Initial filesystem design synopsis.

[Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>]

    Here is my initial outline of the filesystem design.  It is open
    for discussion.  Please feel to ask questions for anything you do
    not understand.  I do not intend to start coding anything for at
    least two weeks.

    There are currently two rough spots in the design.  First, how to
    handle segment overflows in a multi-master environment.  Such overflows
    can occur when the individual masters or slaves have different historical
    data retention policies.  Second, where to store the regeneratable
    indexes.

    Plus, I need a name for this baby. I can't use DFS, however much I 
    want to, because the term is already over-used.

					-Matt
					Matthew Dillon 
					<dillon@backplane.com>


Feature Summary:

    - On-demand filesystem check and recovery.  No need to scan the entire
      filesystem before going live after a reboot.

    - Infinite snapshots (e.g. on the 30 second filesystem sync), ability
      to collapse snapshots for any time interval as a means of
      recovering space.

    - Multi-master operation, including the ability to self-heal a
      corrupted filesystem by accessing replicated data.  Multi-master 
      means that each replicated store can act as a master.  New
      filesystem ops can run independantly on any of the replicated stores
      and will flow to the others.  This is done by giving the controller
      of each store a certain amount of guarenteed free space that can be
      filled without having to notify other controllers.

    - Infinite logless Replication.  No requirement to keep a log of changes
      for the purposes of replication, meaning that replication targets can
      be offline for 'days' without effecting performance or operation.
      ('mobile' computing, but also replication over slow links for backup
      purposes and other things).  Certain information is still logged,
      but only to streamline performance.

    - 64 bit file space, 64 bit filesystem space.  No space restrictions
      whatsoever.

    - Reliably handles data storage for huge multi-hundred-terrabyte
      filesystems without fear of unrecoverable corruption.

    - Cluster operation - ability to commit data to locally replicated
      store independantly of other replication nodes, with access governed
      by cache coherency protocols.

    - Independant index.  Data is laid out in a highly recoverable fashion,
      independant of index generation.  Indexes can be regenerated from
      scratch and thus indexes can be updated asynchronously.

Segmentation:

    The physical storage backing a filesystem is broken up into large
    1MB-4GB segments (64MB is a typical value).  Each segment is
    self-identifying and contains its own header, data table, and record
    table.  The operating system glues together filesystems and determines
    availability based on the segments it finds.

    All segments on a block device are the same size (power-of-2 restricted)
    and the OS can probe the disk without any other information simply by
    locating a segment header, starting with the largest possible segment
    size.  Segment size is typically chosen such that a scan of all segment
    headers takes no longer then a few seconds.

    - The segment header contains all the information required to associate
      the segment with a filesystem.   Certain information in the segment
      header is updated with an ordered write to 'commit' other work already
      flushed out into the segment.  The segment header also contains a 
      bit indicating whether the segment is 'open' or not, to aid in 
      on-demand recovery.

      The segment ID is stored in the segment header, allowing segments
      to be easily relocated.  That is, the segment's location in the
      physical backing store is irrelevant.

    - The data table consists of pure data, laid out linearly in the forward
      direction within the segment.   Data blocks are variable-sized entities
      containing pure data, with no other identifying information, suitable
      for direct DMA.  The segment header has a simple append index for
      the data table.

    - The record table consists of fixed-sized records and a reference to
      data in the data table.  The record table is built backwards from
      the end of the segment.  The segment header has a simple pre-append
      index for the record table.  Each record consists of:

      * an object identifier (e.g. inode number)
      * creation transid
      * deletion transid (updated in-place)
      * indexing key (offset or filename key)
      * data table reference (offset, bytes)  (up to a 1GB contiguous ref)
      * integrity check

    All data elements are completely identified by the record table.  All
    modifications are historical in nature... that is, no actual data is
    destroyed and one can access a 'snapshot' of the segment as of any
    transaction id (i.e. timestamp) simply by ignoring those records
    marked as deleted prior to the specified time or created after
    the specified time.  

