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Re: [Qemu-devel] [PATCH for-2.12 0/4] qmp dirty bitmap API

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-devel] [PATCH for-2.12 0/4] qmp dirty bitmap API
Date: Thu, 30 Nov 2017 15:10:14 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

18.11.2017 00:35, John Snow wrote:

On 11/17/2017 03:22 AM, Vladimir Sementsov-Ogievskiy wrote:
17.11.2017 06:10, John Snow wrote:
On 11/16/2017 03:17 AM, Vladimir Sementsov-Ogievskiy wrote:
16.11.2017 00:20, John Snow wrote:
On 11/13/2017 11:20 AM, Vladimir Sementsov-Ogievskiy wrote:
Hi all.

There are three qmp commands, needed to implement external backup API.

Using these three commands, client may do all needed bitmap
management by

on backup start we need to do a transaction:
   {disable old bitmap, create new bitmap}

on backup success:
   drop old bitmap

on backup fail:
   enable old bitmap
   merge new bitmap to old bitmap
   drop new bitmap

Can you give me an example of how you expect these commands to be used,
and why they're required?

I'm a little weary about how error-prone these commands might be and the
potential for incorrect usage seems... high. Why do we require them?
It is needed for incremental backup. It looks like bad idea to export
abdicate/reclaim functionality, it is simpler
and clearer to allow user to merge/enable/disable bitmaps by hand.

usage is like this:

1. we have dirty bitmap bitmap0 for incremental backup.

2. prepare image fleecing (create temporary image with backing=our_disk)
3. in qmp transaction:
     - disable bitmap0
     - create bitmap1
     - start image fleecing (backup sync=none our_disk -> temp_disk)
This could probably just be its own command, though:

block-job-fleece node=foobar bitmap=bitmap0 etc=etera etc=etera

Could handle forking the bitmap. I'm not sure what the arguments would
look like, but we could name the NBD export here, too. (Assuming the
server is already started and we just need to create the share.)

Then, we can basically do what mirror does:

(1) Cancel
(2) Complete

Cancel would instruct QEMU to keep the bitmap changes (i.e. roll back),
and Complete would instruct QEMU to discard the changes.

This way we don't need to expose commands like split or merge that will
almost always be dangerous to use over QMP.

In fact, a fleecing job would be really convenient even without a
bitmap, because it'd still be nice to have a convenience command for it.
Using an existing infrastructure and understood paradigm is just a bonus.
1. If I understand correctly, Kevin and Max said in their report in
Prague about new block-job approach,
   using filter nodes, so I'm not sure that this is a good Idea to
introduce now new old-style block-job, where we can
   do without it.

We could do without it, but it might be a lot better to have everything
wrapped up in a command that's easy to digest instead of releasing 10
smaller commands that have to be executed in a very specific way in
order to work correctly.

I'm thinking about the complexity of error checking here with all the
smaller commands, versus error checking on a larger workflow we
understand and can quality test better.

I'm not sure that filter nodes becoming the new normal for block jobs
precludes our ability to use the job-management API as a handle for
managing the lifetime of a long-running task like fleecing, but I'll
check with Max and Kevin about this.

2. there is the following scenario: customers needs a possibility to
create a backup of data changed since some
point in time. So, maintaining several static and one (or several) activ
bitmaps with a possiblity of merge some of them
and create a backup using this merged bitmap may be convenient.

I think the ability to copy bitmaps and issue differential backups would
be sufficient in all cases I could think of...

so, instead of keeping several static bitmaps with ability to merge them,
you propose to keep several active bitmaps and copy them to make a backup?

so, instead of new qmp command for merge, add new qmp command for copy?

in case of static bitmaps, we can implement saving/loading them to the image to free RAM space,
so it is better.

or what do you propose for  [2] ?

Best regards,

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