• Damien Le Moal's avatar
    fs: New zonefs file system · 8dcc1a9d
    Damien Le Moal authored
    
    
    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 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 a zoned device. 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 is reduced to an immutable super block to
    persistently store 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. E.g. file sizes come from the device
    zone type and write pointer offset managed by the device itself.
    
    The zone files created on mount have the following characteristics.
    1) Files representing zones of the same type are grouped together
       under a common sub-directory:
         * For conventional zones, the sub-directory "cnv" is used.
         * 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.
    2) The name of zone files is the number of the file within the zone
       type sub-directory, in order of increasing zone start sector.
    3) The size of conventional zone files is fixed to the device zone size.
       Conventional zone files cannot be truncated.
    4) The size of sequential zone files represent the file's zone write
       pointer position relative to the zone start sector. Truncating these
       files is allowed only down to 0, in which case, the zone is reset to
       rewind the 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).
    5) All read and write operations to files are not allowed beyond the
       file zone size. Any access exceeding the zone size is failed with
       the -EFBIG error.
    6) Creating, deleting, renaming or modifying any attribute of files and
       sub-directories is not allowed.
    7) There are no restrictions on the type of read and write operations
       that can be issued to conventional zone files. Buffered, direct and
       mmap read & write operations are accepted. For sequential zone files,
       there are no restrictions on read operations, but all write
       operations must be direct IO append writes. mmap write of sequential
       files is not allowed.
    
    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.
    
    The mkzonefs tool is used to format zoned block devices for use with
    zonefs. This tool is available on Github at:
    
    git@github.com: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.
    
    Example: the following formats a 15TB host-managed SMR HDD with 256 MB
    zones with the conventional zones aggregation feature enabled.
    
    $ sudo mkzonefs -o aggr_cnv /dev/sdX
    $ sudo 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.
    
    $ sudo mkfs.ext4 /mnt/cnv/0
    $ sudo 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=4K count=1 conv=notrunc oflag=direct
    1+0 records in
    1+0 records out
    4096 bytes (4.1 kB, 4.0 KiB) copied, 0.000452219 s, 9.1 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 IO size for writes and corresponds
    to the device physical sector size.
    
    This code contains contributions from:
    * Johannes Thumshirn <jthumshirn@suse.de>,
    * Darrick J. Wong <darrick.wong@oracle.com>,
    * Christoph Hellwig <hch@lst.de>,
    * Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com> and
    * Ting Yao <tingyao@hust.edu.cn>.
    Signed-off-by: default avatarDamien Le Moal <damien.lemoal@wdc.com>
    Reviewed-by: default avatarDave Chinner <dchinner@redhat.com>
    8dcc1a9d
zonefs.h 4.69 KB