• Javier González's avatar
    lightnvm: physical block device (pblk) target · a4bd217b
    Javier González authored
    This patch introduces pblk, a host-side translation layer for
    Open-Channel SSDs to expose them like block devices. The translation
    layer allows data placement decisions, and I/O scheduling to be
    managed by the host, enabling users to optimize the SSD for their
    specific workloads.
    
    An open-channel SSD has a set of LUNs (parallel units) and a
    collection of blocks. Each block can be read in any order, but
    writes must be sequential. Writes may also fail, and if a block
    requires it, must also be reset before new writes can be
    applied.
    
    To manage the constraints, pblk maintains a logical to
    physical address (L2P) table,  write cache, garbage
    collection logic, recovery scheme, and logic to rate-limit
    user I/Os versus garbage collection I/Os.
    
    The L2P table is fully-associative and manages sectors at a
    4KB granularity. Pblk stores the L2P table in two places, in
    the out-of-band area of the media and on the last page of a
    line. In the cause of a power failure, pblk will perform a
    scan to recover the L2P table.
    
    The user data is organized into lines. A line is data
    striped across blocks and LUNs. The lines enable the host to
    reduce the amount of metadata to maintain besides the user
    data and makes it easier to implement RAID or erasure coding
    in the future.
    
    pblk implements multi-tenant support and can be instantiated
    multiple times on the same drive. Each instance owns a
    portion of the SSD - both regarding I/O bandwidth and
    capacity - providing I/O isolation for each case.
    
    Finally, pblk also exposes a sysfs interface that allows
    user-space to peek into the internals of pblk. The interface
    is available at /dev/block/*/pblk/ where * is the block
    device name exposed.
    
    This work also contains contributions from:
      Matias Bjørling <matias@cnexlabs.com>
      Simon A. F. Lund <slund@cnexlabs.com>
      Young Tack Jin <youngtack.jin@gmail.com>
      Huaicheng Li <huaicheng@cs.uchicago.edu>
    Signed-off-by: 's avatarJavier González <javier@cnexlabs.com>
    Signed-off-by: 's avatarMatias Bjørling <matias@cnexlabs.com>
    Signed-off-by: 's avatarJens Axboe <axboe@fb.com>
    a4bd217b
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