[Xen-users] [XCP] primary/primary DRBD 8.4.0-1 LVM-based shared SR (xcp 1.1) preformance tuning

[Jakob Praher <jakob@xxxxxxxxxxx>]

Hi all,

we have followed the very good HOWTO by
http://wherethebitsroam.com/blogs/jeffw/drbd-xcp-05 and set up DRBD on
XCP 1.1 in primary/primary mode.

It works fine, but I am wondering how to squeeze more performance out of
the system (we currently use a crossover GB Ethernet connection). When
writing a 1 GB file on a guest I get write performance of about 5MB/s
(idle). We have disabled all swap partitions. The Disks are capable of
writing at much higher rate, as well the network connection allows a
much higher rate.

Because of active/active we have to use protocol C, but I am not sure if
this is the real bottleneck.

What are your experiences with performance improving DRBD performance.
Do you have any tricks?

When looking into performance tuning DRBD, I found the max-bio-bvecs
settings as a performance factor. I set it to 1 (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) ->
DomU FS
    # 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;

Best,
Jakob


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