[Info-vax] Re; Spiralog, RMS Journaling (was Re: FREESPADRIFT)

[Johnny Billquist]

On 2016-06-22 09:27, lawrencedo99 at gmail.com wrote:
> On Wednesday, June 22, 2016 at 3:30:03 AM UTC+12, Johnny Billquist wrote:
>> On 2016-06-18 11:19, Lawrence D’Oliveiro wrote:
>>
>>> Time Machine plays fast and loose with your filesystem integrity (multiple hard links to directories), and you pay the price in reliability <http://arstechnica.com/civis/viewtopic.php?t=1252811>.
>>
>> Multiple hard links are not a problem for file system integrity.
>
> Yes they are.

No, they are not. Like I said, you actually have hard links for 
directories all over the place already, you just don't realize it.

> Consider either (or both) of the following questions:
>
> * How do you prevent reference cycles?

What is the problem with that? You already get those if you just follow 
symlinks blindly as well. There is nothing dangerous with cycles. I see 
a lot of people believing that there is some problem with that, without 
really understanding what that problem would be. There are also well 
known algorithms to detect when you are in a cycle.

> * What happens if you don’t?

Nothing interesting. Programs that blindly dive down paths will possibly 
run until they a) run out of memory, or b) get an error because their 
path exceeds MAXPATHLENGTH.

>> (Hint: what do you think . and .. is?)
>
> Those cause their own share of problems--look at all the code in directory-search APIs that have to special-case those useless entries and skip over them...

I think most people would seriously disagree with you over their 
uselessness. :-)
And most code do not special treat them in any way at all. They are 
directory entries just like any other.
What code is special treating them?
(And don't say "ls -A")

There is a different reason why systems do not generally allow users to 
create arbitrary hardlinks to directories these days (but it used to be 
possible back in time). But the reason is something completely 
different. I'll let you think about it for a while, and see if you can 
figure it out. :-)

	Johnny