Docs: Mention that fqdn_rand accepts strings as seed additions

[nfagerlund]

No description provided.

[slippycheeze]

I only just noticed this; that change is technically accurate, but I completely fail to understand what the purpose of the arguments is, or why you would supply it. (...and I understand how a PRNG would be implemented, putting me one up on the readers that this is most important for, I suspect. :)

Would it be possible to augment this with something that explained what that did, and when to use it?

For example, something akin to:

The second argument is a string, which is mixed into the random number seed. This is most often useful to identify the "purpose" of the call, if you use it multiple times for scheduling jobs, so that you don't get a thundering herd of processes starting at the same time -- because the number is unpredictable but identical on each call otherwise.

An example of uses would be:
fqdn_rand(30, "update packages")
fqdn_rand(30, "background fsck")

This helps to schedule your package updates and your background fsck operation at different times on the same machine, while still keeping their timing stable.

(I think those are terrible examples of the sort of jobs that might benefit from this, but I can't think of anything better off-hand.)

[nfagerlund]

Wow, that would be excellent. Does anyone have better examples of the types of jobs one would use this for, or is Daniel being self-deprecating again and those are actually fine? :)

[slippycheeze]

Wow, that would be excellent. Does anyone have better examples of the types of jobs one would use this for, or is Daniel being self-deprecating again and those are actually fine? :)

They are relatively abstract / advanced jobs, which wouldn't be
familiar to most sysadmins or most systems. Unfortunately, I couldn't
think of examples that would fit the bill; I wanted:

• something that isn't part of the core OS, but that you schedule yourself
• something that is "costly" in some way
• two of them

It might actually make sense to use "run the puppet agent" as one of
them -- most people know that is potentially expensive, and it isn't
scheduled by the OS by default. (OTOH, do we recommend cron for
running the agent these days? Maybe it isn't great after all.)

Something like "RAID disk checking" might make sense, or it might not,
depending on the users.

I guess, really, it doesn't matter that much what the examples are,
and you could call them "expensive job one" and "expensive job two" or
something if you wanted.