Re: [Qemu-devel] [PATCH v14 19/19] docs: incremental backup documentation

[Eric Blake <eblake@redhat.com>]

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On 02/20/2015 04:07 PM, John Snow wrote:
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  docs/bitmaps.md | 253 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 253 insertions(+)
>  create mode 100644 docs/bitmaps.md
> 
> diff --git a/docs/bitmaps.md b/docs/bitmaps.md
> new file mode 100644
> index 0000000..7cda146
> --- /dev/null
> +++ b/docs/bitmaps.md
> @@ -0,0 +1,253 @@
> +# Dirty Bitmaps

No copyright/license?  Might be good to add.

Also, I'm a fan of reviewing docs before code (does the design make
sense in isolation, and then did we implement it) rather than last in
the series (we may have faithfully documented our code, but that
includes baking in any design flaws that reviewers are now blind to
because of reading the code)

> +* To create a new bitmap that tracks changes in 32KiB segments:
> +
> +```json
> +{ "execute": "block-dirty-bitmap-add",
> +  "arguments": {
> +    "node": "drive0",
> +    "name": "bitmap0",
> +    "granularity": "32768"

s/"32768"/32768/ (we are using a JSON integer, not string)

> +  }
> +}
> +```
> +
> +### Deletion
> +
> +* Can be performed on a disabled bitmap, but not a frozen one.
> +
> +* Because bitmaps are only unique to the node to which they are attached,
> +you must specify the node/drive name here, too.
> +
> +```json
> +{ "execute": "block-dirty-bitmap-remove",
> +  "arguments": {
> +    "node": "drive0",
> +    "name": "bitmap0",
> +  }

No trailing commas in JSON.

> +}
> +```
> +
> +### Enable/Disable:
> +
> +* Not very useful in current cases, but potentially useful for debugging in the
> +future where we'd like to see what information changed only in a specific
> +time period:
> +
> +* To enable (which is, again, the default state after add)
> +
> +```json
> +{ "execute": "block-dirty-bitmap-enable",
> +  "arguments": {
> +    "node": "drive0",
> +    "name": "bitmap0",
> +  }

and again.

> +}
> +```
> +
> +* To disable:
> +
> +```json
> +{ "execute": "block-dirty-bitmap-disable",
> +  "arguments": {
> +    "node": "drive0",
> +    "name": "bitmap0",
> +  }
> +}
> +```

and again. Also, maybe swap these two, since a bitmap defaults to
enabled (that is, you would logically use disable first, then enable to
re-enable, when testing these out, if there is no way to create an
already-disabled bitmap).

> +
> +### Resetting
> +
> +* Resetting a bitmap will clear all information it holds.
> +* An incremental backup created from an empty bitmap will copy no data,
> +as if nothing has changed.
> +
> +```json
> +{ "execute": "block-dirty-bitmap-clear",
> +  "arguments": {
> +    "node": "drive0",
> +    "name": "bitmap0",
> +  }

I'll quit pointing out broken trailing JSON commas, on the assumption
that you'll fix all of them rather than stopping here :)

> +}
> +```
> +
> +## Transactions
> +
> +### Justification
> +Bitmaps can be safely modified when the VM is paused or halted by using
> +the basic QMP commands. For instance, you might perform the following actions:
> +
> +1. Boot the VM in a paused state.
> +2. Create a full drive backup of drive0
> +3. Create a new bitmap attached to drive0
> +4. resume execution of the VM
> +5. Incremental backups are ready to be created.

Consistency on using trailing '.'?

> +
> +At this point, the bitmap and drive backup would be correctly in sync,
> +and incremental backups made from this point forward would be correctly aligned
> +to the full drive backup.
> +
> +This is not particularly useful if we decide we want to start incremental
> +backups after the VM has been running for a while, which will allow us to
> +perform actions like the following:
> +
> +1. Boot the VM and begin execution
> +2. Using transactions, perform the following operations:
> +    * Create 'bitmap0'
> +    * Create a full drive backup of drive0
> +3. Incremental backups are now ready to be created.

It's unclear whether point 2 is describing two separate transactions, or
one transaction with two steps.  If it's just one, I'd word it:

Using a single transaction, perform the following operations:


> +
> +The star of the show.
> +
> +**Nota Bene!** Only incremental backups of entire drives are supported for now.
> +So despite the fact that you can attach a bitmap to any arbitrary node, they are
> +only currently useful when attached to the root node. This is because
> +drive-backup only supports drives/devices instead of arbitrary nodes.

Maybe it's time to think about adding a node-backup :)  But doesn't have
to be this series.



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


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