[OpenAFS] Re: vos move speed rates

Andrew Deason adeason@sinenomine.net
Tue, 12 Apr 2011 15:59:22 -0500

On Tue, 12 Apr 2011 16:20:25 -0400
Eric Chris Garrison <ecgarris@iupui.edu> wrote:

> I've asked something similar before, but I could use some insight on
> how vos moves actually work.  I have a volume being vos move'd from
> one server to another right now, poking along in the range of about
> 15-20 Mbit/s (on a Gigabit ethernet connection, on the same switch as
> the other server).  Other times, I've seen performance more like I
> expect, something around 300 Mbit/s.

How are you measuring these speeds? Are you looking at the actual link
utilization, or just looking at how long it takes a volume to transfer,
and how large the volume is?

> Do the contents of a volume have an impact on the speed at which they
> are moved?  That is, if it's composed of many smaller files, does the
> metadata cause overhead, slowing the transfer down?

Yes. If you have lots of little files, there is considerably more
overhead, and thus more data to transfer.

You can get a feel for how long a transfer will take based on how large
the dump is for a volume. You can get what the size would be without
having to actually generate the dump by running "vos size -dump". That
includes the dump format overhead, and _should_ be generally indicative
of how fast it will go. It's a better indicator than just the 'vos ex'
size of a volume, anyway.

> What other differences in the contents of a volume might make transfer
> speeds orders of magnitude different?  Are there any tricks to making
> slower volumes move more quickly?

I think that's the only one I'm aware of that people have come across.
Conceivably there may be some odd directory usage patterns that may
result in large directory blobs for very few or small files, but I don't
think I've seen that.

If you find that it's faster to dump the volume to local disk and
transfer it to the other machine some other way (e.g. over ssh), you can
just do that and 'vos restore' on the other side. But that's obviously
not that great.

Otherwise there's currently not much you can really do to help it along,
besides just preventing your volumes from getting that large. There are
tunables for Rx transport in general, but if you're fine on some
volumes, it sounds like that's not your problem.

Andrew Deason