423 lines
20 KiB
ReStructuredText
423 lines
20 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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================================================
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ZoneFS - Zone filesystem for Zoned block devices
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================================================
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Introduction
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============
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zonefs is a very simple file system exposing each zone of a zoned block device
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as a file. Unlike a regular POSIX-compliant file system with native zoned block
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device support (e.g. f2fs), zonefs does not hide the sequential write
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constraint of zoned block devices to the user. Files representing sequential
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write zones of the device must be written sequentially starting from the end
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of the file (append only writes).
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As such, zonefs is in essence closer to a raw block device access interface
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than to a full-featured POSIX file system. The goal of zonefs is to simplify
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the implementation of zoned block device support in applications by replacing
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raw block device file accesses with a richer file API, avoiding relying on
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direct block device file ioctls which may be more obscure to developers. One
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example of this approach is the implementation of LSM (log-structured merge)
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tree structures (such as used in RocksDB and LevelDB) on zoned block devices
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by allowing SSTables to be stored in a zone file similarly to a regular file
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system rather than as a range of sectors of the entire disk. The introduction
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of the higher level construct "one file is one zone" can help reducing the
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amount of changes needed in the application as well as introducing support for
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different application programming languages.
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Zoned block devices
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-------------------
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Zoned storage devices belong to a class of storage devices with an address
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space that is divided into zones. A zone is a group of consecutive LBAs and all
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zones are contiguous (there are no LBA gaps). Zones may have different types.
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* Conventional zones: there are no access constraints to LBAs belonging to
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conventional zones. Any read or write access can be executed, similarly to a
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regular block device.
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* Sequential zones: these zones accept random reads but must be written
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sequentially. Each sequential zone has a write pointer maintained by the
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device that keeps track of the mandatory start LBA position of the next write
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to the device. As a result of this write constraint, LBAs in a sequential zone
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cannot be overwritten. Sequential zones must first be erased using a special
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command (zone reset) before rewriting.
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Zoned storage devices can be implemented using various recording and media
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technologies. The most common form of zoned storage today uses the SCSI Zoned
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Block Commands (ZBC) and Zoned ATA Commands (ZAC) interfaces on Shingled
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Magnetic Recording (SMR) HDDs.
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Solid State Disks (SSD) storage devices can also implement a zoned interface
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to, for instance, reduce internal write amplification due to garbage collection.
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The NVMe Zoned NameSpace (ZNS) is a technical proposal of the NVMe standard
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committee aiming at adding a zoned storage interface to the NVMe protocol.
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Zonefs Overview
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===============
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Zonefs exposes the zones of a zoned block device as files. The files
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representing zones are grouped by zone type, which are themselves represented
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by sub-directories. This file structure is built entirely using zone information
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provided by the device and so does not require any complex on-disk metadata
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structure.
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On-disk metadata
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----------------
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zonefs on-disk metadata is reduced to an immutable super block which
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persistently stores a magic number and optional feature flags and values. On
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mount, zonefs uses blkdev_report_zones() to obtain the device zone configuration
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and populates the mount point with a static file tree solely based on this
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information. File sizes come from the device zone type and write pointer
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position managed by the device itself.
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The super block is always written on disk at sector 0. The first zone of the
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device storing the super block is never exposed as a zone file by zonefs. If
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the zone containing the super block is a sequential zone, the mkzonefs format
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tool always "finishes" the zone, that is, it transitions the zone to a full
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state to make it read-only, preventing any data write.
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Zone type sub-directories
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-------------------------
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Files representing zones of the same type are grouped together under the same
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sub-directory automatically created on mount.
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For conventional zones, the sub-directory "cnv" is used. This directory is
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however created if and only if the device has usable conventional zones. If
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the device only has a single conventional zone at sector 0, the zone will not
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be exposed as a file as it will be used to store the zonefs super block. For
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such devices, the "cnv" sub-directory will not be created.
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For sequential write zones, the sub-directory "seq" is used.
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These two directories are the only directories that exist in zonefs. Users
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cannot create other directories and cannot rename nor delete the "cnv" and
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"seq" sub-directories.
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The size of the directories indicated by the st_size field of struct stat,
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obtained with the stat() or fstat() system calls, indicates the number of files
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existing under the directory.
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Zone files
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----------
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Zone files are named using the number of the zone they represent within the set
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of zones of a particular type. That is, both the "cnv" and "seq" directories
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contain files named "0", "1", "2", ... The file numbers also represent
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increasing zone start sector on the device.
