lvol/doc: update lvol documentation

* Shorthand: lvol

* Type name: struct spdk_lvol

A logical volume is implemented as an SPDK blob created from an lvolstore. An lvol is uniquely identified by its lvol ID and lvolstore UUID from which it was created.

A logical volume is implemented as an SPDK blob created from an lvolstore. An lvol is uniquely identified by its UUID. Lvol additional can have alias name.

## Logical volume block device {#lvol_bdev}

* Type name: struct spdk_lvol_bdev

Representation of an SPDK block device (spdk_bdev) with an lvol implementation.

A logical volume block device translates generic SPDK block device I/O (spdk_bdev_io) operations into the equivalent SPDK blob operations. Combination of lvol ID and lvolstore UUID gives lvol_bdev name in a form "uuid/lvolid". block_size of the created bdev is always 4096, due to blobstore page size. Cluster_size is configurable by parameter.

A logical volume block device translates generic SPDK block device I/O (spdk_bdev_io) operations into the equivalent SPDK blob operations. Combination of lvol name and lvolstore name gives lvol_bdev alias name in a form "lvs_name/lvol_name". block_size of the created bdev is always 4096, due to blobstore page size. Cluster_size is configurable by parameter.

Size of the new bdev will be rounded up to nearest multiple of cluster_size.

By default lvol bdevs claim part of lvol store equal to their set size. When thin provision option is enabled, no space is taken from lvol store until data is written to lvol bdev.

## Snapshots and clone {#lvol_snapshots}

Logical volumes support snapshots and clones functionality. User may at any given time create snapshot of existing logical volume to save a backup of current volume state.

When creating snapshot original volume becomes thin provisioned and saves only incremental differences from its underlying snapshot. This means that every read from unallocated cluster is actually a read from the snapshot and

every write to unallocated cluster triggers new cluster allocation and data copy from corresponding cluster in snapshot to the new cluster in logical volume before the actual write occurs.

User may also create clone of existing snapshot that will be thin provisioned and it will behave in the same way as logical volume from which snapshot is created.

There is no limit of clones and snapshots that may be created as long as there is enough space on logical volume store. Snapshots are read only. Clones may be created only from snapshots.

# Configuring Logical Volumes

There is no static configuration available for logical volumes. All configuration is done trough RPC. Information about logical volumes is kept on block devices.

RPC regarding lvolstore:

```

construct_lvol_store [-h] [-c CLUSTER_SZ] base_name

    Constructs lvolstore on specified bdev. During construction bdev is unmapped

    at initialization and all data is erased. Then original bdev is claimed by

construct_lvol_store [-h] [-c CLUSTER_SZ] bdev_name lvs_name

    Constructs lvolstore on specified bdev with specified name. During

    construction bdev is unmapped at initialization and all data is

    erased. Then original bdev is claimed by

    SPDK, but no additional spdk bdevs are created.

    Returns uuid of created lvolstore.

    Optional paramters:

    -h  show help

    -c  CLUSTER_SZ Specifies the size of cluster. By default its 4MiB.

destroy_lvol_store [-h] uuid

destroy_lvol_store [-h] [-u UUID] [-l LVS_NAME]

    Destroy lvolstore on specified bdev. Removes lvolstore along with lvols on

    it. Note that destroying lvolstore requires using this call, while deleting

    single lvol requires using delete_bdev rpc call.

    it. User can identify lvol store by UUID or its name. Note that destroying

    lvolstore requires using this call, while deleting single lvol requires

    using delete_bdev rpc call.

    optional arguments:

    -h, --help  show help

get_lvol_stores [-h] [NAME]

get_lvol_stores [-h] [-u UUID] [-l LVS_NAME]

    Display current logical volume store list

    optional arguments:

    -h, --help  show help

    NAME, show details of specified lvol store

    -u UUID, --uuid UUID  show details of specified lvol store

    -l LVS_NAME, --lvs_name LVS_NAME  show details of specified lvol store

rename_lvol_store [-h] old_name new_name

    Change logical volume store name

    optional arguments:

    -h, --help  show this help message and exit

```

RPC regarding lvol and spdk bdev:

```

construct_lvol_bdev [-h] [-t] uuid size

    Creates lvol with specified size on lvolstore specified by its uuid.

    Then constructs spdk bdev on top of that lvol and presents it as spdk bdev.

construct_lvol_bdev [-h] [-u UUID] [-l LVS_NAME] [-t] lvol_name size

    Creates lvol with specified size and name on lvolstore specified by its uuid

    or name. Then constructs spdk bdev on top of that lvol and presents it as spdk bdev.

    User may use -t switch to create thin provisioned lvol.

    Returns the name of new spdk bdev

    optional arguments:

    -h, --help  show help

get_bdevs [-h] [-b NAME]

    User can view created bdevs using this call including those created on top of lvols.

    optional arguments:

    Deletes spdk bdev

    optional arguments:

    -h, --help  show help

snapshot_lvol_bdev [-h] lvol_name snapshot_name

    Create a snapshot with snapshot_name of a given lvol bdev.

    optional arguments:

    -h, --help  show help

clone_lvol_bdev [-h] snapshot_name clone_name

    Create a clone with clone_name of a given lvol snapshot.

    optional arguments:

    -h, --help  show help

rename_lvol_bdev [-h] old_name new_name

    Change lvol bdev name

    optional arguments:

    -h, --help  show help

resize_lvol_bdev [-h] name size

    Resize existing lvol bdev

    optional arguments:

    -h, --help  show help

```

# Restrictions

- Nesting logical volumes on each other is not supported.

- Resizing lvol bdev is experimental. Code is present but not used.