• Darrick J. Wong's avatar
    xfs: store in-progress CoW allocations in the refcount btree · 174edb0e
    Darrick J. Wong authored
    Due to the way the CoW algorithm in XFS works, there's an interval
    during which blocks allocated to handle a CoW can be lost -- if the FS
    goes down after the blocks are allocated but before the block
    remapping takes place.  This is exacerbated by the cowextsz hint --
    allocated reservations can sit around for a while, waiting to get
    used.
    
    Since the refcount btree doesn't normally store records with refcount
    of 1, we can use it to record these in-progress extents.  In-progress
    blocks cannot be shared because they're not user-visible, so there
    shouldn't be any conflicts with other programs.  This is a better
    solution than holding EFIs during writeback because (a) EFIs can't be
    relogged currently, (b) even if they could, EFIs are bound by
    available log space, which puts an unnecessary upper bound on how much
    CoW we can have in flight, and (c) we already have a mechanism to
    track blocks.
    
    At mount time, read the refcount records and free anything we find
    with a refcount of 1 because those were in-progress when the FS went
    down.
    Signed-off-by: default avatarDarrick J. Wong <darrick.wong@oracle.com>
    Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
    174edb0e
xfs_mount.c 34.5 KB