linux/Documentation/filesystems/zonefs.rst
<<
>>
Prefs
   1.. SPDX-License-Identifier: GPL-2.0
   2
   3================================================
   4ZoneFS - Zone filesystem for Zoned block devices
   5================================================
   6
   7Introduction
   8============
   9
  10zonefs is a very simple file system exposing each zone of a zoned block device
  11as a file. Unlike a regular POSIX-compliant file system with native zoned block
  12device support (e.g. f2fs), zonefs does not hide the sequential write
  13constraint of zoned block devices to the user. Files representing sequential
  14write zones of the device must be written sequentially starting from the end
  15of the file (append only writes).
  16
  17As such, zonefs is in essence closer to a raw block device access interface
  18than to a full-featured POSIX file system. The goal of zonefs is to simplify
  19the implementation of zoned block device support in applications by replacing
  20raw block device file accesses with a richer file API, avoiding relying on
  21direct block device file ioctls which may be more obscure to developers. One
  22example of this approach is the implementation of LSM (log-structured merge)
  23tree structures (such as used in RocksDB and LevelDB) on zoned block devices
  24by allowing SSTables to be stored in a zone file similarly to a regular file
  25system rather than as a range of sectors of the entire disk. The introduction
  26of the higher level construct "one file is one zone" can help reducing the
  27amount of changes needed in the application as well as introducing support for
  28different application programming languages.
  29
  30Zoned block devices
  31-------------------
  32
  33Zoned storage devices belong to a class of storage devices with an address
  34space that is divided into zones. A zone is a group of consecutive LBAs and all
  35zones are contiguous (there are no LBA gaps). Zones may have different types.
  36
  37* Conventional zones: there are no access constraints to LBAs belonging to
  38  conventional zones. Any read or write access can be executed, similarly to a
  39  regular block device.
  40* Sequential zones: these zones accept random reads but must be written
  41  sequentially. Each sequential zone has a write pointer maintained by the
  42  device that keeps track of the mandatory start LBA position of the next write
  43  to the device. As a result of this write constraint, LBAs in a sequential zone
  44  cannot be overwritten. Sequential zones must first be erased using a special
  45  command (zone reset) before rewriting.
  46
  47Zoned storage devices can be implemented using various recording and media
  48technologies. The most common form of zoned storage today uses the SCSI Zoned
  49Block Commands (ZBC) and Zoned ATA Commands (ZAC) interfaces on Shingled
  50Magnetic Recording (SMR) HDDs.
  51
  52Solid State Disks (SSD) storage devices can also implement a zoned interface
  53to, for instance, reduce internal write amplification due to garbage collection.
  54The NVMe Zoned NameSpace (ZNS) is a technical proposal of the NVMe standard
  55committee aiming at adding a zoned storage interface to the NVMe protocol.
  56
  57Zonefs Overview
  58===============
  59
  60Zonefs exposes the zones of a zoned block device as files. The files
  61representing zones are grouped by zone type, which are themselves represented
  62by sub-directories. This file structure is built entirely using zone information
  63provided by the device and so does not require any complex on-disk metadata
  64structure.
  65
  66On-disk metadata
  67----------------
  68
  69zonefs on-disk metadata is reduced to an immutable super block which
  70persistently stores a magic number and optional feature flags and values. On
  71mount, zonefs uses blkdev_report_zones() to obtain the device zone configuration
  72and populates the mount point with a static file tree solely based on this
  73information. File sizes come from the device zone type and write pointer
  74position managed by the device itself.
  75
  76The super block is always written on disk at sector 0. The first zone of the
  77device storing the super block is never exposed as a zone file by zonefs. If
  78the zone containing the super block is a sequential zone, the mkzonefs format
  79tool always "finishes" the zone, that is, it transitions the zone to a full
  80state to make it read-only, preventing any data write.
  81
  82Zone type sub-directories
  83-------------------------
  84
  85Files representing zones of the same type are grouped together under the same
  86sub-directory automatically created on mount.
