• Chris Mason's avatar
    Btrfs: Add zlib compression support · c8b97818
    Chris Mason authored
    
    
    This is a large change for adding compression on reading and writing,
    both for inline and regular extents.  It does some fairly large
    surgery to the writeback paths.
    
    Compression is off by default and enabled by mount -o compress.  Even
    when the -o compress mount option is not used, it is possible to read
    compressed extents off the disk.
    
    If compression for a given set of pages fails to make them smaller, the
    file is flagged to avoid future compression attempts later.
    
    * While finding delalloc extents, the pages are locked before being sent down
    to the delalloc handler.  This allows the delalloc handler to do complex things
    such as cleaning the pages, marking them writeback and starting IO on their
    behalf.
    
    * Inline extents are inserted at delalloc time now.  This allows us to compress
    the data before inserting the inline extent, and it allows us to insert
    an inline extent that spans multiple pages.
    
    * All of the in-memory extent representations (extent_map.c, ordered-data.c etc)
    are changed to record both an in-memory size and an on disk size, as well
    as a flag for compression.
    
    From a disk format point of view, the extent pointers in the file are changed
    to record the on disk size of a given extent and some encoding flags.
    Space in the disk format is allocated for compression encoding, as well
    as encryption and a generic 'other' field.  Neither the encryption or the
    'other' field are currently used.
    
    In order to limit the amount of data read for a single random read in the
    file, the size of a compressed extent is limited to 128k.  This is a
    software only limit, the disk format supports u64 sized compressed extents.
    
    In order to limit the ram consumed while processing extents, the uncompressed
    size of a compressed extent is limited to 256k.  This is a software only limit
    and will be subject to tuning later.
    
    Checksumming is still done on compressed extents, and it is done on the
    uncompressed version of the data.  This way additional encodings can be
    layered on without having to figure out which encoding to checksum.
    
    Compression happens at delalloc time, which is basically singled threaded because
    it is usually done by a single pdflush thread.  This makes it tricky to
    spread the compression load across all the cpus on the box.  We'll have to
    look at parallel pdflush walks of dirty inodes at a later time.
    
    Decompression is hooked into readpages and it does spread across CPUs nicely.
    Signed-off-by: default avatarChris Mason <chris.mason@oracle.com>
    c8b97818