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All read and write operations to zone files are not allowed beyond the file
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maximum size, that is, beyond the zone capacity. Any access exceeding the zone
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capacity is failed with the -EFBIG error.
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Creating, deleting, renaming or modifying any attribute of files and
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sub-directories is not allowed.
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The number of blocks of a file as reported by stat() and fstat() indicates the
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capacity of the zone file, or in other words, the maximum file size.
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Conventional zone files
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-----------------------
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The size of conventional zone files is fixed to the size of the zone they
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represent. Conventional zone files cannot be truncated.
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These files can be randomly read and written using any type of I/O operation:
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buffered I/Os, direct I/Os, memory mapped I/Os (mmap), etc. There are no I/O
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constraint for these files beyond the file size limit mentioned above.
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Sequential zone files
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---------------------
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The size of sequential zone files grouped in the "seq" sub-directory represents
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the file's zone write pointer position relative to the zone start sector.
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Sequential zone files can only be written sequentially, starting from the file
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end, that is, write operations can only be append writes. Zonefs makes no
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attempt at accepting random writes and will fail any write request that has a
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start offset not corresponding to the end of the file, or to the end of the last
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write issued and still in-flight (for asynchronous I/O operations).
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Since dirty page writeback by the page cache does not guarantee a sequential
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write pattern, zonefs prevents buffered writes and writeable shared mappings
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on sequential files. Only direct I/O writes are accepted for these files.
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zonefs relies on the sequential delivery of write I/O requests to the device
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implemented by the block layer elevator. An elevator implementing the sequential
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write feature for zoned block device (ELEVATOR_F_ZBD_SEQ_WRITE elevator feature)
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must be used. This type of elevator (e.g. mq-deadline) is set by default
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for zoned block devices on device initialization.
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There are no restrictions on the type of I/O used for read operations in
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sequential zone files. Buffered I/Os, direct I/Os and shared read mappings are
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all accepted.
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Truncating sequential zone files is allowed only down to 0, in which case, the
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zone is reset to rewind the file zone write pointer position to the start of
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the zone, or up to the zone capacity, in which case the file's zone is
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transitioned to the FULL state (finish zone operation).
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Format options
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--------------
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Several optional features of zonefs can be enabled at format time.
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* Conventional zone aggregation: ranges of contiguous conventional zones can be
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aggregated into a single larger file instead of the default one file per zone.
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* File ownership: The owner UID and GID of zone files is by default 0 (root)
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but can be changed to any valid UID/GID.
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* File access permissions: the default 640 access permissions can be changed.
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IO error handling
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-----------------
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Zoned block devices may fail I/O requests for reasons similar to regular block
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devices, e.g. due to bad sectors. However, in addition to such known I/O
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failure pattern, the standards governing zoned block devices behavior define
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additional conditions that result in I/O errors.
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* A zone may transition to the read-only condition (BLK_ZONE_COND_READONLY):
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While the data already written in the zone is still readable, the zone can
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no longer be written. No user action on the zone (zone management command or
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read/write access) can change the zone condition back to a normal read/write
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state. While the reasons for the device to transition a zone to read-only
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state are not defined by the standards, a typical cause for such transition
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would be a defective write head on an HDD (all zones under this head are
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changed to read-only).
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* A zone may transition to the offline condition (BLK_ZONE_COND_OFFLINE):
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An offline zone cannot be read nor written. No user action can transition an
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offline zone back to an operational good state. Similarly to zone read-only
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transitions, the reasons for a drive to transition a zone to the offline
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condition are undefined. A typical cause would be a defective read-write head
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on an HDD causing all zones on the platter under the broken head to be
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inaccessible.
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* Unaligned write errors: These errors result from the host issuing write
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requests with a start sector that does not correspond to a zone write pointer
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position when the write request is executed by the device. Even though zonefs
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enforces sequential file write for sequential zones, unaligned write errors
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may still happen in the case of a partial failure of a very large direct I/O
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operation split into multiple BIOs/requests or asynchronous I/O operations.
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If one of the write request within the set of sequential write requests
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issued to the device fails, all write requests queued after it will
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become unaligned and fail.
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* Delayed write errors: similarly to regular block devices, if the device side
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write cache is enabled, write errors may occur in ranges of previously
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completed writes when the device write cache is flushed, e.g. on fsync().
