Re: [Gnu-arch-users] Idea for a feature

[James Blackwell]

On Sat, Dec 27, 2003 at 11:39:34AM +1100, Robert Collins wrote:

James Blackwell write something like:
> Why not add a =meta-info/last_changed file to archives?"

Then Robert Collins wrote:

> My first concern would be race conditions. All writers would need to
> update the same file. Secondly, if you have different security on
> different branches, you'd need to allow everyone with any write access
> to alter meta-data, which is IMO not desirable.


I think it would probably be ok. No matter what happens, we always end up
with the most recent date. if either finishes first, the latter one wins
and gives us the date/time we want. If they both finish at the same time (smp?),
we end up writing the same date/time twice and we still end up with what
we want. Way off in the distance, the only thing mirrors care about is
"Is that date there newer than my date here?"

Your second objection is certainly more serious. Though I can imagine that
situation existing, I don't think it would be a common one. I would
imagine that in most cases, rather than working in an archive in which
permissions are restricted (except for a certain branch), people would
create their own archives.

Maybe if I illustrated what we'd be saving. I ran netwatch and ran my
updatemirror script. I then syncronized against 34 mirrors. At the minor
extreme, archive-mirror takes 10-20k of bandwith. On the opposite extreme,
a large mirror takes 2.6 megabytes. As a very ballpark guess, I'd average
a mirror sync at about 200k for both directions, combined. Though this
isn't very significant for me (34 mirrors * 3 times an hour * 24 hours a
day * 200 kilobytes =~ 478 megabytes),  it would be very significant for
users on dialup.  Each time a user on dialup wants to update their mirror,
they have to perform a 200k download, which works out to about 30 seconds.

Much more significant is the cost of running a mirror. jgoerzen has a huge
mirror (about 450 megabytes) of the linux kernel. Every time anybody,
anywhere, updates a mirror, he's stuck serving 2.6 megabytes of data that
essentially says "nothing changed". If he ends up with 100 people
mirroring his archive 3 times a day in which nothing ever changes, he's going
to use up **17 gigabytes** of bandwidth each month.



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