Swapping to a large sparse file

[jd1008]

On 01/17/2015 03:52 PM, Chris Murphy wrote:
> On Fri, Jan 16, 2015 at 7:22 PM, jd1008 <jd1008 at gmail.com> wrote:
>> On 01/16/2015 07:11 PM, John Morris wrote:
>>> On Fri, 2015-01-16 at 14:26 -0700, jd1008 wrote:
>>>
>>>> In older "traditional" practices, swap space was normally
>>>> about twice the ram size. Today, with some systems having
>>>> 64 and even 128GB and even larger RAM, it becomes interesting
>>>> how big swap space should be. Where is the cutoff for performance?
>>>> Paging in and out 128GB memory space could prove to be itself a
>>>> performance bottleneck on very busy or memory bound servers.
>>> My advice is don't bother unless you know you need it.  I find 512MB or
>>> 1GB to be plenty of swap.  You need some swap just so the system can
>>> ditch memory that was used once to initialize code but isn't accessed
>>> again and other similar things that can be safely tossed to swap and
>>> forgot about.  But if the system is actually swapping hundreds of
>>> megabytes in and out you will quickly be in a world of pain.  Plus most
>>> of the time when that sort of memory pressure hits it is a runaway
>>> process that the OOM killer will eventually take out and having a lot of
>>> swap only increases how long you suffer with an almost totally
>>> unresponsive machine until that happens.  If you are swapping and it
>>> isn't a runaway process or an exception to process a one off huge
>>> dataset it is a sign you need to bite the bullet and get more ram.  If
>>> you know you are going to need a lot of swap to get through some script
>>> you banged out that allocates memory like mad, just add an extra
>>> swapfile on a temporary basis and drop it when you are done.  You are
>>> allowed to have multiple swap files, partitions or any combination of
>>> them within sensible limits.
>>>
>>>
>> Well, I need at least 8GB of swap if I want to hibernate, and often
>> I need to hibernate so I can let an important app continue where
>> it left off (apps that do not depend on an internet continuous connection)
>>
> Does hibernation (suspend to disk) even work with a swapfile? It seems
> to depend on the root file system becoming available, so that
> /etc/fstab can state what/where the hibernation file is located. But
> for a loop mounted swap file, in addition the loop needs to be setup
> first. Before that time the swap device doesn't exist. So this seems
> difficult.
>
> If you want to hibernate you need 1x RAM at least. The installer
> (specifically python-blivet) has laptop hibernation specific code to
> create a swap partition that's 2x RAM. This code doesn't get used by
> default in Fedora.
>
> So if you need to hibernate I think you're best off with a swap
> partition that's 2x RAM. It's suggested it might work with less than
> that, but the recommendation is to account for the possibility some
> swap is in use at the time the computer hibernates, so it needs enough
> space to hold what was use for swap and all of RAM.
>
>
Agreed!
The lightweight solution for me is to
get a nand flash stick of 12 or 16GB to remain in the
multi flash reader/writer slot at all times.