Wednesday, 13 January 2016

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

Excerpts from Ben Howard's message of 2016-01-13 04:26:13 -0800:
> All,
>
> On the Ubuntu Cloud Images, we have a request to make /tmp a tmpfs. The
> rationale, from the bug:
> * Performance - much faster read/write access to data in /tmp
> * Security - sensitive data would be cleared from memory on boot,
> rather than written (leaked) to disk -- important for encryption
> scenarios
>
> Since the Ubuntu Cloud Images are used by a wide number of users, I
> wanted to gather feedback and gather consensus on whether or not we
> should make this change.

The two arguments in the bug are:

a) It is more secure
b) It is faster

(a) is only half-correct. _IF_ the system has 0 swap configured, then
tmpfs will never be on disk. But if there is swap, then it will very
likely end up on disk. The answer to this is LUKS, not tmpfs.

(b) is overly general. For some things, this is absolutely true. For
others, the opposite is true. Some tmp usage will overrun the available
RAM and end up using swap directly, which does not have all of the tuning
of the VFS layer, and thus will perform in a much less predictable
manner. Plus, some programs use temporary files specifically because
they know they're about to do something that will be overly expensive
to guarantee RAM speed on, like sorting a file of arbitrary size. The
choice to make in-ram things slower is generally made when the system
owner chooses how much swap to make available. So making those operations
which the developer has chosen to allow to be spooled onto disk faster,
isn't actually a big win. However, failing those operations because of
a lack of space in /tmp is quite frustrating and could be very costly if
they must be retried.

So, the current paradigm is actually the more conservative, more
generally acceptable approach. Making /tmp a tmpfs is an optimization
_for some use cases_, which I think should be fine, but doesn't seem
worth taking a risk with all cloud image use cases.

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