On Wed, May 30, 2012 at 03:08:46PM +0530, Deepak C Shetty wrote:
> Hello All,
>     I have a draft write-up on the VDSM-libstoragemgmt integration.
> I wanted to run this thru' the mailing list(s) to help tune and
> crystallize it, before putting it on the ovirt wiki.
> I have run this once thru Ayal and Tony, so have some of their
> comments incorporated.
> I still have few doubts/questions, which I have posted below with
> lines ending with '?'
> Comments / Suggestions are welcome & appreciated.
> thanx,
> deepak
> [Ccing engine-devel and libstoragemgmt lists as this stuff is
> relevant to them too]
> --------------------------------------------------------------------------------------------------------------
> 1) Background:
> VDSM provides high level API for node virtualization management. It
> acts in response to the requests sent by oVirt Engine, which uses
> VDSM to do all node virtualization related tasks, including but not
> limited to storage management.
> libstoragemgmt aims to provide vendor agnostic API for managing
> external storage array. It should help system administrators
> utilizing open source solutions have a way to programmatically
> manage their storage hardware in a vendor neutral way. It also aims
> to facilitate management automation, ease of use and take advantage
> of storage vendor supported features which improve storage
> performance and space utilization.
> Home Page: http://sourceforge.net/apps/trac/libstoragemgmt/
> libstoragemgmt (LSM) today supports C and python plugins for talking
> to external storage array using SMI-S as well as native interfaces
> (eg: netapp plugin )
> Plan is to grow the SMI-S interface as needed over time and add more
> vendor specific plugins for exploiting features not possible via
> SMI-S or have better alternatives than using SMI-S.
> For eg: Many of the copy offload features require to use vendor
> specific commands, which justifies the need for a vendor specific
> plugin.
> 2) Goals:
>     2a) Ability to plugin external storage array into oVirt/VDSM
> virtualization stack, in a vendor neutral way.
>     2b) Ability to list features/capabilities and other statistical
> info of the array
>     2c) Ability to utilize the storage array offload capabilities
> from oVirt/VDSM.
> 3) Details:
> LSM will sit as a new repository engine in VDSM.
> VDSM Repository Engine WIP @ http://gerrit.ovirt.org/#change,192
> Current plan is to have LSM co-exist with VDSM on the virtualization nodes.
> *Note : 'storage' used below is generic. It can be a file/nfs-export
> for NAS targets and LUN/logical-drive for SAN targets.
> VDSM can use LSM and do the following...
>     - Provision storage
>     - Consume storage
> 3.1) Provisioning Storage using LSM
> Typically this will be done by a Storage administrator.
> oVirt/VDSM should provide storage admin the
>     - ability to list the different storage arrays along with their
> types (NAS/SAN), capabilities, free/used space.
>     - ability to provision storage using any of the array
> capabilities (eg: thin provisioned lun or new NFS export )
>     - ability to manage the provisioned storage (eg: resize/delete storage)

I guess vdsm will need to model a new type of object (perhaps StorageTarget) to
be used for performing the above provisioning operations.  Then, to consume the
provisioned storage, we could create a StorageConnectionRef by passing in a
StorageTarget object and some additional parameters.  Sound about right?

> Once the storage is provisioned by the storage admin, VDSM will have
> to refresh the host(s) for them to be able to see the newly
> provisioned storage.

How would this refresh affect currently connected storage and running VMs?

> 3.1.1) Potential flows:
> Mgmt -> vdsm -> lsm: create LUN + LUN Mapping / Zoning / whatever is
> needed to make LUN available to list of hosts passed by mgmt
> Mgmt -> vdsm: getDeviceList (refreshes host and gets list of devices)
>  Repeat above for all relevant hosts (depending on list passed
> earlier, mostly relevant when extending an existing VG)
> Mgmt -> use LUN in normal flows.
> 3.1.2) How oVirt Engine will know which LSM to use ?
> Normally the way this works today is that user can choose the host
> to use (default today is SPM), however there are a few flows where
> mgmt will know which host to use:
> 1. extend storage domain (add LUN to existing VG) - Use SPM and make
> sure *all* hosts that need access to this SD can see the new LUN
> 2. attach new LUN to a VM which is pinned to a specific host - use this host
> 3. attach new LUN to a VM which is not pinned - use a host from the
> cluster the VM belongs to and make sure all nodes in cluster can see
> the new LUN

You are still going to need to worry about locking the shared storage resource.
Will libstoragemgmt have storage clustering support baked in or will we continue
to rely on SPM?  If the latter is true, most/all of these operations would still
need to be done by SPM if I understand correctly.

> Flows for which there is no clear candidate (Maybe we can use the
> SPM host itself which is the default ?)
> 1. create a new disk without attaching it to any VM
> 2. create a LUN for a new storage domain

Yes, SPM would seem correct to me.

> 3.2) Consuming storage using LSM
> Typically this will be done by a virtualization administrator
> oVirt/VDSM should allow virtualization admin to
>     - Create a new storage domain using the storage on the array.
>     - Be able to specify whether VDSM should use the storage offload
> capability (default) or override it to use its own internal logic.

If vdsm can make the right decisions, I would prefer that vdsm decides when to 
hardware offload and when to use software algorithms without administrator
intervention.  It's another case where oVirt can provide value-add by
simplifying the configuration and providing optimal performance.