  87
  88For conventional zones, the sub-directory "cnv" is used. This directory is
  89however created if and only if the device has usable conventional zones. If
  90the device only has a single conventional zone at sector 0, the zone will not
  91be exposed as a file as it will be used to store the zonefs super block. For
  92such devices, the "cnv" sub-directory will not be created.
  93
  94For sequential write zones, the sub-directory "seq" is used.
  95
  96These two directories are the only directories that exist in zonefs. Users
  97cannot create other directories and cannot rename nor delete the "cnv" and
  98"seq" sub-directories.
  99
 100The size of the directories indicated by the st_size field of struct stat,
 101obtained with the stat() or fstat() system calls, indicates the number of files
 102existing under the directory.
 103
 104Zone files
 105----------
 106
 107Zone files are named using the number of the zone they represent within the set
 108of zones of a particular type. That is, both the "cnv" and "seq" directories
 109contain files named "0", "1", "2", ... The file numbers also represent
 110increasing zone start sector on the device.
 111
 112All read and write operations to zone files are not allowed beyond the file
 113maximum size, that is, beyond the zone capacity. Any access exceeding the zone
 114capacity is failed with the -EFBIG error.
 115
 116Creating, deleting, renaming or modifying any attribute of files and
 117sub-directories is not allowed.
 118
 119The number of blocks of a file as reported by stat() and fstat() indicates the
 120capacity of the zone file, or in other words, the maximum file size.
 121
 122Conventional zone files
 123-----------------------
 124
 125The size of conventional zone files is fixed to the size of the zone they
 126represent. Conventional zone files cannot be truncated.
 127
 128These files can be randomly read and written using any type of I/O operation:
 129buffered I/Os, direct I/Os, memory mapped I/Os (mmap), etc. There are no I/O
 130constraint for these files beyond the file size limit mentioned above.
 131
 132Sequential zone files
 133---------------------
 134
 135The size of sequential zone files grouped in the "seq" sub-directory represents
 136the file's zone write pointer position relative to the zone start sector.
 137
 138Sequential zone files can only be written sequentially, starting from the file
 139end, that is, write operations can only be append writes. Zonefs makes no
 140attempt at accepting random writes and will fail any write request that has a
 141start offset not corresponding to the end of the file, or to the end of the last
 142write issued and still in-flight (for asynchronous I/O operations).
 143
 144Since dirty page writeback by the page cache does not guarantee a sequential
 145write pattern, zonefs prevents buffered writes and writeable shared mappings
 146on sequential files. Only direct I/O writes are accepted for these files.
 147zonefs relies on the sequential delivery of write I/O requests to the device
 148implemented by the block layer elevator. An elevator implementing the sequential
 149write feature for zoned block device (ELEVATOR_F_ZBD_SEQ_WRITE elevator feature)
 150must be used. This type of elevator (e.g. mq-deadline) is set by default
 151for zoned block devices on device initialization.
 152
 153There are no restrictions on the type of I/O used for read operations in
 154sequential zone files. Buffered I/Os, direct I/Os and shared read mappings are
 155all accepted.
 156
 157Truncating sequential zone files is allowed only down to 0, in which case, the
 158zone is reset to rewind the file zone write pointer position to the start of
 159the zone, or up to the zone capacity, in which case the file's zone is
 160transitioned to the FULL state (finish zone operation).
 161
 162Format options
 163--------------
 164
 165Several optional features of zonefs can be enabled at format time.
 166
 167* Conventional zone aggregation: ranges of contiguous conventional zones can be
 168  aggregated into a single larger file instead of the default one file per zone.
 169* File ownership: The owner UID and GID of zone files is by default 0 (root)
 170  but can be changed to any valid UID/GID.
 171* File access permissions: the default 640 access permissions can be changed.
 172
 173IO error handling
 174-----------------
 175
 176Zoned block devices may fail I/O requests for reasons similar to regular block
 177devices, e.g. due to bad sectors. However, in addition to such known I/O
 178failure pattern, the standards governing zoned block devices behavior define
 179additional conditions that result in I/O errors.
