The loop isn't that big of a deal -- but you could eliminate it
entirely if you just index the in-memory snapshot table via the
SnapshotNamespace variant instead of just indexing snapshots by name
(e.g. ImageCtx::snap_ids key switches from a string to a namespace).
This would be required anyway since you might otherwise have duplicate
names between namespaces.
On Wed, Jan 4, 2017 at 1:29 PM, Victor Denisov <vdenisov@xxxxxxxxxxxx> wrote:
> It looks like next CDM is only next month. Let's try to figure it out in email.
>
>> Since you know which images are linked to the group and you know which
>> snapshots are in the group and which group snapshots are in the image,
>> you can reconcile any issues using the details in the
>> GroupSnapshotNamespace -- there shouldn't be any need to depend on the
>> actual snapshot name (it could technically just be a randomly assigned
>> UUID).
>
> Let's say I have a consistency group and a snapshot of this group: CG
> and CGSNAP.
> Images in this snapshot I'll define as:
> CG.images[0] - image1
> CG.images[1] - image2
> CG.images[2] - image3
>
> Image snapshots in cg snapshot will be:
> CG.CGSNAP.snaps[0] - reference to snapshot of image 1
> CG.CGSNAP.snaps[1] - reference to snapshot of image 2
>
> Imagine that this snapshot was created, but wasn't finalized.
> CG.CGSNAP.state == PENDING.
> CG.CGSNAP.snaps.length == 0;
> I'll be writing in pseudo code.
>
> Now, let's say we want to remove this pending CGSNAP. This is the code
> how it's currently implemented:
>
> for (image: CG.images) {
> snap_name = image.id + "_" + CG.CGSNAP.id + "_" + CG.id // This name
> is unique because of uniqueness of the tuple (image.id, CG.CGSNAP.id,
> CG.id)
> remove_image_snapshot(snap_name);
> }
> remove_cg_snap(CGSNAP);
>
> However, if we don't rely on the name then this is how I envision the code:
>
> for (image: CG.images) {
> for (snap: image.snaps) {
> if (snap.namespace.cg_id == CG.id && snap.namespace.cg_snap_id ==
> CG.CGSNAP.id) { // this is our snapshot
> remove_image_snapshot(snap.name);
> }
> }
> }
> remove_cg_snap(CGSNAP);
>
> In this solution I don't like the internal loop.
> What do you think?
>
> Thanks,
> Victor.
>
>
> On Tue, Jan 3, 2017 at 6:56 PM, Jason Dillaman <jdillama@xxxxxxxxxx> wrote:
>> After starting the process of creating a group snapshot, you will
>> already have all the necessary data for the group snapshot namespace
>> [1] (group pool, group id, and group snapshot id) and the group
>> snapshot should be persistently recorded to disk as
>> GROUP_SNAPSHOT_STATE_PENDING.
>>
>> Looking at the snapshot create state machine [2], I don't see any
>> place that a crash (or similar failure) would matter before the actual
>> image snapshot record is created atomically. You would pass the fully
>> populated GroupSnapshotNamespace to snap_create, and if the snapshot
>> is created, it's linked to the group via that namespace and any
>> failures afterwards don't matter since they are linked -- if the
>> snapshot fails to be created, it isn't linked to the group but the
>> snapshot doesn't exist either so there isn't anything to clean up.
>>
>> Since you know which images are linked to the group and you know which
>> snapshots are in the group and which group snapshots are in the image,
>> you can reconcile any issues using the details in the
>> GroupSnapshotNamespace -- there shouldn't be any need to depend on the
>> actual snapshot name (it could technically just be a randomly assigned
>> UUID).
>>
>> Perhaps we could talk about this at a future RBD standup meeting that
>> you are able to join (or the next CDM).
>>
>> [1] https://github.com/ceph/ceph/blob/master/src/cls/rbd/cls_rbd_types.h#L249
>> [2] https://github.com/ceph/ceph/blob/master/src/librbd/operation/SnapshotCreateRequest.h#L28
>>
>> On Tue, Jan 3, 2017 at 7:40 PM, Victor Denisov <vdenisov@xxxxxxxxxxxx> wrote:
>>> Let's say we start creating a group snapshot.
>>> We invoke async snap_create method in Operations class.
>>> When we invoke this method we provide it with the snapshot name.
>>>
>>> While we are wating for the response we can be aborted.