> 4) VDSM potential changes:
> 4.1) How to represent a VM disk, 1 LUN = 1 VMdisk or 1 LV = 1 VMdisk
> ? which bring another question...1 array == 1 storage domain OR 1
> LUN/nfs-export on the array == 1 storage domain ?

Saggi has mentioned some ideas on this topic so I will encourage him to explain
his thoughts here.

> Pros & Cons of each...
> 1 array == 1 storage domain
>     - Each new vmdisk (aka volume) will be a new lun/file on the array.
>     - Easier to exploit offload capabilities, as they are available
> at the LUN/File granularity
>     - Will there be any issues where there will be too many
> LUNs/Files ... any maxluns limit on linux hosts that we might hit ?
>         -- VDSM has been tested with 1K LUNs and it worked fine - ayal
>     - Storage array limitations on the number of LUNs can be a
> downside here.
>     - Would it be ok to share the array for hosting another storage
> domain if need be ?
>         -- Provided the existing domain is not utilising all of the
> free space
>         -- We can create new LUNs and hand it over to anyone needed ?
>     -- Changes needed in VDSM to work with raw LUNs, today it only
> has support for consuming LUNs via VG/LV.
> 1 LUN/nfs-export on the array == 1 storage domain
>     - How to represent a new vmdisk (aka vdsm volume) if its a LUN
> provisioned using SAN target ?
>         -- Will it be VG/LV as is done today for block domains ?
>         -- If yes, then it will be difficult to exploit offload
> capabilities, as they are at LUN level, not at LV level.
>     - Each new vmdisk will be a new file on the nfs-export, assuming
> offload capability is available at the file level, so this should
> work for NAS targets ?
>     - Can use the storage array for hosting multiple storage domains.
>         -- Provision one more LUN and use it for another storage
> domain if need be.
>     - VDSM already supports this today, as part of block storage
> domains for LUNs case.
> Note that we will allow user to do either one of the two options
> above, depending on need.
> 4.2) Storage domain metadata will also include the
> features/capabilities of the storage array as reported by LSM.
>         - Capabilities (taken via LSM) will be stored in the domain
> metadata during storage domain create flow.
>         - Need changes in oVirt engine as well ( see 'oVirt Engine
> potential changes' section below )

Do we want to store the exact hw capabilities or some set of vdsm chosen feature
bits that are set at create time based on the discovered hw capabilities?  The
difference would be that vdsm could choose which features to enable at create
time and update those features later if needed.

> 4.3) VDSM to poll LSM for array capabilities on a regular basis ?
> Per ayal:
>     - If we have a 'storage array' entity in oVirt Engine (see
> 'oVirt Engine potential changes' section below ) then we can have a
> 'refresh capabilities' button/verb.
>     - We can periodically query the storage array.
>     - Query LSM before running operations (sounds redundant to me,
> but if it's cheap enough it could be simplest).
>     Probably need a combination of 1+2 (query at very low frequency
> - 1/hour or 1/day + refresh button)

This problem can be aleviated by the abstraction I suggested above.  Then, LSM
can be queried only when we may want to adjust the policy connected with a
particular storage target.

> 5) oVirt Engine potential changes - as described by ayal :
>     - We will either need a new 'storage array' entity in engine to
> keep credentials, or, in case of storage array as storage domain,
> just keep this info as part of the domain at engine level.
>     - Have a 'storage array' entity in oVirt Engine to support
> 'refresh capabilities' as a button/verb.
>     - When user during storage provisioning, selects a LUN exported
> from a storage array (via LSM), the oVirt Engine would know from
> then onwards that this LUN is being served via LSM.
>       It would then  be able to query the  capabilities of the LUN
> and show it to the virt admin during storage consumption flow.
> 6) Potential flows:
>     - Create snapshot flow
>         -- VDSM will check the snapshot offload capability in the
> domain metadata
>         -- If available, and override is not configured, it will use
> LSM to offload LUN/File snapshot
>         -- If override is configured or capability is not available,
> it will use its internal logic to create
>            snapshot (qcow2).
>     - Copy/Clone vmdisk flow
>         -- VDSM will check the copy offload capability in the domain
> metadata
>         -- If available, and override is not configured, it will use
> LSM to offload LUN/File copy
>         -- If override is configured or capability is not available,
> it will use its internal logic to create
>            snapshot (eg: dd cmd in case of LUN).
> 7) LSM potential changes:
>     - list features/capabilities of the array. Eg: copy offload,
> thin prov. etc.
>     - list containers (aka pools) (present in LSM today)
>     - Ability to list different types of arrays being managed, their
> capabilities and used/free space
>     - Ability to create/list/delete/resize volumes ( LUN or exports,
> available in LSM as of today)
>     - Get monitoring info with object (LUN/snapshot/volume) as
> optional parameter for specific info. eg: container/pool free/used
> space, raid type etc.
> Need to make sure above info is listed in a coherent way across
> arrays (number of LUNs, raid type used? free/total per
> container/pool, per LUN?. Also need I/O statistics wherever
> possible.
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Adam Litke <a...@us.ibm.com>
IBM Linux Technology Center
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