On Wed, 08 Aug 2012 21:51:50 -0400, Edison Su <edison...@citrix.com>
wrote:
From: Mice Xia
[mailto:mice_xia@**tcloudcomputing.com<mice_...@tcloudcomputing.com>
]
...
For following scenarios I need some suggestions:
a) 'vm snapshot, detach volume and attach it to another VM, rollback
snapshot',
This is not a problem. A volume and it's (several) snapshots are described
in the HYPERVISOR's native metadata file closely associated with the
volume. If the hypervisor doesn't actually manage that relationship and
associated housekeeping tasks, CloudStack has no business trying to
implement it. The option is simply not available to the user.
Cloud automation is NOT about inventing new things, it's about automating
what is currently possible within the capabilities of the particular
hypervisor and storage mixture at hand. Where cloud software goes wrong is
thinking it's the master of everything. It's NOT, and never will be. All
configuration and runtime state must be pulled from the actual devices
(hypervisors, storage arrays, network gear, etc.) because things will
change underneath it. It is unforgivably naive to assume there won't be
outside influence. Of course we'd all like to pretend that the cloud knows
all and that it is always authoritative, but if you write the software
under that assumption, the users will be mighty ticked off when it's not
the case and things break left and right.
b) 'vm snapshot, detach and destroy volume, rollback snapshot',
That's not a valid operation. A snapshot is ALWAYS associated with it's
parent 'disk'. If you delete the parent, all snapshots are deleted with it.
Three candidate solutions that I can figure out now,
1) disallow detach volumes if specified VM has VM snapshots.
uh, no.
2) allow to snapshot/rollback, for a), this will result in two volumes,
one attached to anther VM, one attached to VM that rollback from
snapshot; for
This is a matter for the hypervisor or storage array. AFAIK even ESX
doesn't let you do this even if the parent disk is marked RO since it puts
an exclusive lock on the volume. Storage arrays pull this off by
thin-cloning the source so the VM thinks it has it's own disk even when it
didn't start out that way.
Again, Cloudstack isn't about doing storage trickery. It's about making
the proper calls to the hypervisor or (maybe?) the storage array to do such
fancy cloning. So if you want to provision a VM and attach a particular
volume/snapshot sequence that isn't already locked for use, then no
problem; treat it like you own it outright and launch. Any successive
creation/destruction of snapshots is the purview of that one active VM and
doesn't require any fancy footwork. If on the other hand the source
volume+snapshot is already open for use, then you have to ask the storage
provider for a thick or thin clone and you lose the ability to go back in
time without trashing the private copy and re-attaching/re-cloning to the
original.
The use case for cloud is 99.95% "dumb". Let's not complicate the
situation unnecessarily. If qcow/rbd/lvm can't do it with their standard
command set, then it's not available. If you're using 3par/emc/netapp as
the storage and they do have the proper calls, then you can permit such
things. Actually, let me rephrase that. Any fancy cloning or snapshotting
calls MUST be sent to the hypervisor only!! It is up to the hypervisor to
issue qcow/rbd/lvm/netapp/emc commands to get it done. Why? Hypervisor
vendors actually test this stuff and have the resources and incentive to
make sure it works. Cloud software again has no business trying to take
over the hypervisor's role. If KVM/Xen are deficient in some aspect, then
fix the hypervisor; do not try to use cloud software to band-aid the matter.
This early in the CloudStack lifecycle, complicated or exotic operations
should probably require deliberate manual involvement anyhow.
For VM based snapshot, we should not allow user to dynamically
change(attach or detach) VM's disks if there are VM-based snapshot taken on
this VM.
again, no. It is not up to the cloud to manage/track volume/snapshot
state. It instead queries the hypervisor for that information.
It seems to me (admittedly a very new member here) that there is enormous
feature creep being introduced. Cloud automation is really fairly
straightforward.
for any hypervisor
where is it (IP, logical/physical location)
what flavor (KVM, Xen, ESX) and version - tells us which command set to
use
what network gear is attached and what ports need to be modified for to
add/drop vlan membership at the switch level
what is it's utilization
for any storage provider
how to interface with it
what storage containers are defined
to which hypervisors should they be attached, and attachment parameters
for any VM
to which account does it belong
which disks does it need and on what storage container to place them
where to find the master disk templates (if any)
what network bridge/portgroups it needs to have interfaces on
what MAC to assign the various interfaces (or have Hypervisor
auto-assign)
what VLANs to assign to interface or interface aliases
what IP address to assign to the interfaces
are there any locality or performance factors to influence selection of
deployment destination
Basically all the cloud does is dynamically create what amounts to a
VMware .vmx file or a libvirt instantiation script and run it against the
chosen hypervisor host. And if needed prep the hypervisor with network
underpinnings and attach appropriate storage in order to run the guest.
Yes, I realize I simplified a bit but there isn't a whole lot more to it.
