZONE_COND_OFFLINE):
+  An offline zone cannot be read nor written. No user action can transition an
+  offline zone back to an operational good state. Similarly to zone read-only
+  transitions, the reasons for a drive to transition a zone to the offline
+  condition are undefined. A typical cause would be a defective read-write head
+  on an HDD causing all zones on the platter under the broken head to be
+  inaccessible.
+
+* Unaligned write errors: These errors result from the host issuing write
+  requests with a start sector that does not correspond to a zone write pointer
+  position when the write request is executed by the device. Even though zonefs
+  enforces sequential file write for sequential zones, unaligned write errors
+  may still happen in the case of a partial failure of a very large direct I/O
+  operation split into multiple BIOs/requests or asynchronous I/O operations.
+  If one of the write request within the set of sequential write requests
+  issued to the device fails, all write requests after queued after it will
+  become unaligned and fail.
+
+* Delayed write errors: similarly to regular block devices, if the device side
+  write cache is enabled, write errors may occur in ranges of previously
+  completed writes when the device write cache is flushed, e.g. on fsync().
+  Similarly to the previous immediate unaligned write error case, delayed write
+  errors can propagate through a stream of cached sequential data for a zone
+  causing all data to be dropped after the sector that caused the error.
+
+All I/O errors detected by zonefs are notified to the user with an error code
+return for the system call that trigered or detected the error. The recovery
+actions taken by zonefs in response to I/O errors depend on the I/O type (read
+vs write) and on the reason for the error (bad sector, unaligned writes or zone
+condition change).
+
+* For read I/O errors, zonefs does not execute any particular recovery action,
+  but only if the file zone is still in a good condition and there is no
+  inconsistency between the file inode size and its zone write pointer position.
+  If a problem is detected, I/O error recovery is executed (see below table).
+
+* For write I/O errors, zonefs I/O error recovery is always executed.
+
+* A zone condition change to read-only or offline also always triggers zonefs
+  I/O error recovery.
+
+Zonefs minimal I/O error recovery may change a file size and a file access
+permissions.
+
+* File size changes:
+  Immediate or delayed write errors in a sequential zone file may cause the file
+  inode size to be inconsistent with the amount of data successfully written in
+  the file zone. For instance, the partial failure of a multi-BIO large write
+  operation will cause the zone write pointer to advance partially, even though
+  the entire write operation will be reported as failed to the user. In such
+  case, the file inode size must be advanced to reflect the zone write pointer
+  change and eventually allow the user to restart writing at the end of the
+  file.
+  A file size may also be reduced to reflect a delayed write error detected on
+  fsync(): in this case, the amount of data effectively written in the zone may
+  be less than originally indicated by the file inode size. After such I/O
+  error, zonefs always fixes a file inode size to reflect the amount of data
+  persistently stored in the file zone.
+
+* Access permission changes:
+  A zone condition change to read-only is indicated with a change in the file
+  access permissions to render the file read-only. This disables changes to the
+  file attributes and data modification. For offline zones, all permissions
+  (read and write) to the file are disabled.
+
+Further action taken by zonefs I/O error recovery can be controlled by the user
+with the "errors=xxx" mount option. The table below summarizes the result of
+zonefs I/O error processing depending on the mount option and on the zone
+conditions.
+
+    +--------------+-----------+-----------------------------------------+
+    |              |           |            Post error state             |
+    | "errors=xxx" |  device   |                 access permissions      |
+    |    mount     |   zone    | file         file          device zone  |
+    |    option    | condition | size     read    write    read    write |
+    +--------------+-----------+-----------------------------------------+
+    |              | good      | fixed    yes     no       yes     yes   |
+    | remount-ro   | read-only | fixed    yes     no       yes     no    |
+    | (default)    | offline   |   0      no      no       no      no    |
+    +--------------+-----------+-----------------------------------------+
+    |              | good      | fixed    yes     no       yes     yes   |
+    | zone-ro      | read-only | fixed    yes     no       yes     no    |
+    |              | offline   |   0      no      no       no      no    |
+    +--------------+-----------+-----------------------------------------+
+    |              | good      |   0      no      no       yes     yes   |
+    | zone-offline | read-only |   0      no      no       yes     no    |
+    |              | offline   |   0      no      no       no      no    |
+    +--------------+-----------+-----------------------------------------+
+    |              | good      | fixed    yes     yes      yes     yes   |
+    | repair       | read-only | fixed    yes     no       yes     no    |
+    |              | offline   |   0      no      no       no      no    |
+    +--------------+-----------+-----------------------------------------+
+
+Further notes:
+* The "errors=remount-ro" mount option is the default behavior of zonefs I/O
+  error processing if no errors mount option is specified.
+* With the "errors=remount-ro" mount option, the change of the file access
+  permissions to read-only applies to all files. The file system is remounted
+  read-only.
+* Access permission and file size changes due to the device transitioning zones
+  to the offline condition are permanent. Remounting or reformating the device
+  with mkfs.zonefs (mkzonefs) will not change back offline zone files to a good
+  state.
+* File access permission changes to read-only due to the device transitioning
+  zones to the read-only condition are permanent. Remounting or reformating
+  the device will not re-enable file write access.
+* File access permission changes implied by the remount-ro, zone-ro and
+  zone-offline mount options are temporary for zones in a good condition.
+  Unmounting and remounting the file system will restore the previous default
+  (format time values) access rights to the files affected.
+* The repair mount option triggers only the minimal set of I/O error recovery
+  actions, that is, file size fixes for zones in a good condition. Zones
+  indicated as being read-only or offline by the device still imply changes to
+  the zone file access permissions as noted in the table above.
+
+Mount options
+-------------
+
+zonefs define the "errors=<behavior>" mount option to allow the user to specify
+zonefs behavior in response to I/O errors, inode size inconsistencies or zone
+condition chages. The defined behaviors are as follow:
+* remount-ro (default)
+* zone-ro
+* zone-offline
+* repair
+
+The I/O error actions defined for each behavior is detailed in the previous
+section.
+
+Zonefs User Space Tools
+=======================
+
+The mkzonefs tool is used to format zoned block devices for use with zonefs.
+This tool is available on Github at:
+
+https://github.com/damien-lemoal/zonefs-tools
+
+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 aggregated 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, 0.00044121 s, 9.3 MB/s
+
+# ls -l /mnt/seq/0
+-rw-r----- 1 root root 4096 Nov 25 13:23 /mnt/seq/0
+
+The written file can be truncated to the zone size, preventing 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 I/O size for writes and corresponds to the device physical sector size.

MAINTAINERS

@@ -18311,6 +18311,7 @@ L:	linux-fsdevel@vger.kernel.org
 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@ieee.org>