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block: add QAPI command to allow live backing file change

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This allows a user to make a live change to the backing file recorded in
an open image.

The image file to modify can be specified 2 ways:

1) image filename
2) image node-name

Note: this does not cause the backing file itself to be reopened; it
merely changes the backing filename in the image file structure, and
in internal BDS structures.

It is the responsibility of the user to pass a filename string that
can be resolved when the image chain is reopened, and the filename
string is not validated.

A good analogy for this command is that it is a live version of
'qemu-img rebase -u', with respect to changing the backing file string.

[Jeff is offline so I respun this patch in his absence.  Dropped image
filename since using node-name is preferred and this is a new command.
No need to introduce the limitations of finding images by filename.
--Stefan]

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Jeff Cody 2014-07-01 09:52:16 +02:00 • committed by Stefan Hajnoczi
parent 4e855baabf
commit fa40e65622
3 changed files with 144 additions and 0 deletions
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79
blockdev.c
View file

@ -2367,6 +2367,85 @@ void qmp_block_job_complete(const char *device, Error **errp)
block_job_complete(job, errp);
}
void qmp_change_backing_file(const char *device,
const char *image_node_name,
const char *backing_file,
Error **errp)
{
BlockDriverState *bs = NULL;
BlockDriverState *image_bs = NULL;
Error *local_err = NULL;
bool ro;
int open_flags;
int ret;
/* find the top layer BDS of the chain */
bs = bdrv_find(device);
if (!bs) {
error_set(errp, QERR_DEVICE_NOT_FOUND, device);
return;
}
image_bs = bdrv_lookup_bs(NULL, image_node_name, &local_err);
if (local_err) {
error_propagate(errp, local_err);
return;
}
if (!image_bs) {
error_setg(errp, "image file not found");
return;
}
if (bdrv_find_base(image_bs) == image_bs) {
error_setg(errp, "not allowing backing file change on an image "
"without a backing file");
return;
}
/* even though we are not necessarily operating on bs, we need it to
* determine if block ops are currently prohibited on the chain */
if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_CHANGE, errp)) {
return;
}
/* final sanity check */
if (!bdrv_chain_contains(bs, image_bs)) {
error_setg(errp, "'%s' and image file are not in the same chain",
device);
return;
}
/* if not r/w, reopen to make r/w */
open_flags = image_bs->open_flags;
ro = bdrv_is_read_only(image_bs);
if (ro) {
bdrv_reopen(image_bs, open_flags | BDRV_O_RDWR, &local_err);
if (local_err) {
error_propagate(errp, local_err);
return;
}
}
ret = bdrv_change_backing_file(image_bs, backing_file,
image_bs->drv ? image_bs->drv->format_name : "");
if (ret < 0) {
error_setg_errno(errp, -ret, "Could not change backing file to '%s'",
backing_file);
/* don't exit here, so we can try to restore open flags if
* appropriate */
}
if (ro) {
bdrv_reopen(image_bs, open_flags, &local_err);
if (local_err) {
error_propagate(errp, local_err); /* will preserve prior errp */
}
}
}
void qmp_blockdev_add(BlockdevOptions *options, Error **errp)
{
QmpOutputVisitor *ov = qmp_output_visitor_new();