Re: [Qemu-devel] [Qemu-block] [PATCH] rbd: Detect rbd image resizes and propagate them

[John Snow <jsnow@redhat.com>]

On 09/13/2017 12:44 PM, Adam Wolfe Gordon wrote:
> Register a watcher with rbd so that we get notified when an image is
> resized. Propagate resizes to parent block devices so that guest devices
> get resized without user intervention.
> 
> Signed-off-by: Adam Wolfe Gordon <awg@digitalocean.com>
> ---
> Hello!
> 
> We are using this patch in production at DigitalOcean and have tested it fairly
> extensively. However, we use the same block device configuration everywhere, so
> I'd be interested to get an answer to the question I've left in the code:>
>> /* NOTE: This assumes there's only one layer between us and the
>>    block-backend. Is this always true? */
> 

Even if it were always true presently, the fact that the block layer
storage graph can be configured in arbitrary ways suggests that it might
not always be true in the future.

Probably in practice MOST drives are configured like
[BB]<--[Format]<--[Protocol], but the block layer is adding a lot of fun
stuff like filter nodes for throttling or debugging, so it's probably
too much to assert that the length of the chain will ALWAYS be exactly two.

Can I play dumb and ask what you are trying to accomplish? The concept
of a drive deciding to resize itself is not something I have a model for
understanding, presently...

> If that isn't true, this patch will need a bit of work to traverse up the block
> device tree and find the block-backend.
> 
> Of course, any other feedback would be welcome too - this is my first foray into
> the qemu code.
> 
> Regards,
> Adam
> 
>  block/rbd.c | 80 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 80 insertions(+)
> 
> diff --git a/block/rbd.c b/block/rbd.c
> index 144f350e1f..1c9fcbec1f 100644
> --- a/block/rbd.c
> +++ b/block/rbd.c
> @@ -96,6 +96,8 @@ typedef struct BDRVRBDState {
>      rbd_image_t image;
>      char *image_name;
>      char *snap;
> +    uint64_t watcher_handle;
> +    uint64_t size_bytes;
>  } BDRVRBDState;
>  
>  static char *qemu_rbd_next_tok(char *src, char delim, char **p)
> @@ -540,6 +542,67 @@ out:
>      return rados_str;
>  }
>  
> +/* BH for when rbd notifies us of an update. */
> +static void qemu_rbd_update_bh(void *arg)
> +{
> +    BlockDriverState *parent, *bs = arg;
> +    BDRVRBDState *s = bs->opaque;
> +    bool was_variable_length;
> +    uint64_t new_size_bytes;
> +    int64_t new_parent_len;
> +    BdrvChild *c;
> +    int r;
> +
> +    r = rbd_get_size(s->image, &new_size_bytes);
> +    if (r < 0) {
> +        error_report("error reading size for %s: %s", s->name, strerror(-r));
> +        return;
> +    }
> +
> +    /* Avoid no-op resizes on non-resize notifications. */
> +    if (new_size_bytes == s->size_bytes) {
> +        error_printf("skipping non-resize rbd cb\n");
> +        return;
> +    }
> +
> +    /* NOTE: This assumes there's only one layer between us and the
> +       block-backend. Is this always true? */
> +    parent = bs->inherits_from;
> +    if (parent == NULL) {
> +        error_report("bs %s does not have parent", bdrv_get_device_or_node_name(bs));
> +        return;
> +    }
> +
> +    /* Force parents to re-read our size. */
> +    was_variable_length = bs->drv->has_variable_length;
> +    bs->drv->has_variable_length = true;
> +    new_parent_len = bdrv_getlength(parent);
> +    if (new_parent_len < 0) {
> +        error_report("getlength failed on parent %s", bdrv_get_device_or_node_name(parent));
> +        bs->drv->has_variable_length = was_variable_length;
> +        return;
> +    }
> +    bs->drv->has_variable_length = was_variable_length;
> +
> +    /* Notify block backends that that we have resized.
> +       Copied from bdrv_parent_cb_resize. */
> +    QLIST_FOREACH(c, &parent->parents, next_parent) {
> +        if (c->role->resize) {
> +            c->role->resize(c);
> +        }
> +    }
> +
> +    s->size_bytes = new_size_bytes;
> +}
> +
> +/* Called from non-qemu thread - careful! */
> +static void qemu_rbd_update_cb(void *arg)
> +{
> +    BlockDriverState *bs = arg;
> +
> +    aio_bh_schedule_oneshot(bdrv_get_aio_context(bs), qemu_rbd_update_bh, bs);
> +}
> +
>  static int qemu_rbd_open(BlockDriverState *bs, QDict *options, int flags,
>                           Error **errp)
>  {
> @@ -672,9 +735,25 @@ static int qemu_rbd_open(BlockDriverState *bs, QDict *options, int flags,
>          }
>      }
>  
> +    r = rbd_update_watch(s->image, &s->watcher_handle, qemu_rbd_update_cb, bs);
> +    if (r < 0) {
> +        error_setg_errno(errp, -r, "error registering watcher on %s", s->name);
> +        goto failed_watch;
> +    }
> +
> +    r = rbd_get_size(s->image, &s->size_bytes);
> +    if (r < 0) {
> +        error_setg_errno(errp, -r, "error reading size for %s", s->name);
> +        goto failed_sz;
> +    }
> +
>      qemu_opts_del(opts);
>      return 0;
>  
> +failed_sz:
> +    rbd_update_unwatch(s->image, s->watcher_handle);
> +failed_watch:
> +    rbd_close(s->image);
>  failed_open:
>      rados_ioctx_destroy(s->io_ctx);
>  failed_shutdown:
> @@ -712,6 +791,7 @@ static void qemu_rbd_close(BlockDriverState *bs)
>  {
>      BDRVRBDState *s = bs->opaque;
>  
> +    rbd_update_unwatch(s->image, s->watcher_handle);
>      rbd_close(s->image);
>      rados_ioctx_destroy(s->io_ctx);
>      g_free(s->snap);
>