• Lukas Czerner's avatar
    ext4: introduce reserved space · 27dd4385
    Lukas Czerner authored
    Currently in ENOSPC condition when writing into unwritten space, or
    punching a hole, we might need to split the extent and grow extent tree.
    However since we can not allocate any new metadata blocks we'll have to
    zero out unwritten part of extent or punched out part of extent, or in
    the worst case return ENOSPC even though use actually does not allocate
    any space.
    
    Also in delalloc path we do reserve metadata and data blocks for the
    time we're going to write out, however metadata block reservation is
    very tricky especially since we expect that logical connectivity implies
    physical connectivity, however that might not be the case and hence we
    might end up allocating more metadata blocks than previously reserved.
    So in future, metadata reservation checks should be removed since we can
    not assure that we do not under reserve.
    
    And this is where reserved space comes into the picture. When mounting
    the file system we slice off a little bit of the file system space (2%
    or 4096 clusters, whichever is smaller) which can be then used for the
    cases mentioned above to prevent costly zeroout, or unexpected ENOSPC.
    
    The number of reserved clusters can be set via sysfs, however it can
    never be bigger than number of free clusters in the file system.
    
    Note that this patch fixes the failure of xfstest 274 as expected.
    Signed-off-by: default avatarLukas Czerner <lczerner@redhat.com>
    Signed-off-by: default avatar"Theodore Ts'o" <tytso@mit.edu>
    Reviewed-by: default avatarCarlos Maiolino <cmaiolino@redhat.com>
    27dd4385
ext4.txt 27 KB