    Certain updates to the object table occur in-place.  In particular,
    the deletion transaction id is updated in-place.  However, such
    updates are not considered 'committed' until the segment header itself
    is updated with the latest committed transaction id and a recovery
    operation will undo any deletion transaction id greater then the
    committed transaction id in the segment header, as well as free
    any uncommitted objects and their related data.

    The entire filesystem can be recovered from just the record and data
    tables.  Indexing and cross-segment spanning is described in later
    sections.

Physical space recovery:

    Physical space recovery requires actually destroying records marked
    as deleted.  Snapshots can be collapsed by destroying records whos
    creation AND deletion id's fall within the collapsed space.  The oldest
    data is freed up by destroying records whos deletion id is less then
    the terminal timestamp. 

    Record destruction creates holes in both the data table and the record
    table.  Any holes adjacent to the data table append point or the record
    table prepend point are immediately recovered by adjusting the 
    appropriate indices in the segment header.  The operating system may
    cache a record of non-adjacent holes (in memory) and reuse the space,
    and can also generate an in-memory index of available holes on the
    fly when space is very tight (which requires scanning the record table),
    but otherwise the recovery of any space not adjacent to the data table
    append point requires a performance reorganization of the segment.

Locating Data objects, and the Master Record.

    Data objects are basically the amalgamation of all records with
    the same 64 bit object identifier.  The top N bits of this identifier
    indicate the segment the master record for the data object resides in.
    All 64 bits are made available to userland.

    The filesystem needs to be free to relocate the master record for
    a data object on the fly.  Relocation is a dynamic process whereby 
    the master record in a segment becomes a forwarding record to another
    segment.  Any references to a forwarding record is adjusted on the fly
    in order to collapse the chain, and any intermediate forwarding records
    can be physically destroyed once all references to them have been
    adjusted.  However, the ORIGINAL forwarding record must be retained
    for all time in order to maintain the uniqueness of the originally
    assigned user-visible inode number and to give us a way to locate
    the master record given the inode number.  We cannot change the inode
    number.  Overhead is minimal.

    A forwarding record is created in two situations: (1) To move the 
    master record to improve performance and (2) If the current segment
    does not have sufficient space to increase the size the master record
    if it becomes necessary to do so.

    A forwarding record is a degenerate case of a master record and the
    forwarding information is embedded in the record itself, with no
    data table reference at all.  The space used is only the space required
    to store a record table entry.

    The master record for a data object is a special record in the record
    table.  It is special because it is NOT part of the historical data
    store.  The creation and deletion transaction ids represent the creation
    or deletion of the entire data object, and the data block contains
    information on where to find the other bits and pieces of the data
    object (in particular, if the data object spans more then one segment).
    A CORRUPTED MASTER RECORD CAN BE COMPLETELY RECONSTRUCTED BY SCANNING
    THE FILESYSTEM.

    The master record can be thought of as a persistent cache.  It gives
    the filesystem a way to quickly locate all the segments that might
    contain records for the data object and reserves a limited amount of
    space for the filesystem to store direct references to data residing
    in the same segment as the master record.

    Master record updates are fairly rare.  For the most part a master
    record must only be updated if a file or directory grows into a
    new segment.

Performance reorganization:

    Segments can be repacked in order to clean up fragmentation and 
    reorganize data and records for more optimal access.  Repacking is
    accomplished by copying the segment's data into a wholely new
    segment on the physical disk, then destroying the old segment.

    Since a segment is identified by its header the actual location of
    the segment on the physical media is irrelevant.

    For example, master records can wind up strewn about the record
    table for a segment.  Repacking would pack all of those master
    records next to each other.

    Similarly, a file or directory might have bits and pieces of data
    strewn about a segment.  Repacking would de-fragment those entities,
    as well as pack together the data related to small files and 
    separate out the larger chunks related to larger files.

