MFV/ZoL: Implement large_dnode pool feature

MFV/ZoL: Implement large_dnode pool feature

commit 50c957f702ea6d08a634e42f73e8a49931dd8055
Author: Ned Bass <bass6@llnl.gov>
Date:   Wed Mar 16 18:25:34 2016 -0700

    Implement large_dnode pool feature

    Justification
    -------------

    This feature adds support for variable length dnodes. Our motivation is
    to eliminate the overhead associated with using spill blocks.  Spill
    blocks are used to store system attribute data (i.e. file metadata) that
    does not fit in the dnode's bonus buffer. By allowing a larger bonus
    buffer area the use of a spill block can be avoided.  Spill blocks
    potentially incur an additional read I/O for every dnode in a dnode
    block. As a worst case example, reading 32 dnodes from a 16k dnode block
    and all of the spill blocks could issue 33 separate reads. Now suppose
    those dnodes have size 1024 and therefore don't need spill blocks.  Then
    the worst case number of blocks read is reduced to from 33 to two--one
    per dnode block. In practice spill blocks may tend to be co-located on
    disk with the dnode blocks so the reduction in I/O would not be this
    drastic. In a badly fragmented pool, however, the improvement could be
    significant.

    ZFS-on-Linux systems that make heavy use of extended attributes would
    benefit from this feature. In particular, ZFS-on-Linux supports the
    xattr=sa dataset property which allows file extended attribute data
    to be stored in the dnode bonus buffer as an alternative to the
    traditional directory-based format. Workloads such as SELinux and the
    Lustre distributed filesystem often store enough xattr data to force
    spill bocks when xattr=sa is in effect. Large dnodes may therefore
    provide a performance benefit to such systems.

    Other use cases that may benefit from this feature include files with
    large ACLs and symbolic links with long target names. Furthermore,
    this feature may be desirable on other platforms in case future
    applications or features are developed that could make use of a
    larger bonus buffer area.

    Implementation
    --------------

    The size of a dnode may be a multiple of 512 bytes up to the size of
    a dnode block (currently 16384 bytes). A dn_extra_slots field was
    added to the current on-disk dnode_phys_t structure to describe the
    size of the physical dnode on disk. The 8 bits for this field were
    taken from the zero filled dn_pad2 field. The field represents how
    many "extra" dnode_phys_t slots a dnode consumes in its dnode block.
    This convention results in a value of 0 for 512 byte dnodes which
    preserves on-disk format compatibility with older software.

    Similarly, the in-memory dnode_t structure has a new dn_num_slots field
    to represent the total number of dnode_phys_t slots consumed on disk.
    Thus dn->dn_num_slots is 1 greater than the corresponding
    dnp->dn_extra_slots. This difference in convention was adopted
    because, unlike on-disk structures, backward compatibility is not a
    concern for in-memory objects, so we used a more natural way to
    represent size for a dnode_t.

    The default size for newly created dnodes is determined by the value of
    a new "dnodesize" dataset property. By default the property is set to
    "legacy" which is compatible with older software. Setting the property
    to "auto" will allow the filesystem to choose the most suitable dnode
    size. Currently this just sets the default dnode size to 1k, but future
    code improvements could dynamically choose a size based on observed
    workload patterns. Dnodes of varying sizes can coexist within the same
    dataset and even within the same dnode block. For example, to enable
    automatically-sized dnodes, run

     # zfs set dnodesize=auto tank/fish

    The user can also specify literal values for the dnodesize property.
    These are currently limited to powers of two from 1k to 16k. The
    power-of-2 limitation is only for simplicity of the user interface.
    Internally the implementation can handle any multiple of 512 up to 16k,
    and consumers of the DMU API can specify any legal dnode value.

    The size of a new dnode is determined at object allocation time and
    stored as a new field in the znode in-memory structure. New DMU
    interfaces are added to allow the consumer to specify the dnode size
    that a newly allocated object should use. Existing interfaces are
    unchanged to avoid having to update every call site and to preserve
    compatibility with external consumers such as Lustre. The new
    interfaces names are given below. The versions of these functions that
    don't take a dnodesize parameter now just call the _dnsize() versions
    with a dnodesize of 0, which means use the legacy dnode size.

    New DMU interfaces:
      dmu_object_alloc_dnsize()
      dmu_object_claim_dnsize()
      dmu_object_reclaim_dnsize()

    New ZAP interfaces:
      zap_create_dnsize()
      zap_create_norm_dnsize()
      zap_create_flags_dnsize()
      zap_create_claim_norm_dnsize()
      zap_create_link_dnsize()

    The constant DN_MAX_BONUSLEN is renamed to DN_OLD_MAX_BONUSLEN. The
    spa_maxdnodesize() function should be used to determine the maximum
    bonus length for a pool.

