• Chris Mason's avatar
    Btrfs: move data checksumming into a dedicated tree · d20f7043
    Chris Mason authored
    
    
    Btrfs stores checksums for each data block.  Until now, they have
    been stored in the subvolume trees, indexed by the inode that is
    referencing the data block.  This means that when we read the inode,
    we've probably read in at least some checksums as well.
    
    But, this has a few problems:
    
    * The checksums are indexed by logical offset in the file.  When
    compression is on, this means we have to do the expensive checksumming
    on the uncompressed data.  It would be faster if we could checksum
    the compressed data instead.
    
    * If we implement encryption, we'll be checksumming the plain text and
    storing that on disk.  This is significantly less secure.
    
    * For either compression or encryption, we have to get the plain text
    back before we can verify the checksum as correct.  This makes the raid
    layer balancing and extent moving much more expensive.
    
    * It makes the front end caching code more complex, as we have touch
    the subvolume and inodes as we cache extents.
    
    * There is potentitally one copy of the checksum in each subvolume
    referencing an extent.
    
    The solution used here is to store the extent checksums in a dedicated
    tree.  This allows us to index the checksums by phyiscal extent
    start and length.  It means:
    
    * The checksum is against the data stored on disk, after any compression
    or encryption is done.
    
    * The checksum is stored in a central location, and can be verified without
    following back references, or reading inodes.
    
    This makes compression significantly faster by reducing the amount of
    data that needs to be checksummed.  It will also allow much faster
    raid management code in general.
    
    The checksums are indexed by a key with a fixed objectid (a magic value
    in ctree.h) and offset set to the starting byte of the extent.  This
    allows us to copy the checksum items into the fsync log tree directly (or
    any other tree), without having to invent a second format for them.
    Signed-off-by: default avatarChris Mason <chris.mason@oracle.com>
    d20f7043