Re: Hammer: Transactional file updates

[Michael Neumann <mneumann@xxxxxxxx>]

Martin Schut wrote:
> On Fri, 01 Aug 2008 17:36:13 +0200, Michael Neumann <mneumann@ntecs.de> 
> wrote:
>
> > Jasse Jansson wrote:
> > >  On Aug 1, 2008, at 1:09 PM, Michael Neumann wrote:
> > >
> > > > Hi,
> > > >
> > > > So Hammer does not guarantee "transctional consistency" of data in case
> > > > of a crash, only that of meta-data, right?
> > > >
> > > > Is there a method to guarantee the write to be transactional, so that
> > > > I either have the previous "version" of the file or the version that I
> > > > wrote? Like this:
> > > >
> > > >   fd = open(file);  // behaves like START TRANSACTION
> > > >   read(fd, ...);
> > > >   write(fd, ...);
> > > >   close(fd);        // behaves like COMMIT
> > > >
> > > > That would be incredible cool (and very useful!) and could play well 
> > > > due
> > > > to Hammers historical nature.
> > >  You are talking about COW (copy on write), right.
> > > It slows things down, but it's cool.
> >
> > Well, Hammer's history-retention IMHO is similar, just that it is less
> > fine-grained than COW.
> >
> > I'm not sure how Hammer internally works, but I think that a single
> > write() can generate a new version of that file (at least if you wait 30
> > seconds). What I'd like to have is the ability to do multiple writes to
> > that file and once I reach the end of my write transaction switch the
> > file to use that new version. During that period of time, other readers
> > would see the old content. Basically like a transaction in a database,
> > but provided by Hammer so that I don't have to reimplement transactions
> > myself ;-).
>
> Sounds cool, however what to do once a application crashes. I suppose 
> you will not commit. But then, a lot of log-files are not written at 
> all. Also utils like tail -f won't work to.

It shouldn't be default behaviour. One would have to specify:

  fd = open("file", O_TRANSACT);

If the application would crash, the file would be in the previous state.

Regards,

  Michael