Re: [Qemu-devel] [PATCH v2 2/7] block: add live block commit functionality

[Eric Blake <eblake@redhat.com>]

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On 09/25/2012 10:29 AM, Jeff Cody wrote:
> This adds the live commit coroutine.  This iteration focuses on the
> commit only below the active layer, and not the active layer itself.
> 
> The behaviour is similar to block streaming; the sectors are walked
> through, and anything that exists above 'base' is committed back down
> into base.  At the end, intermediate images are deleted, and the
> chain stitched together.  Images are restored to their original open
> flags upon completion.
> 
> Signed-off-by: Jeff Cody <jcody@redhat.com>

> +void commit_start(BlockDriverState *bs, BlockDriverState *base,
> +                 BlockDriverState *top, int64_t speed,
> +                 BlockErrorAction on_error, BlockDriverCompletionFunc *cb,
> +                 void *opaque, Error **errp)
> +{
> +    CommitBlockJob *s;
> +    BlockReopenQueue *reopen_queue = NULL;
> +    int orig_overlay_flags;
> +    int orig_base_flags;
> +    BlockDriverState *overlay_bs;
> +    Error *local_err = NULL;
> +
> +    if ((on_error == BLOCK_ERR_STOP_ANY ||
> +         on_error == BLOCK_ERR_STOP_ENOSPC) &&
> +        !bdrv_iostatus_is_enabled(bs)) {
> +        error_set(errp, QERR_INVALID_PARAMETER_COMBINATION);
> +        return;
> +    }
> +
> +    overlay_bs = bdrv_find_overlay(bs, top);
> +
> +    if (overlay_bs == NULL) {
> +        error_setg(errp, "Could not find overlay image for %s:", top->filename);
> +        return;
> +    }
> +
> +    orig_base_flags    = bdrv_get_flags(base);
> +    orig_overlay_flags = bdrv_get_flags(overlay_bs);

I think you are missing a check here that base is on the backing chain
of top.  See also my comments to 5/7.

> +
> +    /* convert base_bs & overlay_bs to r/w, if necessary */
> +    if (!(orig_base_flags & BDRV_O_RDWR)) {
> +        reopen_queue = bdrv_reopen_queue(reopen_queue, base,
> +                                         orig_base_flags | BDRV_O_RDWR);
> +    }
> +    if (!(orig_overlay_flags & BDRV_O_RDWR)) {
> +        reopen_queue = bdrv_reopen_queue(reopen_queue, overlay_bs,
> +                                         orig_overlay_flags | BDRV_O_RDWR);
> +    }
> +    if (reopen_queue) {
> +        bdrv_reopen_multiple(reopen_queue, &local_err);

Is it valid to make a no-op call, such as:

{ "execute":"block-commit", "arguments":{
  "device":"drive0", "top":"base", "base":"base" }}

If so, should we do an early exit here, rather than temporarily changing
base to R/W just to change it back to R/O?

If not, should we be rejecting it up front (again, back to the question
of failing if 'base' is not a backing file of 'top', even if both 'top'
and 'base' are backing files of 'device').

-- 
Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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