 180
 181* A zone may transition to the read-only condition (BLK_ZONE_COND_READONLY):
 182  While the data already written in the zone is still readable, the zone can
 183  no longer be written. No user action on the zone (zone management command or
 184  read/write access) can change the zone condition back to a normal read/write
 185  state. While the reasons for the device to transition a zone to read-only
 186  state are not defined by the standards, a typical cause for such transition
 187  would be a defective write head on an HDD (all zones under this head are
 188  changed to read-only).
 189
 190* A zone may transition to the offline condition (BLK_ZONE_COND_OFFLINE):
 191  An offline zone cannot be read nor written. No user action can transition an
 192  offline zone back to an operational good state. Similarly to zone read-only
 193  transitions, the reasons for a drive to transition a zone to the offline
 194  condition are undefined. A typical cause would be a defective read-write head
 195  on an HDD causing all zones on the platter under the broken head to be
 196  inaccessible.
 197
 198* Unaligned write errors: These errors result from the host issuing write
 199  requests with a start sector that does not correspond to a zone write pointer
 200  position when the write request is executed by the device. Even though zonefs
 201  enforces sequential file write for sequential zones, unaligned write errors
 202  may still happen in the case of a partial failure of a very large direct I/O
 203  operation split into multiple BIOs/requests or asynchronous I/O operations.
 204  If one of the write request within the set of sequential write requests
 205  issued to the device fails, all write requests queued after it will
 206  become unaligned and fail.
 207
 208* Delayed write errors: similarly to regular block devices, if the device side
 209  write cache is enabled, write errors may occur in ranges of previously
 210  completed writes when the device write cache is flushed, e.g. on fsync().
 211  Similarly to the previous immediate unaligned write error case, delayed write
 212  errors can propagate through a stream of cached sequential data for a zone
 213  causing all data to be dropped after the sector that caused the error.
 214
 215All I/O errors detected by zonefs are notified to the user with an error code
 216return for the system call that triggered or detected the error. The recovery
 217actions taken by zonefs in response to I/O errors depend on the I/O type (read
 218vs write) and on the reason for the error (bad sector, unaligned writes or zone
 219condition change).
 220
 221* For read I/O errors, zonefs does not execute any particular recovery action,
 222  but only if the file zone is still in a good condition and there is no
 223  inconsistency between the file inode size and its zone write pointer position.
 224  If a problem is detected, I/O error recovery is executed (see below table).
 225
 226* For write I/O errors, zonefs I/O error recovery is always executed.
 227
 228* A zone condition change to read-only or offline also always triggers zonefs
 229  I/O error recovery.
 230
 231Zonefs minimal I/O error recovery may change a file size and file access
 232permissions.
 233
 234* File size changes:
 235  Immediate or delayed write errors in a sequential zone file may cause the file
 236  inode size to be inconsistent with the amount of data successfully written in
 237  the file zone. For instance, the partial failure of a multi-BIO large write
 238  operation will cause the zone write pointer to advance partially, even though
 239  the entire write operation will be reported as failed to the user. In such
 240  case, the file inode size must be advanced to reflect the zone write pointer
 241  change and eventually allow the user to restart writing at the end of the
 242  file.
 243  A file size may also be reduced to reflect a delayed write error detected on
 244  fsync(): in this case, the amount of data effectively written in the zone may
 245  be less than originally indicated by the file inode size. After such I/O
 246  error, zonefs always fixes the file inode size to reflect the amount of data
 247  persistently stored in the file zone.
 248
 249* Access permission changes:
 250  A zone condition change to read-only is indicated with a change in the file
 251  access permissions to render the file read-only. This disables changes to the
 252  file attributes and data modification. For offline zones, all permissions
 253  (read and write) to the file are disabled.
