Re: [Qemu-devel] [PATCH 2/8] block: add live block commit functionality

[Eric Blake <eblake@redhat.com>]

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


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