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Similarly to the previous immediate unaligned write error case, delayed write
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errors can propagate through a stream of cached sequential data for a zone
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causing all data to be dropped after the sector that caused the error.
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All I/O errors detected by zonefs are notified to the user with an error code
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return for the system call that triggered or detected the error. The recovery
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actions taken by zonefs in response to I/O errors depend on the I/O type (read
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vs write) and on the reason for the error (bad sector, unaligned writes or zone
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condition change).
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* For read I/O errors, zonefs does not execute any particular recovery action,
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but only if the file zone is still in a good condition and there is no
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inconsistency between the file inode size and its zone write pointer position.
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If a problem is detected, I/O error recovery is executed (see below table).
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* For write I/O errors, zonefs I/O error recovery is always executed.
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* A zone condition change to read-only or offline also always triggers zonefs
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I/O error recovery.
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Zonefs minimal I/O error recovery may change a file size and file access
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permissions.
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* File size changes:
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Immediate or delayed write errors in a sequential zone file may cause the file
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inode size to be inconsistent with the amount of data successfully written in
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the file zone. For instance, the partial failure of a multi-BIO large write
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operation will cause the zone write pointer to advance partially, even though
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the entire write operation will be reported as failed to the user. In such
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case, the file inode size must be advanced to reflect the zone write pointer
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change and eventually allow the user to restart writing at the end of the
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file.
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A file size may also be reduced to reflect a delayed write error detected on
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fsync(): in this case, the amount of data effectively written in the zone may
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be less than originally indicated by the file inode size. After such I/O
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error, zonefs always fixes the file inode size to reflect the amount of data
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persistently stored in the file zone.
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* Access permission changes:
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A zone condition change to read-only is indicated with a change in the file
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access permissions to render the file read-only. This disables changes to the
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file attributes and data modification. For offline zones, all permissions
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(read and write) to the file are disabled.
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Further action taken by zonefs I/O error recovery can be controlled by the user
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with the "errors=xxx" mount option. The table below summarizes the result of
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zonefs I/O error processing depending on the mount option and on the zone
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conditions::
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+--------------+-----------+-----------------------------------------+
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| | | Post error state |
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| "errors=xxx" | device | access permissions |
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| mount | zone | file file device zone |
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| option | condition | size read write read write |
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+--------------+-----------+-----------------------------------------+
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| | good | fixed yes no yes yes |
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| remount-ro | read-only | as is yes no yes no |
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| (default) | offline | 0 no no no no |
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+--------------+-----------+-----------------------------------------+
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| | good | fixed yes no yes yes |
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| zone-ro | read-only | as is yes no yes no |
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| | offline | 0 no no no no |
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+--------------+-----------+-----------------------------------------+
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| | good | 0 no no yes yes |
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| zone-offline | read-only | 0 no no yes no |
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| | offline | 0 no no no no |
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+--------------+-----------+-----------------------------------------+
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| | good | fixed yes yes yes yes |
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| repair | read-only | as is yes no yes no |
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| | offline | 0 no no no no |
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+--------------+-----------+-----------------------------------------+
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Further notes:
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* The "errors=remount-ro" mount option is the default behavior of zonefs I/O
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error processing if no errors mount option is specified.
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* With the "errors=remount-ro" mount option, the change of the file access
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permissions to read-only applies to all files. The file system is remounted
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read-only.
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* Access permission and file size changes due to the device transitioning zones
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to the offline condition are permanent. Remounting or reformatting the device
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with mkfs.zonefs (mkzonefs) will not change back offline zone files to a good
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state.
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* File access permission changes to read-only due to the device transitioning
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zones to the read-only condition are permanent. Remounting or reformatting
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the device will not re-enable file write access.
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* File access permission changes implied by the remount-ro, zone-ro and
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zone-offline mount options are temporary for zones in a good condition.
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Unmounting and remounting the file system will restore the previous default
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(format time values) access rights to the files affected.
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* The repair mount option triggers only the minimal set of I/O error recovery
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actions, that is, file size fixes for zones in a good condition. Zones
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indicated as being read-only or offline by the device still imply changes to
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the zone file access permissions as noted in the table above.
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Mount options
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-------------
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zonefs define the "errors=<behavior>" mount option to allow the user to specify
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zonefs behavior in response to I/O errors, inode size inconsistencies or zone
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condition changes. The defined behaviors are as follow:
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* remount-ro (default)
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* zone-ro
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* zone-offline
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* repair
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The run-time I/O error actions defined for each behavior are detailed in the
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previous section. Mount time I/O errors will cause the mount operation to fail.
