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Re: [PATCH v3 1/6] block: add bitmap-populate job

From: Eric Blake
Subject: Re: [PATCH v3 1/6] block: add bitmap-populate job
Date: Wed, 2 Sep 2020 10:58:00 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0

[reviving an old thread]

On 6/22/20 4:44 PM, Eric Blake wrote:
On 6/19/20 11:16 PM, Vladimir Sementsov-Ogievskiy wrote:
19.06.2020 22:56, Eric Blake wrote:
From: John Snow <jsnow@redhat.com>

This job copies the allocation map into a bitmap. It's a job because
there's no guarantee that allocation interrogation will be quick (or
won't hang), so it cannot be retrofitted into block-dirty-bitmap-merge.

It was designed with different possible population patterns in mind,
but only top layer allocation was implemented for now.

Signed-off-by: John Snow <jsnow@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

+{ 'struct': 'BlockDirtyBitmapPopulate',
+  'base': 'BlockDirtyBitmap',
+  'data': { 'job-id': 'str',
+            'pattern': 'BitmapPattern',

As written, "pattern":"allocate-top" is rather limited - it can only grab allocation from the top node. Being able to grab the allocation from a specific node may indeed be more useful. Another bitmap patterns that might be useful would be an all-one pattern (create a bitmap that treats the entire disk as dirty). I also remember brainstorming with John the question of whether we want bitmap-populate to have different mask modes: does the population perform an overwrite (the bitmap now matches the source pattern exactly, even if some bits were set and others cleared), a union merge (any bits already set in the bitmap remain set, additional bits are set according to pattern), or even a difference (any bits already cleared in the bitmap remain clear, while bits in the pattern can also clear additional bits in the bitmap).

If I understand Peter's goals, the initial libvirt use is a union mode (keep bits in the bitmap that are already set, but set additional bits according to the population pattern).

+            '*on-error': 'BlockdevOnError',
+            '*auto-finalize': 'bool',
+            '*auto-dismiss': 'bool' } }

Peter said about a possibility of populating several target bitmaps simultaneously.

What about such a generalized semantics:

Merge all sources to each target

@targets: list of bitmaps to be populated by the job
{ 'struct': 'BlockDirtyBitmapPopulate',
   'data': { <common job fields>,
             'targets': ['BlockDirtyBitmap'],
             'sources': ['BitmapPopulateSource'] } }

We still need the 'pattern' argument (the idea being that if we have: Base <- Active, we want to be able to merge in the allocation map of Active into bitmaps stored in Base as part of a commit operation, whether that is active commit of a live guest or offline commit while the guest is offline).  Having an array for 'targets' to merge into is fine, but for 'sources', it's less a concern about selecting from multiple sources, and more a concern about selecting the allocation pattern to be merged in (libvirt wants to merge the same allocation pattern into each bitmap in Base).  Generalizing things to allow the merge of more than one source at once might not hurt, but I'm not sure we need it yet.

But when it comes to multiple destinations or multiple sources, while it seems like it might be a convenience factor, I also worry that it is over-engineering. See more below...

But there are other patterns that we may want to support: an all-ones pattern, or maybe a pattern that tracks known-zeros instead of allocation.

@bitmap: specify dirty bitmap to be merged to target bitamp(s)
@node: specify a node name, which allocation-map is to be merged to target bitmap(s)
{ 'alternate': 'BitmapPopulateSource',
   'data': { 'bitmap': 'BlockDirtyBitmap',
             'node': 'str' } }

This design is clever in that it lets us merge in both existing bitmaps and using a node-name for merging in an allocation map instead of a bitmap; but it limits us to only one pattern.  Better might be something where we supply a union (hmm, we've had proposals in the past for a default value to the discriminator to allow it to be optional, so I'll proceed as if we will finally implement that):

{ 'enum': 'BitmapPattern', 'data': [ 'bitmap', 'allocation-top' ] }
{ 'union': 'BitmapPopulateSource',
   'base': { '*pattern': 'BitmapPattern' },
   'discriminator': { 'name': 'pattern', 'default': 'bitmap' },
   'data': { 'bitmap': 'BitmapPopulateSource',
             'allocation-top': { 'node': 'str' } } }

so that you can then do:

{ "execute": "block-dirty-bitmap-populate",
   "arguments": { "targets": [ { "node": "base", "name": "b1" },
                               { "node": "base", "name": "b2" } ],
         "sources": [ { "pattern": "allocation-top", "node": "top" } ]
   } }

to merge in the allocation information of top into multiple bitmaps of base at once,

Hmm, I left out the mandatory "job-id" parameter here; one of the key points of the new command is that some patterns (like allocation) may involve potentially lengthy I/O, so we need a job-id (the existing block-dirty-bitmap-merge does not). But since the existing block-dirty-bitmap-merge supports multiple sources to one destination, supporting multiple patterns to one destination tracked by a single job id does have some appeal.

or conversely, do:

{ "execute": "block-dirty-bitmap-populate",
   "arguments": { "targets": [ { "node": "base", "name": "b1" } ],
         "sources": [ { "pattern": "bitmap",
                        "node": "top", "name": "b1" } ]
   } }
{ "execute": "block-dirty-bitmap-populate",
   "arguments": { "targets": [ { "node": "base", "name": "b2" } ],
         "sources": [ { "node": "top", "name": "b2" } ]
   } }

and of course, wrap this in a "transaction" to ensure that it all succeeds or fails as a unit, rather than messing up one bitmap if another fails, while also allowing future extension for additional patterns.

We already have transactions that let us perform multiple destinations as a group. So what is the difference in the end results between merging one source into two separate destinations in one command spelled this way:

# proposal with many:many bitmap populate
{ "execute": "block-dirty-bitmap-populate",
  "arguments": { "job-id": "job0",
                 "targets": [ { "node": "base", "name": "b1" },
                              { "node": "base", "name": "b2" } ],
                 "source": { "pattern": "allocation", "node": "top" } } }
wait for job to complete

vs. spelled this way:

# patch as written with 1:1 bitmap-populate, but tweak to source
{ "execute": "block-dirty-bitmap-add",
  "arguments": { "node": "top", "name": "tmp" } }
{ "execute": "block-dirty-bitmap-populate",
  "arguments": { "job-id": "job0",
                 "node": "top", "name": "tmp",
                 "source": { "pattern": "allocation", "node": "top" } } }
wait for job to complete
{ "execute": "transaction",
  "arguments": { "actions": [
    { "type": "block-dirty-bitmap-merge", "node": "base", "name": "b1",
        "bitmaps": [ { "node": "top", "name": "tmp" } ] },
    { "type": "block-dirty-bitmap-merge", "node": "base", "name": "b2",
        "bitmaps": [ { "node": "top", "name": "tmp" } ] }
] } }

- so, we can merge several bitmaps together with several allocation maps into several target bitmaps. (I remember, we also said about a possibility of starting several populating jobs, populating into   same bitmap, I think it may be substituted by one job with several sources. Still, it's not hard to   allow to use target bitmaps in a several jobs simultaneously and this is not about the QAPI interface)

Will this simplify things in libvirt?

Peter, in your preliminary experiments with block-dirty-bitmap-populate, did you ever need to start more than one job to a single bitmap destination, or was it merely starting multiple jobs because you had multiple destinations but always just a single source?

I guess I'm struggling in posting a v4 in part because I don't have a good answer to what is easiest for Peter to use.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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