 254
 255Further action taken by zonefs I/O error recovery can be controlled by the user
 256with the "errors=xxx" mount option. The table below summarizes the result of
 257zonefs I/O error processing depending on the mount option and on the zone
 258conditions::
 259
 260    +--------------+-----------+-----------------------------------------+
 261    |              |           |            Post error state             |
 262    | "errors=xxx" |  device   |                 access permissions      |
 263    |    mount     |   zone    | file         file          device zone  |
 264    |    option    | condition | size     read    write    read    write |
 265    +--------------+-----------+-----------------------------------------+
 266    |              | good      | fixed    yes     no       yes     yes   |
 267    | remount-ro   | read-only | as is    yes     no       yes     no    |
 268    | (default)    | offline   |   0      no      no       no      no    |
 269    +--------------+-----------+-----------------------------------------+
 270    |              | good      | fixed    yes     no       yes     yes   |
 271    | zone-ro      | read-only | as is    yes     no       yes     no    |
 272    |              | offline   |   0      no      no       no      no    |
 273    +--------------+-----------+-----------------------------------------+
 274    |              | good      |   0      no      no       yes     yes   |
 275    | zone-offline | read-only |   0      no      no       yes     no    |
 276    |              | offline   |   0      no      no       no      no    |
 277    +--------------+-----------+-----------------------------------------+
 278    |              | good      | fixed    yes     yes      yes     yes   |
 279    | repair       | read-only | as is    yes     no       yes     no    |
 280    |              | offline   |   0      no      no       no      no    |
 281    +--------------+-----------+-----------------------------------------+
 282
 283Further notes:
 284
 285* The "errors=remount-ro" mount option is the default behavior of zonefs I/O
 286  error processing if no errors mount option is specified.
 287* With the "errors=remount-ro" mount option, the change of the file access
 288  permissions to read-only applies to all files. The file system is remounted
 289  read-only.
 290* Access permission and file size changes due to the device transitioning zones
 291  to the offline condition are permanent. Remounting or reformatting the device
 292  with mkfs.zonefs (mkzonefs) will not change back offline zone files to a good
 293  state.
 294* File access permission changes to read-only due to the device transitioning
 295  zones to the read-only condition are permanent. Remounting or reformatting
 296  the device will not re-enable file write access.
 297* File access permission changes implied by the remount-ro, zone-ro and
 298  zone-offline mount options are temporary for zones in a good condition.
 299  Unmounting and remounting the file system will restore the previous default
 300  (format time values) access rights to the files affected.
 301* The repair mount option triggers only the minimal set of I/O error recovery
 302  actions, that is, file size fixes for zones in a good condition. Zones
 303  indicated as being read-only or offline by the device still imply changes to
 304  the zone file access permissions as noted in the table above.
 305
 306Mount options
 307-------------
 308
 309zonefs define the "errors=<behavior>" mount option to allow the user to specify
 310zonefs behavior in response to I/O errors, inode size inconsistencies or zone
 311condition changes. The defined behaviors are as follow:
 312
 313* remount-ro (default)
 314* zone-ro
 315* zone-offline
 316* repair
 317
 318The run-time I/O error actions defined for each behavior are detailed in the
 319previous section. Mount time I/O errors will cause the mount operation to fail.
 320The handling of read-only zones also differs between mount-time and run-time.
 321If a read-only zone is found at mount time, the zone is always treated in the
 322same manner as offline zones, that is, all accesses are disabled and the zone
 323file size set to 0. This is necessary as the write pointer of read-only zones
 324is defined as invalib by the ZBC and ZAC standards, making it impossible to
 325discover the amount of data that has been written to the zone. In the case of a
 326read-only zone discovered at run-time, as indicated in the previous section.
 327The size of the zone file is left unchanged from its last updated value.
 328
 329A zoned block device (e.g. an NVMe Zoned Namespace device) may have limits on
 330the number of zones that can be active, that is, zones that are in the
 331implicit open, explicit open or closed conditions.  This potential limitation
 332translates into a risk for applications to see write IO errors due to this
 333limit being exceeded if the zone of a file is not already active when a write
 334request is issued by the user.
 335
 336To avoid these potential errors, the "explicit-open" mount option forces zones
 337to be made active using an open zone command when a file is opened for writing
 338for the first time. If the zone open command succeeds, the application is then
 339guaranteed that write requests can be processed. Conversely, the
 340"explicit-open" mount option will result in a zone close command being issued
 341to the device on the last close() of a zone file if the zone is not full nor
 342empty.
 343
 344Zonefs User Space Tools
 345=======================
 346
 347The mkzonefs tool is used to format zoned block devices for use with zonefs.
