• Jonathan Brassow's avatar
    dm log: userspace add luid to distinguish between concurrent log instances · 7ec23d50
    Jonathan Brassow authored
    Device-mapper userspace logs (like the clustered log) are
    identified by a universally unique identifier (UUID).  This
    identifier is used to associate requests from the kernel to
    a specific log in userspace.  The UUID must be unique everywhere,
    since multiple machines may use this identifier when communicating
    about a particular log, as is the case for cluster logs.
    Sometimes, device-mapper/LVM may re-use a UUID.  This is the
    case during pvmoves, when moving from one segment of an LV
    to another, or when resizing a mirror, etc.  In these cases,
    a new log is created with the same UUID and loaded in the
    "inactive" slot.  When a device-mapper "resume" is issued,
    the "live" table is deactivated and the new "inactive" table
    becomes "live".  (The "inactive" table can also be removed
    via a device-mapper 'clear' command.)
    The above two issues were colliding.  More than one log was being
    created with the same UUID, and there was no way to distinguish
    between them.  So, sometimes the wrong log would be swapped
    out during the exchange.
    The solution is to create a locally unique identifier,
    'luid', to go along with the UUID.  This new identifier is used
    to determine exactly which log is being referenced by the kernel
    when the log exchange is made.  The identifier is not
    universally safe, but it does not need to be, since
    create/destroy/suspend/resume operations are bound to a specific
    machine; and these are the operations that make up the exchange.
    Signed-off-by: default avatarJonathan Brassow <jbrassow@redhat.com>
    Signed-off-by: default avatarAlasdair G Kergon <agk@redhat.com>
dm-log-userspace-transfer.h 467 Bytes