Re: [Qemu-devel] [Qemu-block] [PATCH v4 1/2] live-block-ops.txt: Rename, rewrite, and improve it

[Alberto Garcia]

On Wed 28 Jun 2017 04:58:00 PM CEST, Kashyap Chamarthy wrote:
> +Once a 'mirror' job has started, there are two possible actions when a
> +``drive-mirror`` job is active:
> +
> +1. Issuing the command ``block-job-cancel``: will, after completing
> +   synchronization of the content from the disk image chain to the
> +   target image, [E] -- create a point-in-time (which is at the time of
> +   *triggering* the cancel command) copy, contained in image [E], of the
> +   backing file.

A point-in-time [...] copy [...] of the backing file ? That would be a
copy of the whole chain or the topmost image (depending on the sync
mode).

> +The ``"sync": "full"``, from the above, means: copy the *entire* chain
> +to the destination.

I think it's in general a good idea to describe the different sync modes
before doing this, because they're not completely obvious when you first
try to use these commands.

> +Notes on ``blockdev-mirror``
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  [...]
> +(1) Create the target image (using ``qemu-img``), say, backup.qcow2

If we're doing a mirror here, backup.qcow2 is probably not the best
name :-)

> +Live disk backup --- ``drive-backup`` and ``blockdev-backup``
> +-------------------------------------------------------------
> +
> +The ``drive-backup`` (and its newer equivalent ``blockdev-backup``) allows
> +you to create a point-in-time snapshot.
> +
> +In this case, the point-in-time is when you *start* the ``drive-backup``
> +(or its newer equivalent ``blockdev-backup``) command.
> +
> +Currently, there are four different types of synchronization modes:
> +
> +(1) ``full`` -- Synchronize the content of entire disk image chain to
> +    the target
> +(2) ``top`` -- Synchronize only the contents of the top-most disk image
> +    in the chain to the target
> +(3) ``none`` -- Synchronize only the new writes from this point on
> +(4) ``incremental`` -- Synchronize content that is described by the
> +    dirty bitmap

Here's the kind of summary of the different sync modes that I was
talking about earlier.

One note about sync=none, though. If I'm not wrong drive-backup makes a
point-in-time copy of the data at the moment when the command is issued
(that's one difference from drive-mirror). Therefore drive-backup never
synchronizes "new writes from this point on".

I don't think that drive-backup sync=none copies any data then, can
anyone clarify?

The rest of the document looks good to me, thanks!

Berto