Re: Need for HW-clock independent timestamps

[Kalin KOZHUHAROV <me.kalin@gmail.com>]

On Sat, May 12, 2018 at 12:07 AM, Axel Neumann <neumann@cgws.de> wrote:
> We have the following chicken-egg problem:
> We are using WG on openwrt devices which do not have a hardware clock so
> that time is resetted after each reboot.
> Because internet access shall be routed via WG tunnels the internet and
> network-time services (NTP) is not available unless WG works properly.
> But, guess, to reconnect, WG needs a greater time than before it
> rebooted :-(
>
> I heard that a suggested solution is to periodically save the current
> time to filesystem and then fix it during boot based on the last saved
> one. But all embedded devices use flash memory with a limited amount
> (about 10^5) of write cycles before they become unstable which would
> destroy the flash within weeks.
>
Good/Hard problem!

> Any ideas how to circumvent this problem?
>
I only see 2 ways:
* hardware: add a cheap RTC (best), or storage (USB, SD)
* policy: add exception for ntpdate on base UDP (not WG) (or have
dedicated server/s for that, which you control).

BTW, while flash write cycles are indeed limited, you are writing a
very small data (that causes a single block to be rewritten).
So a device with say 32 MB(8000 x 4K blocks) will last about 9000
years if you write one block once per hour until 10% (=800) of its
cells are past 10^5 writes.
(or 3 years if you write once per second!)
But my knowledge says 10^3-10^4 write cycles (so still 90-900 years for 1/hour).

If you have a large number of devices of the same hardware, I'd
suggest a "kill test" of one of them, by rewriting its flash. You can
further use specific file-systems optimized for NAND/NOR flash.

Cheers,
Kalin.