On Wed, 18 Mar 2015 23:39:11 -0600 Eric Mei <meijia@xxxxxxxxx> wrote: > From: Eric Mei <eric.mei@xxxxxxxxxxx> > > When array is degraded, read data landed on failed drives will result in > reading rest of data in a stripe. So a single sequential read would > result in same data being read twice. > > This patch is to avoid chunk aligned read for degraded array. The > downside is to involve stripe cache which means associated CPU overhead > and extra memory copy. > > Signed-off-by: Eric Mei <eric.mei@xxxxxxxxxxx> > --- > drivers/md/raid5.c | 15 ++++++++++++--- > 1 files changed, 12 insertions(+), 3 deletions(-) > > diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c > index cd2f96b..763c64a 100644 > --- a/drivers/md/raid5.c > +++ b/drivers/md/raid5.c > @@ -4180,8 +4180,12 @@ static int raid5_mergeable_bvec(struct mddev *mddev, > unsigned int chunk_sectors = mddev->chunk_sectors; > unsigned int bio_sectors = bvm->bi_size >> 9; > > - if ((bvm->bi_rw & 1) == WRITE) > - return biovec->bv_len; /* always allow writes to be > mergeable */ > + /* > + * always allow writes to be mergeable, read as well if array > + * is degraded as we'll go through stripe cache anyway. > + */ > + if ((bvm->bi_rw & 1) == WRITE || mddev->degraded) > + return biovec->bv_len; > > if (mddev->new_chunk_sectors < mddev->chunk_sectors) > chunk_sectors = mddev->new_chunk_sectors; > @@ -4656,7 +4660,12 @@ static void make_request(struct mddev *mddev, > struct bio * bi) > > md_write_start(mddev, bi); > > - if (rw == READ && > + /* > + * If array is degraded, better not do chunk aligned read because > + * later we might have to read it again in order to reconstruct > + * data on failed drives. > + */ > + if (rw == READ && mddev->degraded == 0 && > mddev->reshape_position == MaxSector && > chunk_aligned_read(mddev,bi)) > return; Thanks for the patch. However this sort of patch really needs to come with some concrete performance numbers. Preferably both sequential reads and random reads. I agree that sequential reads are likely to be faster, but how much faster are they? I imagine that this might make random reads a little slower. Does it? By how much? Thanks, NeilBrown
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