[Devel] Re: [BUG][cryo] Create file on restart ?

[Serge E. Hallyn]

Quoting Matt Helsley (matthltc at us.ibm.com):
> 
> On Wed, 2008-07-16 at 14:26 -0700, sukadev at us.ibm.com wrote:
> > Serge E. Hallyn [serue at us.ibm.com] wrote:
> > | Quoting sukadev at us.ibm.com (sukadev at us.ibm.com):
> > | > Serge E. Hallyn [serue at us.ibm.com] wrote:
> > | > | Quoting sukadev at us.ibm.com (sukadev at us.ibm.com):
> > | > | > 
> > | > | > cryo does not (cannot ?) recreate files if the application created
> > | > | 
> > | > | I think that's for the best.
> > | > | 
> > | > | Don't you?
> > | > 
> > | > I can understand that configuration or data files should exist, but
> > | > not sure about temporary or log files that an application created
> > | > upon start-up and expects to be present. Should the admin find
> > | > out about them and create them by hand before restart ?
> > | 
> > | I think the admin should have set the destination environment such that
> > | the task is restarted in the same network fs in the same directory, with
> > | no files having been deleted.
> 
> [Assuming Serge meant: s/network fs/network, fs,/]

Well no I meant a network filesystem - at least if you're migrating apps
around a cluster.

> > or new files created ? For instance if the application was checkpointed
> > before it created a temporary file with O_EXCL flag, that temporary
> > file must not exist when restarting ?
> 
> 	I think that's not a problem given my assumptions above. The filesystem
> that the application restarts in would be the same because the admin
> should have set up the restart environment as Serge suggested. The admin
> can't rely on restart in an alternate environment. However, given
> knowledge of the application and environment, using an alternate
> environment may be a risk the admin is willing to take.

Yup.  But Suka is right that in the case of the checkpointed app
continuing to run for a bit before being killed and restarted, it could
get out of whack with respect to the file system.

> > | Am I wrong?
> > 
> > So we take a snapshot of the FS and checkpoint the application. Do they
> > need to be atomic ?
> 
> 	If all the applications in a container are frozen then I think we can
> get fs snapshots consistent with checkpointed applications.
> Otherwise, yes, I think we'd be gambling that the checkpointed
> application isn't interacting with another, running, application via an
> intermittently-shared file.

What fun :)

I wonder whether the experience of users of c/r on sgi and cray could
teach us anything here.

-serge
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