On 18.05.20 18:26, Peter Krempa wrote: > On Wed, May 13, 2020 at 16:56:10 +0200, Max Reitz wrote: >> This command allows mapping block node names to aliases for the purpose >> of block dirty bitmap migration. >> >> This way, management tools can use different node names on the source >> and destination and pass the mapping of how bitmaps are to be >> transferred to qemu (on the source, the destination, or even both with >> arbitrary aliases in the migration stream). >> >> Suggested-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com> >> Signed-off-by: Max Reitz <mre...@redhat.com> >> --- > > [...] > >> --- >> qapi/migration.json | 36 ++++++++++++++++++++ >> migration/block-dirty-bitmap.c | 60 ++++++++++++++++++++++++++++++++-- >> 2 files changed, 94 insertions(+), 2 deletions(-) > > I just started to write some quick and dirty hacks to test use of this > infrastructure in libvirt. I have 2 quick observations for now: > >> >> diff --git a/qapi/migration.json b/qapi/migration.json >> index d5000558c6..97037ea635 100644 >> --- a/qapi/migration.json >> +++ b/qapi/migration.json >> @@ -1621,3 +1621,39 @@ >> ## >> { 'event': 'UNPLUG_PRIMARY', >> 'data': { 'device-id': 'str' } } >> + >> +## >> +# @MigrationBlockNodeMapping: >> +# >> +# Maps a block node name to an alias for migration. >> +# >> +# @node-name: A block node name. >> +# >> +# @alias: An alias name for migration (for example the node name on >> +# the opposite site). >> +# >> +# Since: 5.1 >> +## >> +{ 'struct': 'MigrationBlockNodeMapping', >> + 'data': { >> + 'node-name': 'str', >> + 'alias': 'str' >> + } } > > We'd probably like a > 'nodename:bitmapname' -> 'alias' mapping so that we can select which > bitmaps to migrate and where to migrate them to.
Sure, entirely doable. I think Vladimir is right that we’d want separate node and bitmap aliases then, though, because the migration stream has both fields. (Also, if we only had a single alias, you’d always need to call migrate-set-bitmap-(node-)mapping on both ends to unpack that alias. With separate node and bitmap aliases, it suffices to call it on one end, just like the version in thie patch.) > The specific use case > is following: > > Libvirt supports migration without shared storage (NFS/etc) where we > copy the disk images prior to the memory migration using qemu's NBD > server and the blockdev-mirror job. By default and the most simple way > which doesn't require users fudging with the disk images and copying > backing images themselves is that we flatten all the backing chain > during the copy ("sync":"full"). In this mode we'll need to do some > merging of the bitmaps prior to finalizing the copy. > > It's not a problem to do it ourselves, but in the end we'll need to copy > only certain bitmaps which will be created temporarily on the source to > the destination where they'll be persisted. > > For now (until I implement the use of the dirty-bitmap-populate blockjob > which I'm also doing in parallel with this kind of) when creating a > snapshot we create a new active bitmap in the overlay for every active > bitmap in the snapshotted image. > > When flattening we'll then need to merge the appropriate ones. As said > it's not a problem to prepare all the bitmaps before but we then don't > need to migrate all of them. > > By the way, that brings me to another question: > > Is there any difference of handling of persistent and non-persistent > bitmaps? Specifically I'm curious what's the correct approach to do the > shared storage migration case when the source and destination image is > the same one. Should we also instruct to migrate the active ones? or are > they migrated by writing them to the image and reloading them? I hope Vladimir has answered your question sufficiently extensively. :) >> +## >> +# @migrate-set-bitmap-node-mapping: > > qemu-5.0 deprecated a bunch of migrate-set- specific commands in favor > of migrate-set-parameters. Is it worth/necessary to add a new command > here? I wasn’t aware of that. It would probably indeed make sense from a user’s perspective. It would make the implementation rather different, though, because instead of putting the mapping locally (and statically) into migration/block-dirty-bitmap.c, it would require putting it into the central MigrationState. Which isn’t to say it’d be worse. I suppose it’d be better, actually, but I’ll have to try to say for sure. You also suggested “setting nothing will clear the mapping” in your second mail. That would be a weird default. Right now, the default for all migration parameters is to leave them as-is, so it would be different. The first question would be: What do you mean by “clear the mapping”? Reset it to the original identity mapping? Or actually clear it, so that no bitmap is migrated? I presume the former, because the latter can easily be achieved by passing an empty array. I understand that it seems to make sense to be able to return to the original identity mapping, but is there actually a use for this? After you have started using a custom mapping, wouldn’t you always use custom mappings? If there is a use for it, I think the better way to do it would be to use an alternate type where an explicit null resets the mapping to the identity mapping. Max
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