• Vishal Verma's avatar
    nd_btt: atomic sector updates · 5212e11f
    Vishal Verma authored
    BTT stands for Block Translation Table, and is a way to provide power
    fail sector atomicity semantics for block devices that have the ability
    to perform byte granularity IO. It relies on the capability of libnvdimm
    namespace devices to do byte aligned IO.
    The BTT works as a stacked blocked device, and reserves a chunk of space
    from the backing device for its accounting metadata. It is a bio-based
    driver because all IO is done synchronously, and there is no queuing or
    asynchronous completions at either the device or the driver level.
    The BTT uses 'lanes' to index into various 'on-disk' data structures,
    and lanes also act as a synchronization mechanism in case there are more
    CPUs than available lanes. We did a comparison between two lane lock
    strategies - first where we kept an atomic counter around that tracked
    which was the last lane that was used, and 'our' lane was determined by
    atomically incrementing that. That way, for the nr_cpus > nr_lanes case,
    theoretically, no CPU would be blocked waiting for a lane. The other
    strategy was to use the cpu number we're scheduled on to and hash it to
    a lane number. Theoretically, this could block an IO that could've
    otherwise run using a different, free lane. But some fio workloads
    showed that the direct cpu -> lane hash performed faster than tracking
    'last lane' - my reasoning is the cache thrash caused by moving the
    atomic variable made that approach slower than simply waiting out the
    in-progress IO. This supports the conclusion that the driver can be a
    very simple bio-based one that does synchronous IOs instead of queuing.
    Cc: Andy Lutomirski <luto@amacapital.net>
    Cc: Boaz Harrosh <boaz@plexistor.com>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Jens Axboe <axboe@fb.com>
    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: Christoph Hellwig <hch@lst.de>
    Cc: Neil Brown <neilb@suse.de>
    Cc: Jeff Moyer <jmoyer@redhat.com>
    Cc: Dave Chinner <david@fromorbit.com>
    Cc: Greg KH <gregkh@linuxfoundation.org>
    [jmoyer: fix nmi watchdog timeout in btt_map_init]
    [jmoyer: move btt initialization to module load path]
    [jmoyer: fix memory leak in the btt initialization path]
    [jmoyer: Don't overwrite corrupted arenas]
    Signed-off-by: default avatarVishal Verma <vishal.l.verma@linux.intel.com>
    Signed-off-by: default avatarDan Williams <dan.j.williams@intel.com>