On 09/14/2012 07:41 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. >
> +
> +enum {
> + /*
> + * Size of data buffer for populating the image file. This should be
> large
> + * enough to process multiple clusters in a single call, so that
> populating
> + * contiguous regions of the image is efficient.
> + */
> + COMMIT_BUFFER_SIZE = 512 * 1024, /* in bytes */
I'm guessing you will add a followup patch that depends on Paolo's
series for controlling the granularity of this buffer? Or is it less
important for the commit case?
> +
> +static void coroutine_fn commit_run(void *opaque)
> +{
> + ret = base_len = bdrv_getlength(base);
> + if (base_len < 0) {
> + goto exit_restore_reopen;
> + }
> +
> + if (base_len < s->common.len) {
> + ret = bdrv_truncate(base, s->common.len);
> + if (ret) {
> + goto exit_restore_reopen;
> + }
> + }
Question: is it valid to have a qcow2 file whose size is smaller than
it's backing image? Suppose I have base[1M] <- mid[2M] <- top[3M] <-
active[3M], and request to commit top into base. This bdrv_truncate()
means I will now have:
base[3M] <- mid[2M] <- top[3M] <- active[3M].
If I then abort the commit operation at this point, then we have the
situation of 'mid' reporting a smaller size than 'base' - which may make
'mid' invalid. And even if it is valid, what happens if I now request
to commit 'mid' into 'base', but 'base' already had data written past
the 2M mark before I aborted the first operation?
I'm worried that you may have to bdrv_truncate() the entire chain to
keep it consistent, which is more complex because it requires more r/w
files.
--
Eric Blake ebl...@redhat.com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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