Saturday, 16 January 2016

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

On Fri, Jan 15, 2016 at 2:25 AM, Seth Arnold <[email protected]> wrote:
> On Thu, Jan 14, 2016 at 12:27:58PM +0200, Dustin Kirkland wrote:
>> Moreover, just 'sudo apt-get install swapspace' and watch as swapfiles
>> are created/deleted as needed. If your root disk is lvm-encrypted,
>> then obviously such swap files are encrypted, too.
>
> I've been severely skeptical of the swapspace package:
>
> - Swap is used when the system is already under pressure; a few hundred
> megs is great and probably for the best but if the system is actively
> pushing beyond that then it's being pushed too hard.
>
> - If the swap space is going to be allocated on the fly, that means the
> disk blocks have to zeroed on the fly, when the system is under
> pressure, rather than at some leisurely time beforehand.
>
> - If the swap space is allocated on a filesystem, it's probably being
> allocated from a fragmented filesystem that's 90% full rather than a
> nice contiguous block of space as it would with a swap partition.
>
> - Accessing further into a file may involve loading multiple indirect
> blocks from disk into unswappable kernel memory. A swap partition does
> not require indirection blocks.
>
> - If the swap space allocated from a filesystem pushes the filesystem to
> 95% full (or whatever is left after accounting for reserved blocks),
> programs will error and almost nothing handles "disk full" errors
> gracefully. Swap partitions do not cause surprise gigabyte losses in
> free space.
>
> - Swap files can't be allocated from btrfs filesystems and probably
> shouldn't be allocated from zfs filesystems either. (Swap on zvols,
> maybe.)
>
> Perhaps the swapspace package uses some tasteful tunables to mitigate
> against my concerns but the end result is that it contributes extra load,
> extra IO pressure, and extra uncertainty at a time when the system is
> already experiencing too much load, too much IO pressure, and too much
> uncertainty.
>
> The risks and downsides of swapspace feel like a lot compared to the
> slight hassle of having the installer make a swap partition.

I count 4 "if's", 3 "probably's", 2 "should/would's", and 1 "maybe" in
that reply :-)

Perhaps try it out?

I've been running it and /tmp on tmpfs for several years (since before
~precise) on my desktop on an encrypted LVM partition. My machine has
a lot of memory (16GB), though I do push it hard), and have never
noticed a swapspace-related problem. I've also used this combination
on hundreds of servers, and several production systems.

Here's /etc/swapspace.conf, with its tunables...

http://paste.ubuntu.com/14516155/

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