Wednesday, 20 January 2016

Re: RFC on Cloud Images: Make /tmp a tmpfs

On Wed, Jan 20, 2016 at 5:45 PM, Robie Basak <[email protected]> wrote:
> On Wed, Jan 20, 2016 at 05:27:51AM +0100, Dustin Kirkland wrote:
>> > I'd like to see even some rudimentary experiments done with realistic
>> > workloads before saying this is a better idea than leaving things as
>> > they are. We've all speculated and provided anecdotal evidence enough to
>> > warrant such an investigation for those who speculate it will be a
>> > worthwhile change.
>>
>> Sure, done! You can find a detailed statistical analysis, as well as
>> the raw data for your download and treatment at:
>
> This is useful. Thank you for this research!
>
>> http://blog.dustinkirkland.com/2016/01/data-driven-analysis-tmp-on-tmpfs.html
>
> Are we sure that using /dev/zero is a fair test? I hope this isn't
> shortcutted somehow in the tmpfs case.

No, I'm not sure. But I also can't find any "faster" source of data
than /dev/zero :-) /dev/urandom can't keep up, nor can any /dev/

Have a look at this page to understand how benchmarking with dd works,
and what it actually does:

https://romanrm.net/dd-benchmark

I've also copied Colin King, who is the king of performance
benchmarking, in my book :-)

>> Based on a statistical analysis of 502 physical and virtual servers
>> running production and test services at Canonical (including
>> databases, websites, OpenStack, ubuntu.com, launchpad.net, et al.),
>> 96.6% of them could fit all of the data they currently have in /tmp,
>> entirely in half of the free memory available in the system. That
>> ratio goes up to 99.2% of the systems surveyed (i.e., all but 4) when
>> one takes into account both free available memory and available swap.
>> The remaining 4 systems are are currently using [101 GB, 42 GB, 13 GB,
>> and 10 GB] of swap, respectively, and are themselves somewhat special
>> cases.
>
> Even if they are special cases, surely that's something we need to
> consider for our users? If your data is representative, isn't that
> around 1% of users who will be impacted or broken somehow by this change
> in defaults?

In fact it's our job as Ubuntu developers to make intelligent,
informed, data-driven decisions about opinionated defaults that cover
the vast majority of our users and clearly document solutions to
problems affecting the remainder. We do this all the time, in all
aspects of Ubuntu, from upstream project versions, to default package
sets and default configuration options!

It's important that we do so carefully, tastefully, scientifically,
and that we course correct gracefully when we're wrong ;-)

> What would be the guidance for 1) users; and 2) upstreams; if they want
> large temporary filesystem space after this change? Would that be to use
> /var/tmp in all relevant cases? And for upstreams, is this something
> that they will accept that they can do universally, or is it behaviour
> that they have to differentiate depending on the distro upon which they
> are running?

Good question... Solutions to insufficient available space in /tmp on
tmpfs include any and all of the following:

(a) commenting out the "tmpfs /tmp tmpfs rw,nosuid,nodev" line in /etc/fstab
(b) setting $TMPDIR to /var/tmp (or elsewhere) in your shell profile
(c) pointing your application at /var/tmp (or elsewhere)
(d) allocating sufficiently large swap partition(s) or swap file(s)
to overflow into
(e) using the swapspace package to dynamically grow/shrink swap on demand

>> Moreover, Ubuntu is hardly the first Linux/UNIX distribution that has
>> considered putting /tmp on tmpfs by default. Solaris has used a tmpfs
>> since 1994. Fedora moved to /tmp on tmpfs in 2012, as did ArchLinux.
>> Things seem to be working okay there...
>
> This is really useful to know, thanks.
>
> Robie

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