----- Original Message -----
> I've been working on refactoring the storageDomain\images system in
> VDSM.
> Apart from facilitating various features I've also been trying to
> make adding new SD types easier and making the image manipulation
> bits consistent across domain implementation.
> 
> Currently in order to create a new domain type you have to create a
> new StoageDomain,Image and Volume objects and implement all the
> logic to manipulate them. Apart from being cumbersome and redundant
> it also make mixed clustered very hard to do.
> 
> On of the big changes I put in is separating the image manipulation
> with the actual storage work.
> 
> Instead of each domain type implementing createImage and co you have
> one class responsible for all the image manipulation in the cluster.
> 
> All you have to do facilitate a new storage type is to create a
> domain engine.
> 
> A domain engine is a python class that implement a minimal interface.
> 1. It has to be able to create resize and delete a slab (slab being a
> block of writable storage like a lun\lv\file)
> 2. It has to be able to create and delete tags (tags are pointers to
> slabs)
> 
> The above function are very easy to implement and require very little
> complexity. All the heavy lifting (image manipulation, cleaning,
> transaction, atomic operations, etc) is managed by the Image Manager
> that just uses this unified interface to interact with the different
> storage types)
> 
> In cases where a domain might have a special non-standard features I
> introduce the concept of capabilities. A domain engine can declare
> support for certain capabilities (eg. native snapshotting) and
> implement additional interfaces. If the image manager sees that the
> domain implements a capability it will use it if not it will use a
> default implementation that uses the default must have verbs. This
> is similar to just having drawLine and having drawRect. This is done
> automatically and at runtime.
> 
> I like to compare this to how OpenGL will use software rendering if a
> certain standard feature is not implemented by the card so you might
> get a slower but still correct result.
> 
> Now, libstorage is another way to abstract interactions and
> capabilities for different storage types and have a unified API for
> accessing them.
> 
> Building a repo engine on top of libstorage is completely possible.
> But as you can see this creates a redundant layer of abstractions in
> the libstorage side.
> 
> As I see it if you just want to have you storage supported by ovirt
> creating a repo engine is simpler as you can use high level concepts
> and I do plan to have engines run as their own processes so you
> could use whatever licence, language and storage server API you
> choose.

Regardless of the point of the thread below, it's probably worth emphasizing 
why you want each domain to have its own process.

> 
> Also libstorage will have to keep it's abstraction at a much lower
> level. This means exposing target specific flags and abilities.
> eWhile this is good in concept it will mean that the repo engine
> wrapping libstorage will have to juggle all those flags and calls
> instead of having different distinct class for each storage type
> with it's own specific hacks in place.
> 
> Just as a current example, we currently use the same "engine" for
> nfs3 and nfs4. This means that when we are running on nfs4 we are
> still doing all the hacks that are meant to circumvent issues with
> v3 being stateless. This is no longer relevant as v4 is stateful.
> And what about SAMBA? or gluster? You got to have special hacks for
> boths
> 
> What I'm saying is that if in the relatively simple world of NAS
> where we have a proven abstraction (file access commands, POSIX). We
> can't find a way to create a 1 class to rule them all. How can we
> expect to have a sane solution for the crazy world of SAN.
> 
> I'm not saying we shouldn't create an engine for libstorage, just
> that we should treat it like we treat sharefs. As a simple generic
> non bullet proof\optimized implementation.

As far as I understand it the idea is for libStorage to support capabilities.  
The point is to avoid having to know how to run storage array specific 
commands.  Even if we end up having specialized domains I think we would still 
want to use libStorage to manipulate the storage array.
I would see this as bringing SAN to something similar to the POSIX semantics 
(which as you noted are not perfect) so that our complexity would be reduced 
and we could focus on higher level logic.
Take into consideration that doing it otherwise could mean that we would need a 
storage domain type per storage array *model* (i.e. if Netapp has 10 different 
models with a bit different semantics...)



> 
> Let the flaming commence!
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