[time-nuts] Re: Clock display on Linux systems?

[Lux, Jim]

On 12/8/21 2:15 PM, Bill Dailey wrote:
> You can also set them up so they don’t write to the SD once everything is set.  SD’s will last forever like this.  Basically read only and RAM disk.


yes indeed - these days, with lots o'RAM on a rPi, you should boot off 
the SD (or eMMC) and run out of RAM.  For a "clock" application, you 
could probably structure your writes to SD (for nonvolatile storage of 
logs, etc.) so that you limit the number of writes. If you log once an 
hour that's just under 9000 writes/year.

Typical MLC flash is good for at least 10,000 erase cycles on a page. 
Writing data to an erased page (or the part that's not already written) 
doesn't wear it out, but changing data in the middle of a file does, 
because you have to erase it (consuming life), and then rewrite.

There are Journaling File Systems that deal with this, but I doubt 
they're compatible with the wear leveling systems in commercial SD 
cards. Basically, the SD card has a controller that exposes a 
generalized interface, with the wear leveling hidden from you, and if 
it's hidden, then the JFS doesn't really know how to manage the device.

I don't know, though, it's a fertile ground - and someone may have a 
nice JFS for a common distro for RPi and SD card.


If you want to get real down and dirty, there are also clever schemes 
that write all ones or zeros (depending on the device), instead of 
erasing, and then the reader of the file knows that this means "not 
used" - Much like the RUBOUT character on paper tape, or a similar 
scheme used with PROMS where you don't want to erase it.