• Joe Thornber's avatar
    dm cache: add passthrough mode · 2ee57d58
    Joe Thornber authored
    "Passthrough" is a dm-cache operating mode (like writethrough or
    writeback) which is intended to be used when the cache contents are not
    known to be coherent with the origin device.  It behaves as follows:
    
    * All reads are served from the origin device (all reads miss the cache)
    * All writes are forwarded to the origin device; additionally, write
      hits cause cache block invalidates
    
    This mode decouples cache coherency checks from cache device creation,
    largely to avoid having to perform coherency checks while booting.  Boot
    scripts can create cache devices in passthrough mode and put them into
    service (mount cached filesystems, for example) without having to worry
    about coherency.  Coherency that exists is maintained, although the
    cache will gradually cool as writes take place.
    
    Later, applications can perform coherency checks, the nature of which
    will depend on the type of the underlying storage.  If coherency can be
    verified, the cache device can be transitioned to writethrough or
    writeback mode while still warm; otherwise, the cache contents can be
    discarded prior to transitioning to the desired operating mode.
    Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
    Signed-off-by: default avatarHeinz Mauelshagen <heinzm@redhat.com>
    Signed-off-by: default avatarMorgan Mears <Morgan.Mears@netapp.com>
    Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
    2ee57d58
dm-cache-metadata.c 29.3 KB