    These are a few noteworthy changes to key functions:

    * The prototype for dnode_hold_impl() now takes a "slots" parameter.
      When the DNODE_MUST_BE_FREE flag is set, this parameter is used to
      ensure the hole at the specified object offset is large enough to
      hold the dnode being created. The slots parameter is also used
      to ensure a dnode does not span multiple dnode blocks. In both of
      these cases, if a failure occurs, ENOSPC is returned. Keep in mind,
      these failure cases are only possible when using DNODE_MUST_BE_FREE.

      If the DNODE_MUST_BE_ALLOCATED flag is set, "slots" must be 0.
      dnode_hold_impl() will check if the requested dnode is already
      consumed as an extra dnode slot by an large dnode, in which case
      it returns ENOENT.

    * The function dmu_object_alloc() advances to the next dnode block
      if dnode_hold_impl() returns an error for a requested object.
      This is because the beginning of the next dnode block is the only
      location it can safely assume to either be a hole or a valid
      starting point for a dnode.

    * dnode_next_offset_level() and other functions that iterate
      through dnode blocks may no longer use a simple array indexing
      scheme. These now use the current dnode's dn_num_slots field to
      advance to the next dnode in the block. This is to ensure we
      properly skip the current dnode's bonus area and don't interpret it
      as a valid dnode.

    zdb
    ---
    The zdb command was updated to display a dnode's size under the
    "dnsize" column when the object is dumped.

    For ZIL create log records, zdb will now display the slot count for
    the object.

    ztest
    -----
    Ztest chooses a random dnodesize for every newly created object. The
    random distribution is more heavily weighted toward small dnodes to
    better simulate real-world datasets.

    Unused bonus buffer space is filled with non-zero values computed from
    the object number, dataset id, offset, and generation number.  This
    helps ensure that the dnode traversal code properly skips the interior
    regions of large dnodes, and that these interior regions are not
    overwritten by data belonging to other dnodes. A new test visits each
    object in a dataset. It verifies that the actual dnode size matches what
    was stored in the ztest block tag when it was created. It also verifies
    that the unused bonus buffer space is filled with the expected data
    patterns.

    ZFS Test Suite
    --------------
    Added six new large dnode-specific tests, and integrated the dnodesize
    property into existing tests for zfs allow and send/recv.

    Send/Receive
    ------------
    ZFS send streams for datasets containing large dnodes cannot be received
    on pools that don't support the large_dnode feature. A send stream with
    large dnodes sets a DMU_BACKUP_FEATURE_LARGE_DNODE flag which will be
    unrecognized by an incompatible receiving pool so that the zfs receive
    will fail gracefully.

    While not implemented here, it may be possible to generate a
    backward-compatible send stream from a dataset containing large
    dnodes. The implementation may be tricky, however, because the send
    object record for a large dnode would need to be resized to a 512
    byte dnode, possibly kicking in a spill block in the process. This
    means we would need to construct a new SA layout and possibly
    register it in the SA layout object. The SA layout is normally just
    sent as an ordinary object record. But if we are constructing new
    layouts while generating the send stream we'd have to build the SA
    layout object dynamically and send it at the end of the stream.

    For sending and receiving between pools that do support large dnodes,
    the drr_object send record type is extended with a new field to store
    the dnode slot count. This field was repurposed from unused padding
    in the structure.

    ZIL Replay
    ----------
    The dnode slot count is stored in the uppermost 8 bits of the lr_foid
    field. The bits were unused as the object id is currently capped at
    48 bits.

    Resizing Dnodes
    ---------------
    It should be possible to resize a dnode when it is dirtied if the
    current dnodesize dataset property differs from the dnode's size, but
    this functionality is not currently implemented. Clearly a dnode can
    only grow if there are sufficient contiguous unused slots in the
    dnode block, but it should always be possible to shrink a dnode.
    Growing dnodes may be useful to reduce fragmentation in a pool with
    many spill blocks in use. Shrinking dnodes may be useful to allow
    sending a dataset to a pool that doesn't support the large_dnode
    feature.

    Feature Reference Counting
    --------------------------
    The reference count for the large_dnode pool feature tracks the
    number of datasets that have ever contained a dnode of size larger
    than 512 bytes. The first time a large dnode is created in a dataset
    the dataset is converted to an extensible dataset. This is a one-way
    operation and the only way to decrement the feature count is to
    destroy the dataset, even if the dataset no longer contains any large
    dnodes. The complexity of reference counting on a per-dnode basis was
    too high, so we chose to track it on a per-dataset basis similarly to
    the large_block feature.

    Signed-off-by: Ned Bass <bass6@llnl.gov>
    Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
    Closes #3542