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- Introduction of an External Storage Interface.

Introduction of an External Storage Interface

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Overview

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To extend the shared block storage template and give Ganeti the ability

to control and manipulate external storage (provisioning, removal,

growing, etc.) we need a more generic approach. The generic method for

supporting external shared storage in Ganeti will be to have an

ExtStorage provider for each external shared storage hardware type. The

ExtStorage provider will be a set of files (executable scripts and text

files), contained inside a directory which will be named after the

provider. This directory must be present across all nodes of a nodegroup

(Ganeti doesn't replicate it), in order for the provider to be usable by

Ganeti for this nodegroup (valid). The external shared storage hardware

should also be accessible by all nodes of this nodegroup too.

An “ExtStorage provider” will have to provide the following methods:

- Create a disk

- Remove a disk

- Grow a disk

- Attach a disk to a given node

- Detach a disk from a given node

- Verify its supported parameters

The proposed ExtStorage interface borrows heavily from the OS

interface and follows a one-script-per-function approach. An ExtStorage

provider is expected to provide the following scripts:

- `create`

- `remove`

- `grow`

- `attach`

- `detach`

- `verify`

All scripts will be called with no arguments and get their input via

environment variables. A common set of variables will be exported for

all commands, and some of them might have extra ones.

- `VOL_NAME`: The name of the volume. This is unique for Ganeti and it

  uses it to refer to a specific volume inside the external storage.

- `VOL_SIZE`: The volume's size in mebibytes.

- `VOL_NEW_SIZE`: Available only to the `grow` script. It declares the

  new size of the volume after grow (in mebibytes).

- `EXTP_name`: ExtStorage parameter, where `name` is the parameter in

  upper-case (same as OS interface's `OSP_*` parameters).

All scripts except `attach` should return 0 on success and non-zero on

error, accompanied by an appropriate error message on stderr. The

`attach` script should return a string on stdout on success, which is

the block device's full path, after it has been successfully attached to

the host node. On error it should return non-zero.

Implementation

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To support the ExtStorage interface, we will introduce a new disk

template called `ext`. This template will implement the existing Ganeti

disk interface in `lib/bdev.py` (create, remove, attach, assemble,

shutdown, grow), and will simultaneously pass control to the external

scripts to actually handle the above actions. The `ext` disk template

will act as a translation layer between the current Ganeti disk

interface and the ExtStorage providers.

We will also introduce a new IDISK_PARAM called `IDISK_PROVIDER =

provider`, which will be used at the command line to select the desired

ExtStorage provider. This parameter will be valid only for template

`ext` e.g.::

 gnt-instance add -t ext --disk=0:size=2G,provider=sample_provider1

The Extstorage interface will support different disks to be created by

different providers. e.g.::

 gnt-instance add -t ext --disk=0:size=2G,provider=sample_provider1

                         --disk=1:size=1G,provider=sample_provider2

                         --disk=2:size=3G,provider=sample_provider1

Finally, the ExtStorage interface will support passing of parameters to

the ExtStorage provider. This will also be done per disk, from the

command line::

 gnt-instance add -t ext --disk=0:size=1G,provider=sample_provider1,

                                  param1=value1,param2=value2

The above parameters will be exported to the ExtStorage provider's

scripts as the enviromental variables:

- `EXTP_PARAM1 = str(value1)`

- `EXTP_PARAM2 = str(value2)`

We will also introduce a new Ganeti client called `gnt-storage` which

will be used to diagnose ExtStorage providers and show information about

them, similarly to the way  `gnt-os diagose` and `gnt-os info` handle OS

definitions.

external storage providers (see below), e.g.::

The pool, in which to put the new instance's disk, will be defined at

the command line during `instance add`. This will become possible by

replacing the IDISK_PROVIDER parameter with a new one, called `IDISK_POOL

= pool`. The cmdlib logic will then look at the cluster-level mapping

dictionary to determine the ExtStorage provider for the given pool.

gnt-storage

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The `gnt-storage` client can be extended to support pool management

(creation/modification/deletion of pools, connection/disconnection of

pools to nodegroups, etc.). It can also be extended to diagnose and

provide information for internal disk templates too, such as lvm and

drbd.