Hi!
We recently noticed a big performance hit when upgrading distro kernel
versions (Ubuntu trusty->xenial) for bulk io ("dd if=/dev/zero of=/dev/sdb
bs=256k" as easy reproducable case), performance dropping from 1.8GB/s to
0.6GB/s on our raidsets. With some fiddling we found this to come from a
change between mainline kernels 3.18.21 and 3.18.22.
Christian Erhardt of helpful Ubuntu support helped pin this down to a
particular change where newer kernels trust what the HW raid controller
advertises for max_sectors_kb, even though it seems like the controller
doesn't like getting big merged IOs larger than 1024k.
Our discussion, data and conclusion is documented in:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1668557
Posting here to raise some awareness and maybe providing some feedback to
relevant maintainers for how to handle this case, where the hardware raid
controller seems to be able to technically handle large IOs, but at a
significant performance hit.
We're applying the workaround of capping max_sectors_kb to something
performance-sensible like the old value of 512, and hopefully this mail
can help someone else facing the same issues.
/Mattias Wadenstein
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