Re: [Qemu-devel] [PATCH] migration: adding migration to/from a file

[Uri Lublin]

Anthony Liguori wrote:
> Uri Lublin wrote:
> > Migration to file, reuses migration-to-fd.
> > Migration from file, uses qemu-fopen directly.
> >
> > The saved state-file should be used only once and removed (or used
> > with -snapshot, or a the disk-image should be copied), as the
> > disk image is not saved, only the VM state.
> >
> > Also there is not point of doing a _live_ migration to file (except
> > for debugging migration code), so I recommend to stop the VM before 
> > migrating its state to a file.
> >
> > An advantage migration-to-file over savevm/loadvm is that for the latter
> > a qcow2 is a requirement, while the former works for any image-format.
> >
> > Signed-off-by: Uri Lublin <address@hidden>
> > ---

> > +
> > +MigrationState *file_start_outgoing_migration(const char *filename,
> > +                                              int64_t bandwidth_limit,
> > +                                              int async)
> > +{
> > +    FdMigrationState *s;
> > +    int fd;
> > +
> > +    s = qemu_mallocz(sizeof(*s));
> > +    if (s == NULL) {
> > +        perror("file_migration: qemu_mallocz failed");
> > +        term_printf("file_migration: qemu_mallocz failed");
> > +        goto err1;
> > +    }
> > +
> > +    fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, 0600);
> > +    if (fd < 0) {
> > +        perror("file_migration: failed to open filename");
> > +        term_printf("file_migration: failed to open filename %s\n", 
> > filename);
> > +        goto err2;
> > +    }
> >   
>
> The migration code assumes that the file descriptor used is 
> non-blocking.  In general, open() on a file system cannot produce a 
> non-blocking file descriptor.

I can call fcntl with F_SETFL and O_NONBLOCK.

> You could either use the posix-aio code to implement migration to a file 
> or you could introduce a fork()'d process that wrote to a file from 
> stdin.  Although this is basically just exec dd of=.

We started with "exec dd..." for saving state in a file. It's obviously not the 
best solution, as it needs to fork/exec/pipe instead of just open a file.

Thanks,
   Uri.