[Dovecot] SIS Implementation

[Timo Sirainen]

On Fri, 2009-08-14 at 12:40 -0700, Jason Fesler wrote:
> > Hard links would be the simplest implementation without needing a
> > separate database. Sure you could implement that too if you wanted to.
> 
> It would be worth checking the limits for hard links, and making sure they 
> are suitable for a large mail system using this scheme, without having a 
> fallback plan of some sort.
> 
> Looks like UFS hardlink limit is 32767; ext2 32000; reiser and jfs, 65535.
> http://www.dirvish.org/viewcvs/dirvish_1_2/FAQ.html?rev=2  see
> "Could linking between images be limited by a maximum link count?"

Well, if there are that many copies already it probably doesn't matter
much if the file gets duplicated a few times. The logic can be as easy
as:

if (link(a, b) < 0 && errno == EMLINK) {
  copy(a, tmp);
  rename(tmp, a);
  link(a, b);
}

So the old file with 32k links no longer exists in attachments dir, but
you have a new file which can be linked 32k times more.
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