 348This tool is available on Github at:
 349
 350https://github.com/damien-lemoal/zonefs-tools
 351
 352zonefs-tools also includes a test suite which can be run against any zoned
 353block device, including null_blk block device created with zoned mode.
 354
 355Examples
 356--------
 357
 358The following formats a 15TB host-managed SMR HDD with 256 MB zones
 359with the conventional zones aggregation feature enabled::
 360
 361    # mkzonefs -o aggr_cnv /dev/sdX
 362    # mount -t zonefs /dev/sdX /mnt
 363    # ls -l /mnt/
 364    total 0
 365    dr-xr-xr-x 2 root root     1 Nov 25 13:23 cnv
 366    dr-xr-xr-x 2 root root 55356 Nov 25 13:23 seq
 367
 368The size of the zone files sub-directories indicate the number of files
 369existing for each type of zones. In this example, there is only one
 370conventional zone file (all conventional zones are aggregated under a single
 371file)::
 372
 373    # ls -l /mnt/cnv
 374    total 137101312
 375    -rw-r----- 1 root root 140391743488 Nov 25 13:23 0
 376
 377This aggregated conventional zone file can be used as a regular file::
 378
 379    # mkfs.ext4 /mnt/cnv/0
 380    # mount -o loop /mnt/cnv/0 /data
 381
 382The "seq" sub-directory grouping files for sequential write zones has in this
 383example 55356 zones::
 384
 385    # ls -lv /mnt/seq
 386    total 14511243264
 387    -rw-r----- 1 root root 0 Nov 25 13:23 0
 388    -rw-r----- 1 root root 0 Nov 25 13:23 1
 389    -rw-r----- 1 root root 0 Nov 25 13:23 2
 390    ...
 391    -rw-r----- 1 root root 0 Nov 25 13:23 55354
 392    -rw-r----- 1 root root 0 Nov 25 13:23 55355
 393
 394For sequential write zone files, the file size changes as data is appended at
 395the end of the file, similarly to any regular file system::
 396
 397    # dd if=/dev/zero of=/mnt/seq/0 bs=4096 count=1 conv=notrunc oflag=direct
 398    1+0 records in
 399    1+0 records out
 400    4096 bytes (4.1 kB, 4.0 KiB) copied, 0.00044121 s, 9.3 MB/s
 401
 402    # ls -l /mnt/seq/0
 403    -rw-r----- 1 root root 4096 Nov 25 13:23 /mnt/seq/0
 404
 405The written file can be truncated to the zone size, preventing any further
 406write operation::
 407
 408    # truncate -s 268435456 /mnt/seq/0
 409    # ls -l /mnt/seq/0
 410    -rw-r----- 1 root root 268435456 Nov 25 13:49 /mnt/seq/0
 411
 412Truncation to 0 size allows freeing the file zone storage space and restart
 413append-writes to the file::
 414
 415    # truncate -s 0 /mnt/seq/0
 416    # ls -l /mnt/seq/0
 417    -rw-r----- 1 root root 0 Nov 25 13:49 /mnt/seq/0
 418
 419Since files are statically mapped to zones on the disk, the number of blocks
 420of a file as reported by stat() and fstat() indicates the capacity of the file
 421zone::
 422
 423    # stat /mnt/seq/0
 424    File: /mnt/seq/0
 425    Size: 0             Blocks: 524288     IO Block: 4096   regular empty file
 426    Device: 870h/2160d  Inode: 50431       Links: 1
 427    Access: (0640/-rw-r-----)  Uid: (    0/    root)   Gid: (    0/    root)
 428    Access: 2019-11-25 13:23:57.048971997 +0900
 429    Modify: 2019-11-25 13:52:25.553805765 +0900
 430    Change: 2019-11-25 13:52:25.553805765 +0900
 431    Birth: -
 432
 433The number of blocks of the file ("Blocks") in units of 512B blocks gives the
 434maximum file size of 524288 * 512 B = 256 MB, corresponding to the device zone
 435capacity in this example. Of note is that the "IO block" field always
 436indicates the minimum I/O size for writes and corresponds to the device
 437physical sector size.
 438