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

[Daniel P. Berrange]

On Tue, Jan 20, 2009 at 01:32:24PM +0200, Uri Lublin wrote:
> 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.

IIRC that doesn't have any effect on plain files - they'll still potentially
block on write.

Daniel
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