[openstack-dev] [tc] [all] OpenStack moving both too fast and too slow at the same time

Duncan Thomas duncan.thomas at gmail.com
Tue May 9 16:17:46 UTC 2017

On 5 May 2017 at 23:45, Chris Friesen <chris.friesen at windriver.com> wrote:
> On 05/05/2017 02:04 PM, John Griffith wrote:
>> I'd love some detail on this.  What falls over?

> It's been a while since I looked at it, but the main issue was that with LIO
> as the iSCSI server there is no automatic traffic shaping/QoS between
> guests, or between guests and the host.  (There's no iSCSI server process to
> assign to a cgroup, for example.)
> The throttling in IOPS/Bps is better than nothing, but doesn't really help
> when you don't necessarily know what your total IOPS/bandwidth actually is
> or how many volumes could get created.
> So you have one or more guests that are hammering on the disk as fast as
> they can, combined with disks on the cinder server that maybe aren't as fast
> as they should be, and it ended up slowing down all the other guests.  And
> if the host is using the same physical disks for things like glance
> downloads or image conversion, then a badly-behaved guest can cause
> performance issues on the host as well due to IO congestion.  And if they
> fill up the host caches they can even affect writes to other unrelated
> devices.
> So yes, it wasn't the ideal hardware for the purpose, and there are some
> tuning knobs, but in an ideal world we'd be able to reserve some
> amount/percentage of bandwidth/IOPs for the host and have the rest shared
> equally between all active iSCSI sessions (or unequally via a share
> allocation if desired).

So that's a complaint that it can't do magic with underspecced,
overloaded hardware, plus a request for fair-share I/O or network
scheduling? The latter is maybe something cinder could look at, though
we're limited by the available technologies - array vendors tend to
keep such things proprietary. Note that it is trivial to overload many
SAN too, both the data path and the control path.

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