[Box Backup] backup on nfs-mounted storage

[Ben Summers]

On 19 Oct 2006, at 11:38, Martin Ebourne wrote:

> Ben Summers <ben@fluffy.co.uk> wrote:
>> What happens if you use both? Are the two mechanism independent?
>
> From the manpage on Linux:
>
>        flock(2) does not lock files over NFS.  Use fcntl(2)  
> instead: that does
>        work  over  NFS,  given  a  sufficiently  recent version of  
> Linux and a
>        server which supports locking.
>
>        Since kernel 2.0, flock(2) is implemented as a system call  
> in  its  own
>        right  rather  than  being  emulated  in the GNU C library  
> as a call to
>        fcntl(2).  This yields true BSD  semantics:  there  is  no   
> interaction
>        between the types of lock placed by flock(2) and fcntl(2),  
> and flock(2)
>        does not detect deadlock.
>
> Hence the wariness over new holes...
>
> What does BSD do about file locking over NFS?

Well I think it "works" if you run the right daemons. But "works"  
only means "works as well as NFS locking ever does".

The only thing for which locking is actually required is locking  
accounts against duplicate write access (assuming that we get rid of  
lib/raidfile -- I don't think there's any need for it any more). It  
might be better to do the work to replace all RaidFile operations  
with normal FileStream operations, check that locking can be done  
away with, then implement a better mechanism for locking accounts.

Perhaps something like creating a file called

   lock.hostname.pid

So, to lock, you create a file with your hostname and PID.  
Afterwards, check to see if there is another lock file. If so, delete  
yours and try again. Then you just need a mechanism to check for  
stale locks. Not so easy if the hostname isn't one for your machine  
though.

Ben