Segment Allocation:

    Remember that since the actual physical location of a segment is
    independant of the segment identifier (typically an 8 or 16 bit
    number), the allocation of segment numbers does not have to be
    linear.  The filesystem will typically be generous in its allocation
    of segment numbers in order to allow for spill over growth of files
    into logically adjacent segments, thus simplifying the segment
    range stored in the master record that describes which segments
    might contain data records for a data object.  For example,
    the first segment allocated by the filesystem when using a 16 bit
    segment id would not be 0x0000 or 0xffff, but probably 0x8000.  The
    next segment allocated (when not doing a spill-over) might be 0x4000
    or 0xc000, and so forth.  The entire segment space is used even
    if there are fewer actual (physical) segments.

Large cross-segment objects:

    A Data object can wind up being far larger then a segment.  For that
    matter, even a small data object with a lot of history can wind up being
    far larger then a segment.

    The filesystem does its best to organize the data for such objects
    to reduce the size of the master records required to describe them
    and to separate historical data from current data, to maintain the
    performance of the live filesystem.

Indexing:

    An object's master record generally describes the segments containing
    the data for the object, and may contain direct references to data
    in the same segment as the master record (an optimization or performance 
    reorganization that is desireable for files smaller then a single
    segment).

    However, data objects can grow to be many records due to fragmentation,
    simply being large files, or due to the retention of a great deal of
    history.  The records pertaining to these objects may have to be indexed
    beyond the space the filesystem is willing to reserve in the master
    record in order to improve access.

    To make it clear, indexing is an optimization.  The index is not
    required to recover a segment or to recover a filesystem.  The intent
    is for the master record to always contain sufficient information
    to reduce the number I/O's required to resolve a file access request
    to a reasonable number, even if no index is available.

    Indexing can occur in three places:

    * First, it can occur in the segment itself to organize the records
      in that segment.  Such indexes are usually persistently cached in the
      dead space between the data table and the record table, and the 
      filesystem heuristically determines how much space is needed for
      that.  If the data table or the record table grows into the index
      the filesystem can choose to relocate, regenerate, or shift the index
      data, or to disallow the growth (extend the data object into a new
      segment).  These heuristics are fairly simple.

    * Second, it can occur in the master record to roughly organize the
      data space... for example so the filesystem does not have to check
      all segments if a particular file offset (or offset range) and all
      of its history is known to reside in just one.  The filesystem
      typically is only willing to allow so much space in the master record
      for a data object to store such an index.  If this level of index
      becomes too large it is basically simplified in-place and starts
      requiring the use of the per-segment index to further resolve the
      location of data records for the object.

    * Third, it can be generated and cached in memory.  When dealing with
      very large multi-segment files it may be beneficial to scan
      the relatively few records in the record table for the related segments
      and simply index them in memory, rather then store an index on-disk.

      For example if you are using 64MB segments and have a 20GB file,
      literally only 320 disk accesses (with a few data records read in
      each access) are required to construct an index of the entire 20GB
      file and the index itself would require very little memory.

    Indexes are asynchronous entities.  The 'official' filesystem store is
    the data table and the record table.  The host can cache updates in
    memory, and asynchronously update the performance-improving index.  

    Indexes residing in segments are regenerated if the segment is marked
    open on initial access (due to an unclean shutdown).  This allows
    persistent per-segment indexes to be updated without requiring any
    particular transactional disk synchronization or block ordering.

    Indexes are generally implemented using a B+Tree structure.

Replication:

    Data and record elements are only directly referenced by their offset
    within a segment when referenced from within that same segment.  This
    means that replication is possible on a segment-by-segment basis.

    Segment headers contain asynchronously updated record update log areas
    for deletion events (because these modify the deletion timestamp in
    an existing record rather then append a new record).  These log areas
    are of limited size and can overflow under normal operation.  They are
    ONLY used to streamline replication.  If a replication target is not
    fast enough, it will have to resynchronize by scanning the records
    on the source (which itself doesn't usually take very long to do so it
    isn't considered a big deal).  But otherwise it can check the log area
    and simply pick up where it left off.  The intention is to completely
    support both very slow replication slaves and mostly off-line slaves.

