[DRBD-user] primary/primary via lvm on xen (centos 5) and drbd 8.4 - performance tuning

Jakob Praher jakob at praher.info
Sun Sep 25 10:44:40 CEST 2011

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

Dear list,

When looking into performance tuning DRBD, I found the max-bio-bvecs
settings as a performance factor. I set it to 1 due to the comment below
(which is slow). Is this problem still the case (we are using DRBD
version 8.4.0-1)? Can one enforce the alignment settings for ext3/ext4?

    # In some special circumstances the device mapper stack manages to
    # pass BIOs to DRBD that violate the constraints that are set forth
    # by DRBD's merge_bvec() function and which have more than one bvec.
    # A known example is:
    # phys-disk -> DRBD -> LVM -> Xen -> missaligned partition (63) ->
    # Then you might see "bio would need to, but cannot, be split:" in
    # the Dom0's kernel log.
    # The best workaround is to proper align the partition within
    # the VM (E.g. start it at sector 1024). (Costs 480 KiByte of storage)
    # Unfortunately the default of most Linux partitioning tools is
    # to start the first partition at an odd number (63). Therefore
    # most distribution's install helpers for virtual linux machines will
    # end up with missaligned partitions.
    # The second best workaround is to limit DRBD's max bvecs per BIO
    # (= max-bio-bvecs) to 1. (Costs performance).
    # max-bio-bvecs 1;

Thanks in advance,

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