On 2/23/19 3:00 PM, Eric Blake wrote:
Due to historical back-compat, 'virsh snapshot-create-as' favors internal snapshots (but can't be used on domains with raw storage), while 'virsh snapshot-create-as --disk-only) favors external
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snapshots. What's more, snapshots created with --disk-only while the domain was running are marked as snapshot state 'disk-snapshot', while snapshots created while the domain was offline are marked as snapshot state 'shutdown' (a 'disk-snapshot' image might not be quiescent, while a 'shutdown' snapshot always is).
But this leads to some interesting problems: if we create a --disk-only snapshot of an offline guest, and then immediately try to 'virsh snapshot-create --redefine' using the resulting XML to overwrite the existing snapashot in place, things silently succeed, but 'virsh snapshot-create --redefine --disk-only' fails because the snapshot state is not 'disk-only'. Worse, if we delete the snapshot metadata first and then try to recreate things, omitting --disk-only fails because the verification code wants to force the default of an internal snapshot (which doesn't work with raw disks), and using --disk-only fails because the verification code doesn't see the 'disk-only' state in the snapshot xml - making it impossible to recreate the snapshot metadata. Ideally, the presence or absence of the --disk-only flag, and the presence or absence of an existing snapshot being overwritten, shouldn't matter; if the XML is valid for one situation, it should always be valid to redefine the metadata for that snapshot.
Fix things by uniformly declaring that a redefined snapshot in the 'shutdown' state can permit a disk-only external snapshot (we got it right in only one out of three places in the function).
See also https://bugzilla.redhat.com/1680304; this fixes the domain-agnostic problems mentioned there, but another patch is needed to fix further oddities with the qemu driver. I did not check for sure, but this has probably been broken even prior to when commit 670e86bf (1.1.4) factored redefine prep out of qemu code into the common snapshot_conf code.
Perhaps 709b0f37c (1.0.2)? or older e260e401a5 (1.0.0) The 1.0.2 change seems to be the source of hunk 2 and 3 below, while 1.0.0 is the source of hunk 1.
Signed-off-by: Eric Blake <eblake@redhat.com> --- src/conf/snapshot_conf.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-)
Your explanation seems reasonable and although I don't think I could begin to (re) explain all the various states, conditions, [in]consistencies, etc... Still, consider it Reviewed-by: John Ferlan <jferlan@redhat.com> John [...]