    The only thing that is actually replicated are the record table
    entries and (with the exception of a master record) the data table
    entries.  The data table entry for a master record is never replicated,
    but (re)generated by the replication target.  The replication target
    is responsible for maintaining its own indexes and managing its own
    physical segment space.  This gives any replication target a great
    deal of robustness and flexibility.

    Also note that the physical location of the logical segments on the
    replication target is entirely up to the replication target.  Replication
    is done logically, not physically.

Segment Expansion - Virtual segments

    A replication target may wish to retain historical data that the 
    replication source has not, or destroy historical data that the
    replication source intends to retain.  This can result in the replication
    target running out of room in a logical segment, and can also complicate
    recovery operations if the replication source needs to self-heal a
    corrupted segment.  This presents a problem because all filesystem
    accesses and all replication and recovery activities are segment-centric.

    To deal with this situation a logical segment can be turned into a 
    virtual segment.  A virtual segment is made up of multiple logical
    segments, all with the same identifier plus additional information
    in the segment distinguishing the pieces from each other.  Each logical
    segment is maintained independantly but API accesses check both
    (or all N such segments) when doing lookups, and write to whichever
    one has free space.  

    Virtual segments are pretty standard fare on replication slaves which
    are intended to archive a great deal more history then replication
    masters, but are intended to be very rare features of replication
    masters.  If a virtual segment must be created on a replication master
    the filesystem does all it can to shift data off the virtual segment
    in order to be able to collapse it back into a logical segment.

    Virtual segmentation is not space efficient.

Files and Directories

    As has been described, a data object (which can be a file or directory
    or other filesystem entity) is identified by a data object id, which
    is effectively its inode, and a 64 bit key field.  The key field is
    typically a base offset for a file or a sortable key for a directory
    entry.  Negative numbers are used for meta-data.  For example, -1 will
    be used for the inode data & stat information.  Other negative numbers
    will be used to support other meta-data such as ACLs.

    Since records support variable-length data, the data storage for a file
    is effectively extent-based.  Minimum granularity will be something like
    32 bytes.

    Files are thus fairly straight forward.

    From the point of view of the filesystem each directory entry will
    be a data record.  The data record will basically just contain the
    inode number, type, and filename.  It will be variable-length insofar
    as the filename is variable length.

    Most files will be representable in a single extent (or at least
    can be optimized into a single extent), and so will not depend very
    heavily on indexing.

    Directory lookups WILL depend on the indexing of the directory records
    for efficiency, otherwise every record would have to be scanned to
    lookup a filename.  In this regard a B+Tree is probably the best 
    solution.

Inode Number Uniqueness

    Inode numbers are 64 bits where the top N bits represent the segment
    in which the inode was originally allocated, identifying the master
    record for the data object.  Inode numbers do not change even if
    the master record is relocated and the original master record replaced
    with a forwarding record.  The forwarding record must be retained
    in order to guarentee the uniqueness of the inode number.

    This also allows inode numbers to be allocated on a per-segment basis,
    with 48 bits of uniqueness (one trillion file creates & deletes) within
    each segment.

    The filesystem will always allocate new inode numbers using a sequence
    number maintained in the segment header.  When all 48 bits are used up,
    however, the filesystem must check new inode numbers against existing
    numbers (including any deleted historical objects).  

    Inode number uniqueness over all time can no longer be guarenteed, but
    inode number uniqueness over a period of time CAN still be guarenteed
    by retaining the master record for deleted files for a period of time
    EVEN AFTER THEY HAVE BEEN DESTROYED.  The system operator can specify
    the retention time with the caveat that certain benchmarks might cause
    the disk to fill up or become somewhat less efficient even when 
    historical data is purged.

    It is recommended that any exported or clustered filesystems set the
    inode number retention time to at least a week.  Inode number uniqueness
    is crucial to cluster operation.