On Wed, Apr 18, 2012 at 09:06:36AM -0400, Ayal Baron wrote:
> 
> 
> ----- Original Message -----
> > On Tue, Apr 17, 2012 at 03:38:25PM +0530, Shireesh Anjal wrote:
> > > Hi all,
> > > 
> > > As part of adding Gluster support in ovirt, we need to introduce
> > > some Storage Device management capabilities (on the host). Since
> > > these are quite generic and not specific to Gluster as such, we
> > > think it might be useful to add it as a core vdsm and oVirt
> > > feature.
> > > At a high level, this involves following:
> > > 
> > >  - A "Storage Devices" sub-tab on "Host" entity, displaying
> > > information about all the storage devices*
> > >  - Listing of different types of storage devices of a host
> > >     - Regular Disks and Partitions*
> > >     - LVM*
> > >     - Software RAID*
> > >  - Various actions related to device configuration
> > >     - Partition disks*
> > >     - Format and mount disks / partitions*
> > >     - Create, resize and delete LVM Volume Groups (VGs)
> > >     - Create, resize, delete, format and mount LVM Logical Volumes
> > >     (LVs)
> > >     - Create, resize, delete, partition, format and mount Software
> > > RAID devices
> > >  - Edit properties of the devices
> > >  - UI can be modeled similar to the system-config-lvm tool
> > > 
> > > The items marked with (*) in above list are urgently required for
> > > the Gluster feature, and will be developed first.
> > > 
> > > Comments / inputs welcome.
> > 
> > This seems like a big undertaking, and I would like to understand the
> > complete use case of this. Is it intended to create the block storage
> > devices on top of which a Gluster volume will be created?
> 
> Yes, but not only.
> It could also be used to create the file system on top of which you create a 
> local storage domain (just an example, there are many others, more listed 
> below).
> 
> > 
> > I must tell that we had a bad experience with exposing low level
> > commands over the Vdsm API: A Vdsm storage domain is a VG with some
> > metadata on top. We used to have two API calls for creating a storage
> > domain: one to create the VG and one to add the metadata and make it
> > an
> > SD. But it is pretty hard to handle all the error cases remotely. It
> > proved more useful to have one atomic command for the whole sequence.
> > 
> > I suspect that this would be the case here, too. I'm not sure if
> > using
> > Vdsm as an ssh-replacement for transporting lvm/md/fdisk commands is
> > the
> > best approach.
> 
> I agree, we should either provide added value or figure out a way where we 
> don't need to simply add a verb every time the underlying APIs added 
> something.
> 
> > 
> > It may be better to have a single verb for creating Gluster volume
> > out
> > of block storage devices. Something like: "take these disks,
> > partition
> > them, build a raid, cover with a vg, carve some PVs and make each of
> > them a Gluster volume".
> > 
> > Obviously, it is not simple to define a good language to describe a
> > general architecture of a Gluster voluem. But it would have to be
> > done
> > somewhere - if not in Vdsm then in Engine; and I suspect it would be
> > better done on the local host, not beyond a fragile network link.
> > 
> > Please note that currently, Vdsm makes a lot of effort not to touch
> > LVM
> > metadata of existing VGs on regular "HSM" hosts. All such operations
> > are
> > done on the engine-selected "SPM" host. When implementing this, we
> > must
> > bear in mind these safeguards and think whether we want to break
> > them.
> 
> I'm not sure I see how this is relevant, we allow creating a VG on any host 
> today and that isn't going to change...

We have one painful exception, that alone is no reason to add more. Note
that currently, Engine uses the would-be-spm for vg creation. In the
gluster use case, any host is expected to do this on async timing. It
might be required, but it's not warm and fuzzy.

> 
> In general, we know that we already need to support using a LUN even if it 
> has partitions on it (with force or something).
> 
> We know that we have requirements for more control over the VG that we create 
> e.g. support striping, control over max LV size (limited by pv extent size 
> today) etc.
> 
> We also know that users would like a way not only to use a local dir for a 
> storage domain but also create the directory + fs?

These three examples are storage domain flavors..

> 
> We know that in the gLuster use case we would like the ability to setup samba 
> over the gluster volume as well as iSCSI probably.

Now I do not see the relevance. Configuring gluster and how it exposes
its volume is something other than preparing block storage for gluster
bricks.

> 
> So although I believe that when we create a gluster volume or an ovirt 
> storage domain then indeed we shouldn't need a lot of low level commands, but 
> it would appear to me that not allowing for more control when needed is not 
> going to work and that there are enough use cases which do not involve a 
> gluster volume nor a storage domain to warrant this to be generic.

I'm not against more control; I'm against uncontrollable API such as
runThisLvmCommandAsRoot()
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