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Re: [libvirt] [PATCHv3.5 18/43] snapshot: prevent migration from stranding snapshot data



On 08/30/2011 04:10 PM, Eric Blake wrote:
Migration is another case of stranding metadata.  And since
snapshot metadata is arbitrarily large, there's no way to
shoehorn it into the migration cookie of migration v3.


A future patch will make it possible to manually recreate the
snapshot metadata on the destination.

I'm starting to work on that now, with VIR_DOMAIN_SNAPSHOT_CREATE_REDEFINE; I'm also adding a flag VIR_DOMAIN_SNAPSHOT_CREATE_CURRENT, which can mark a just-redefined snapshot as current. I'll probably stick the redefine earlier in the series, then rebase this patch for migration on top of that.

 But even that is limited,
since if we delete the snapshot metadata prior to migration,
then we won't know the name of the current snapshot to pass
along; and if we delete the snapshot metadata after migration
and use the v3 migration cookie to pass along the name of the
current snapshot, then we need a way to bypass the fact that
this patch refuses migration with snapshot metadata present.

This part of my analysis is still not quite right. The prohibition on snapshot metadata should only be on migration with VIR_MIGRATE_UNDEFINE_SOURCE; otherwise, if the domain is persistent, the domain remains behind to allow migration of the snapshot data after the migration instead of before (besides, transferring snapshot metadata before migrating requires the domain to already exist and be persistent on the target, whereas transferring snapshot metadata after migration allows transferring to either persistent or transient target). For transient domains, and without a migration v4, there is no window of time where the domain is accessible on both source and destination to do the transfer with both hosts seeing the same metadata; it will be up to the management app to save the snapshot dumpxml files pre-migration, then to redefine from those files post-migration, and the metadata is automatically cleaned up on the source when the last reference to the transient domain goes away.

--
Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org


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