>>> As a result we will be able to find the exact image snapshot using only its name
>>> as this was the only information we had at the time of running
>>> snap_create method.
>>>
>>> If snap_create was successful we will be able to find the snapshot
>>> otherwise we will not.
>>> However if we allow renaming snapshots from GroupSnapshotNamespace
>>> then we may not find the snapshot even if it
>>> was created successfully.
>>>
>>> On Fri, Dec 16, 2016 at 6:53 AM, Jason Dillaman <jdillama@xxxxxxxxxx> wrote:
>>>> Can you give a little background on this specific inconsistent case
>>>> you are referring to?
>>>>
>>>> On Thu, Dec 15, 2016 at 7:05 PM, Victor Denisov <vdenisov@xxxxxxxxxxxx> wrote:
>>>>> Yes, but if image's snapshot is renamed then I'm not able to find this
>>>>> snapshot having only group's snapshot in an inconsistent state for
>>>>> example.
>>>>>
>>>>> V.
>>>>>
>>>>> On Thu, Dec 15, 2016 at 7:10 AM, Jason Dillaman <jdillama@xxxxxxxxxx> wrote:
>>>>>> I think I might be confused. When creating a group snapshot, we have
>>>>>> the ConsistencyGroupSnapshot that allows you to store the necessary
>>>>>> linkage between the image's snapshot and its associated group snapshot
>>>>>> [1]. Why not just name the image's snapshots to the same name as the
>>>>>> parent group snapshot name and search the snapshot's metadata to get
>>>>>> the linkage?
>>>>>>
>>>>>> [1] https://github.com/ceph/ceph/blob/master/src/cls/rbd/cls_rbd_types.h#L255
>>>>>>
>>>>>> On Tue, Dec 13, 2016 at 8:03 PM, Victor Denisov <vdenisov@xxxxxxxxxxxx> wrote:
>>>>>>> Jason,
>>>>>>>
>>>>>>> My current implementation of consistency group snapshot feature names
>>>>>>> image snapshots like: <group_pool>_<group_id>_<group_snap_id>
>>>>>>> I rely on this fact when I need to remove a consistency group. It's
>>>>>>> necessary because if some of image snapshots were created, but the
>>>>>>> whole group snapshot operation failed,
>>>>>>> then the only way to find those dangling image snapshots is by this name.
>>>>>>>
>>>>>>> It means that we should forbid renaming snapshots from
>>>>>>> ConsistencyGroupSnapshot namespace.
>>>>>>>
>>>>>>> Another option is to allocate image snapshot ids during the creation
>>>>>>> of group snapshot, but this requires a major rewrite of the whole
>>>>>>> process of snapshot creation for images.
>>>>>>>
>>>>>>> What is your opinion on this?
>>>>>>>
>>>>>>> Thanks,
>>>>>>> V.
>>>>>>>
>>>>>>>
>>>>>>> On Tue, Nov 1, 2016 at 8:01 AM, Jason Dillaman <jdillama@xxxxxxxxxx> wrote:
>>>>>>>> I chatted with Xing on IRC this morning re: Cinder generic groups. It
>>>>>>>> sounds like RBD will need to support preserving the image's
>>>>>>>> consistency group snapshots even if the image is removed from the
>>>>>>>> group. In the OpenStack case, you won't have to worry about the image
>>>>>>>> being deleted while it still has associated group snapshots.
>>>>>>>>
>>>>>>>> We will also want to support being able to clone child images from a
>>>>>>>> group snapshot to ensure that we can thin provision new groups volumes
>>>>>>>> when creating a new group from a group snapshot. This means that the
>>>>>>>> group snapshots should be able to be protected/unprotected just like
>>>>>>>> standard user snapshots.
>>>>>>>>
>>>>>>>> On Mon, Oct 31, 2016 at 9:07 AM, Jason Dillaman <jdillama@xxxxxxxxxx> wrote:
>>>>>>>>> Looking at the Cinder codebase, I don't see any such restriction that
>>>>>>>>> would prevent you from removing a volume from a consistency group that
>>>>>>>>> has associated snapshots. I would double-check on the OpenStack
>>>>>>>>> development mailing list if this is correct and is the intent. Worst
>>>>>>>>> case, the RBD driver could raise an exception if there are still
>>>>>>>>> consistency group snapshots associated to the image.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Fri, Oct 28, 2016 at 6:41 PM, Victor Denisov <vdenisov@xxxxxxxxxxxx> wrote:
>>>>>>>>>> Another thing that bothers me. When we remove an image from a consistency group.
