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

[Anthony Liguori <anthony@codemonkey.ws>]

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 <uril@redhat.com>
> ---
>  Makefile         |    2 +-
>  migration-file.c |  127 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  migration.c      |    4 ++
>  migration.h      |    5 ++
>  4 files changed, 137 insertions(+), 1 deletions(-)
>  create mode 100644 migration-file.c
>
> diff --git a/Makefile b/Makefile
> index 8cbdcda..7e0e6f2 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -81,7 +81,7 @@ OBJS+=usb.o usb-hub.o usb-$(HOST_USB).o usb-hid.o usb-msd.o usb-wacom.o
>  OBJS+=usb-serial.o usb-net.o
>  OBJS+=sd.o ssi-sd.o
>  OBJS+=bt.o bt-host.o bt-vhci.o bt-l2cap.o bt-sdp.o bt-hci.o bt-hid.o usb-bt.o
> -OBJS+=buffered_file.o migration.o migration-tcp.o net.o qemu-sockets.o
> +OBJS+=buffered_file.o migration.o migration-tcp.o migration-file.o net.o qemu-sockets.o
>  OBJS+=qemu-char.o aio.o net-checksum.o savevm.o cache-utils.o
>  
>  ifdef CONFIG_BRLAPI
> diff --git a/migration-file.c b/migration-file.c
> new file mode 100644
> index 0000000..5cde3e2
> --- /dev/null
> +++ b/migration-file.c
> @@ -0,0 +1,127 @@
> +/*
> + * QEMU live migration
> + *
> + * Copyright IBM, Corp. 2008
> + *           Red Hat, Inc. 2008
> + *
> + * Authors:
> + *  Anthony Liguori   <aliguori@us.ibm.com>
> + *  Uri Lublin        <uril@redhat.com>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2.  See
> + * the COPYING file in the top-level directory.
> + *
> + */
> +
> +#include <unistd.h>
> +#include "qemu-common.h"
> +#include "migration.h"
> +#include "sysemu.h"
> +#include "console.h"
> +#include "block.h"
> +
> +//#define DEBUG_MIGRATION_FILE
> +
> +#ifdef DEBUG_MIGRATION_FILE
> +#define dprintf(fmt, ...) \
> +    do { printf("migration-file: " fmt, ## __VA_ARGS__); } while (0)
> +#else
> +#define dprintf(fmt, ...) \
> +    do { } while (0)
> +#endif
> +
> +static int file_close(FdMigrationState *s)
> +{
> +    close(s->fd);
> +}
> +
> +static int file_errno(FdMigrationState *s)
> +{
> +    return errno;
> +}
> +
> +static int file_write(FdMigrationState *s, const void * buf, size_t size)
> +{
> +    return write(s->fd, buf, size);
> +}
> +
> +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.

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=.

It shouldn't be too hard to use posix-aio though.  You just have to be 
clever about implementing the s->write op.

Regards,

Anthony Liguori