[Dovecot] Avoiding Hardlinks (was: Something other than dotlock for uidlist locking?)

[Timo Sirainen]

On 9.8.2006, at 23.44, James Berry wrote:

> In seaching the code for "link(", I find only four affected files/ 
> cases. Here I look into possible implementations for a patch to  
> AVOID_HARDLINKS. Such a patch would dramatically improve  
> performance on Mac OS X with the HFS+ filesystem.

Hmh. The only annoying problem with this is that it should be runtime  
configurable, but there's no clean way to do that without larger  
changes.

> Timo, would you mind commenting on these to make sure my  
> understanding is correct? And for the last one, I need a suggestion  
> on how to proceed.
>
> file-copy.c:  [EASY]
>
> 	Looks like it would be easy to take the already coded case for  
> when try_hardlink is false.

Yes, easy. This case could even be #ifdefed. Although currently the  
only user of file_copy() is copying ssl-parameters.dat from /var/lib/ 
dovecot to /var/run/dovecot, and I don't see it being used elsewhere  
anytime soon.

> file-dotlock.c: [EASY]
>
> 	In dotlock_create, use try_create_lock_excl rather than  
> try_create_lock_hardlink.

This really should rather be configurable from dovecot.conf, but  
there's no simple way to do that, except something like a getenv()  
hack inside the file-dotlock.c code..

I don't think this should be #ifdefed since O_EXCL could break if  
you're using NFS.

> maildir-copy.c: [LOOKS EASY]
>
> 	In maildir_copy(), take the fallback case using mail_storage_copy.

This is the default (maildir_copy_with_hardlinks=no) so no need to do  
anything.

> maildir-save.c: [NEED HELP]
>
> 	In maildir_file_move(), there seems to be no fallback case, and  
> I'm unsure of the desired  symantics, so I could use some  
> suggestions here: can we just fall back into something like the  
> code in file-copy.c?

Like you noticed, this can be changed to just rename(). The only  
reason it's link()+unlink() is because maildir spec says it should be  
done like that. And in case the maildir filenames aren't really  
unique the link()+unlink() method makes sure that mails aren't  
overwritten, but that practically can't happen with Dovecot. This  
code could perhaps also be #ifdefed..
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