• Mustafa Mesanovic's avatar
    dm stripe: implement merge method · 29915202
    Mustafa Mesanovic authored
    Implement a merge function in the striped target.
    
    When the striped target's underlying devices provide a merge_bvec_fn
    (like all DM devices do via dm_merge_bvec) it is important to call down
    to them when building a biovec that doesn't span a stripe boundary.
    
    Without the merge method, a striped DM device stacked on DM devices
    causes bios with a single page to be submitted which results
    in unnecessary overhead that hurts performance.
    
    This change really helps filesystems (e.g. XFS and now ext4) which take
    care to assemble larger bios.  By implementing stripe_merge(), DM and the
    stripe target no longer undermine the filesystem's work by only allowing
    a single page per bio.  Buffered IO sees the biggest improvement
    (particularly uncached reads, buffered writes to a lesser degree).  This
    is especially so for more capable "enterprise" storage LUNs.
    
    The performance improvement has been measured to be ~12-35% -- when a
    reasonable chunk_size is used (e.g. 64K) in conjunction with a stripe
    count that is a power of 2.
    
    In contrast, the performance penalty is ~5-7% for the pathological worst
    case stripe configuration (small chunk_size with a stripe count that is
    not a power of 2).  The reason for this is that stripe_map_sector() is
    now called once for every call to dm_merge_bvec().  stripe_map_sector()
    will use slower division if stripe count isn't a power of 2.
    Signed-off-by: default avatarMustafa Mesanovic <mume@linux.vnet.ibm.com>
    Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
    Signed-off-by: default avatarAlasdair G Kergon <agk@redhat.com>
    29915202
dm-stripe.c 10.1 KB