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The handling of read-only zones also differs between mount-time and run-time.
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If a read-only zone is found at mount time, the zone is always treated in the
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same manner as offline zones, that is, all accesses are disabled and the zone
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file size set to 0. This is necessary as the write pointer of read-only zones
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is defined as invalib by the ZBC and ZAC standards, making it impossible to
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discover the amount of data that has been written to the zone. In the case of a
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read-only zone discovered at run-time, as indicated in the previous section.
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The size of the zone file is left unchanged from its last updated value.
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Zonefs User Space Tools
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=======================
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The mkzonefs tool is used to format zoned block devices for use with zonefs.
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This tool is available on Github at:
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https://github.com/damien-lemoal/zonefs-tools
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zonefs-tools also includes a test suite which can be run against any zoned
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block device, including null_blk block device created with zoned mode.
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Examples
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--------
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The following formats a 15TB host-managed SMR HDD with 256 MB zones
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with the conventional zones aggregation feature enabled::
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# mkzonefs -o aggr_cnv /dev/sdX
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# mount -t zonefs /dev/sdX /mnt
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# ls -l /mnt/
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total 0
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dr-xr-xr-x 2 root root 1 Nov 25 13:23 cnv
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dr-xr-xr-x 2 root root 55356 Nov 25 13:23 seq
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The size of the zone files sub-directories indicate the number of files
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existing for each type of zones. In this example, there is only one
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conventional zone file (all conventional zones are aggregated under a single
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file)::
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# ls -l /mnt/cnv
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total 137101312
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-rw-r----- 1 root root 140391743488 Nov 25 13:23 0
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This aggregated conventional zone file can be used as a regular file::
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# mkfs.ext4 /mnt/cnv/0
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# mount -o loop /mnt/cnv/0 /data
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The "seq" sub-directory grouping files for sequential write zones has in this
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example 55356 zones::
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# ls -lv /mnt/seq
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total 14511243264
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-rw-r----- 1 root root 0 Nov 25 13:23 0
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-rw-r----- 1 root root 0 Nov 25 13:23 1
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-rw-r----- 1 root root 0 Nov 25 13:23 2
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...
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-rw-r----- 1 root root 0 Nov 25 13:23 55354
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-rw-r----- 1 root root 0 Nov 25 13:23 55355
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For sequential write zone files, the file size changes as data is appended at
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the end of the file, similarly to any regular file system::
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# dd if=/dev/zero of=/mnt/seq/0 bs=4096 count=1 conv=notrunc oflag=direct
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1+0 records in
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1+0 records out
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4096 bytes (4.1 kB, 4.0 KiB) copied, 0.00044121 s, 9.3 MB/s
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# ls -l /mnt/seq/0
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-rw-r----- 1 root root 4096 Nov 25 13:23 /mnt/seq/0
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The written file can be truncated to the zone size, preventing any further
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write operation::
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# truncate -s 268435456 /mnt/seq/0
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# ls -l /mnt/seq/0
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-rw-r----- 1 root root 268435456 Nov 25 13:49 /mnt/seq/0
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Truncation to 0 size allows freeing the file zone storage space and restart
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append-writes to the file::
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# truncate -s 0 /mnt/seq/0
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# ls -l /mnt/seq/0
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-rw-r----- 1 root root 0 Nov 25 13:49 /mnt/seq/0
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Since files are statically mapped to zones on the disk, the number of blocks
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of a file as reported by stat() and fstat() indicates the capacity of the file
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zone::
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# stat /mnt/seq/0
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File: /mnt/seq/0
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Size: 0 Blocks: 524288 IO Block: 4096 regular empty file
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Device: 870h/2160d Inode: 50431 Links: 1
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Access: (0640/-rw-r-----) Uid: ( 0/ root) Gid: ( 0/ root)
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Access: 2019-11-25 13:23:57.048971997 +0900
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Modify: 2019-11-25 13:52:25.553805765 +0900
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Change: 2019-11-25 13:52:25.553805765 +0900
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Birth: -
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The number of blocks of the file ("Blocks") in units of 512B blocks gives the
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maximum file size of 524288 * 512 B = 256 MB, corresponding to the device zone
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capacity in this example. Of note is that the "IO block" field always
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indicates the minimum I/O size for writes and corresponds to the device
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physical sector size.
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