>>>>>>>>>> Should we remove all snapshots of this image that were created as part
>>>>>>>>>> of a consistency group snapshot?
>>>>>>>>>>
>>>>>>>>>> The easiest solution would be to remove all snapshots that are in
>>>>>>>>>> GroupSnapshotNamespace and reference this consistency group.
>>>>>>>>>> I looked into cinder docs for this feature:
>>>>>>>>>> http://docs.openstack.org/admin-guide/blockstorage-consistency-groups.html
>>>>>>>>>>
>>>>>>>>>> But it's not clear to me which behavior cinder expects.
>>>>>>>>>>
>>>>>>>>>> Thanks,
>>>>>>>>>> V.
>>>>>>>>>>
>>>>>>>>>> On Wed, Oct 26, 2016 at 6:16 AM, Jason Dillaman <jdillama@xxxxxxxxxx> wrote:
>>>>>>>>>>> In a perfect world, it would be nice to add a new optional to "rbd
>>>>>>>>>>> snap ls" to show all snapshots (with a new column to indicate the
>>>>>>>>>>> associated namespace).
>>>>>>>>>>>
>>>>>>>>>>> On Tue, Oct 25, 2016 at 11:07 PM, Victor Denisov <vdenisov@xxxxxxxxxxxx> wrote:
>>>>>>>>>>>> Question. When we print out snapshots of an image, should the group
>>>>>>>>>>>> snapshots be listed, or should they be marked as special snapshots?
>>>>>>>>>>>>
>>>>>>>>>>>> Thanks,
>>>>>>>>>>>> V.
>>>>>>>>>>>>
>>>>>>>>>>>> On Mon, Oct 10, 2016 at 3:14 PM, Victor Denisov <vdenisov@xxxxxxxxxxxx> wrote:
>>>>>>>>>>>>> Ok. I didn't have any intention to throw exceptions.
>>>>>>>>>>>>> I was more concerned about whether it's ok to allocate and delete
>>>>>>>>>>>>> objects or I should use smart pointers.
>>>>>>>>>>>>>
>>>>>>>>>>>>> On Mon, Oct 10, 2016 at 7:18 AM, Jason Dillaman <jdillama@xxxxxxxxxx> wrote:
>>>>>>>>>>>>>> The only place exceptions are routinely used is within the "::decode"
>>>>>>>>>>>>>> functions. I would prefer to see the code not throwing new exceptions
>>>>>>>>>>>>>> on purpose.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> On Fri, Oct 7, 2016 at 2:26 PM, Victor Denisov <vdenisov@xxxxxxxxxxxx> wrote:
>>>>>>>>>>>>>>> Are any exceptions used in librbd code? Should the code be exception safe?
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>> V.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> On Fri, Sep 16, 2016 at 10:37 AM, Jason Dillaman <jdillama@xxxxxxxxxx> wrote:
>>>>>>>>>>>>>>>> On Thu, Sep 15, 2016 at 7:17 PM, Victor Denisov <vdenisov@xxxxxxxxxxxx> wrote:
>>>>>>>>>>>>>>>>> if (struct_v >= 5) {
>>>>>>>>>>>>>>>>> ::decode(snapshot_namespace, p);
>>>>>>>>>>>>>>>>> } else {
>>>>>>>>>>>>>>>>> snapshot_namespace = cls::rbd::UserSnapshotNamespace();
>>>>>>>>>>>>>>>>> }
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> then code for ::encode function of cls_rbd_snap would change accordingly:
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> instead of
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> boost::apply_visitor(cls::rbd::EncodeSnapshotTypeVisitor(bl),
>>>>>>>>>>>>>>>>> snapshot_namespace);
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> I would do:
>>>>>>>>>>>>>>>>> ::encode(snapshot_namespace, bl);
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> +1 -- looks good to me
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> --
>>>>>>>>>>>>>>>> Jason
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> --
>>>>>>>>>>>>>> Jason
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> --
>>>>>>>>>>> Jason
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> --
>>>>>>>>> Jason
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> --
>>>>>>>> Jason
>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>> Jason
>>>>
>>>>
>>>>
>>>> --
>>>> Jason
>>
>>
>>
>> --
>> Jason
--
Jason
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