On 08/15/2014 11:43 AM, Matthew Booth wrote:
On 15/08/14 16:12, Andrew Laski wrote:
On 08/08/2014 08:42 AM, Nikola ─Éipanov wrote:
On 08/06/2014 07:54 PM, Jay Pipes wrote:
I bring this up on the mailing list because I think Liyi's patch offers
an interesting future direction to the way that we think about our retry
approach in Nova. Instead of having hard-coded or configurable interval
times, I think Liyi's approach of calculating the interval length based
on some input values is a good direction to take.

This indeed is a problem that we've seen bite us a number of times, and
I tried to solve it by proposing [1] but didn't get to work on it
further yet.

Having said that - after thinking about it more, I was not sure I like
my own approach in [1] on the grounds of it being too generic (and
overly elaborate) for the particular problem it is solving.

I was then thinking of something similar to what is proposed here, where
we would have a waiting time that is a function of a value that we could
query Cinder on. Allocation rate proposed here seems to fit this nicely,
but in my mind we would have a way to query cinder about it instead of
hardcoding it, however this can be done later and in cooperation with
the Cinder team.
I like this direction a lot, because the allocation time depends on
Cinder and its backend more than anything.  Excepting perhaps image
transfer speeds.

2) We should deprecate the CONF.block_device_allocate_retries_interval
option only, and keep the CONF.block_device_allocate_retries
configuration option as-is, changing the help text to read something
like "Max number of retries. We calculate the interval of the retry
based on the size of the volume."

I'd go with this as the number of retries can still be useful as a tool
for easy workaround and troubleshooting, but I'd put a big disclaimer
that it is mostly meant for debugging/workaround purposes.
I disagree a bit with having a knob purely for debugging/workaround.  I
think this is a place where it's very useful to have the knob though.
The figures in the review seem sound, but this is probably not a place
where one size is going to fit all.  Until/unless we can get some
coordination between Nova and Cinder on this I think having a knob that
is intended to be used is the right way to go.
FYI, this bug is admittedly only from code inspection, but I think many
interval config variables are currently broken in Nova:

https://bugs.launchpad.net/nova/+bug/1354403

If they've never been used, they're presumably of limited value.

Fair point. Though it's likely they'll be fixed as soon as someone starts tuning them and finds them broken. And it'll be easier to fix them than to remove them and add them in later. Which is not to say that all of them are useful.

But the two values in question here are ones I've been tuning recently and know that they work.


Matt

N.

[1] https://review.openstack.org/#/c/87546/

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