• Tejun Heo's avatar
    block: implement mixed merge of different failfast requests · 80a761fd
    Tejun Heo authored
    Failfast has characteristics from other attributes.  When issuing,
    executing and successuflly completing requests, failfast doesn't make
    any difference.  It only affects how a request is handled on failure.
    Allowing requests with different failfast settings to be merged cause
    normal IOs to fail prematurely while not allowing has performance
    penalties as failfast is used for read aheads which are likely to be
    located near in-flight or to-be-issued normal IOs.
    This patch introduces the concept of 'mixed merge'.  A request is a
    mixed merge if it is merge of segments which require different
    handling on failure.  Currently the only mixable attributes are
    failfast ones (or lack thereof).
    When a bio with different failfast settings is added to an existing
    request or requests of different failfast settings are merged, the
    merged request is marked mixed.  Each bio carries failfast settings
    and the request always tracks failfast state of the first bio.  When
    the request fails, blk_rq_err_bytes() can be used to determine how
    many bytes can be safely failed without crossing into an area which
    requires further retrials.
    This allows request merging regardless of failfast settings while
    keeping the failure handling correct.
    This patch only implements mixed merge but doesn't enable it.  The
    next one will update SCSI to make use of mixed merge.
    Signed-off-by: default avatarTejun Heo <tj@kernel.org>
    Cc: Niel Lambrechts <niel.lambrechts@gmail.com>
    Signed-off-by: default avatarJens Axboe <jens.axboe@oracle.com>
blk-merge.c 11 KB