1# Logical Volumes {#logical_volumes} 2 3The Logical Volumes library is a flexible storage space management system. It provides creating and managing virtual 4block devices with variable size. The SPDK Logical Volume library is built on top of @ref blob. 5 6## Terminology {#lvol_terminology} 7 8### Logical volume store {#lvs} 9 10* Shorthand: lvolstore, lvs 11* Type name: struct spdk_lvol_store 12 13A logical volume store uses the super blob feature of blobstore to hold uuid (and in future other metadata). 14Blobstore types are implemented in blobstore itself, and saved on disk. An lvolstore will generate a UUID on 15creation, so that it can be uniquely identified from other lvolstores. 16By default when creating lvol store data region is unmapped. Optional --clear-method parameter can be passed 17on creation to change that behavior to writing zeroes or performing no operation. 18 19### Logical volume {#lvol} 20 21* Shorthand: lvol 22* Type name: struct spdk_lvol 23 24A logical volume is implemented as an SPDK blob created from an lvolstore. An lvol is uniquely identified by 25its UUID. Lvol additional can have alias name. 26 27### Logical volume block device {#lvol_bdev} 28 29* Shorthand: lvol_bdev 30* Type name: struct spdk_lvol_bdev 31 32Representation of an SPDK block device (spdk_bdev) with an lvol implementation. 33A logical volume block device translates generic SPDK block device I/O (spdk_bdev_io) operations into the 34equivalent SPDK blob operations. Combination of lvol name and lvolstore name gives lvol_bdev alias name in 35a form "lvs_name/lvol_name". block_size of the created bdev is always 4096, due to blobstore page size. 36Cluster_size is configurable by parameter. Size of the new bdev will be rounded up to nearest multiple of 37cluster_size. By default lvol bdevs claim part of lvol store equal to their set size. When thin provision 38option is enabled, no space is taken from lvol store until data is written to lvol bdev. 39By default when deleting lvol bdev or resizing down, allocated clusters are unmapped. Optional --clear-method 40parameter can be passed on creation to change that behavior to writing zeroes or performing no operation. 41 42### Thin provisioning {#lvol_thin_provisioning} 43 44Thin provisioned lvols rely on dynamic cluster allocation (e.g. when the first write operation on a cluster is performed), only space 45required to store data is used and unallocated clusters are obtained from underlying device (e.g. zeroes_dev). 46 47Sample write operations of thin provisioned blob are shown on the diagram below: 48 49 50 51Sample read operations and the structure of thin provisioned blob are shown on the diagram below: 52 53 54 55### Snapshots and clone {#lvol_snapshots} 56 57Logical volumes support snapshots and clones functionality. User may at any given time create snapshot of existing 58logical volume to save a backup of current volume state. When creating snapshot original volume becomes thin provisioned 59and saves only incremental differences from its underlying snapshot. This means that every read from unallocated cluster 60is actually a read from the snapshot and every write to unallocated cluster triggers new cluster allocation and data copy 61from corresponding cluster in snapshot to the new cluster in logical volume before the actual write occurs. 62 63The read operation is performed as shown in the diagram below: 64 65 66The write operation is performed as shown in the diagram below: 67 68 69User may also create clone of existing snapshot that will be thin provisioned and it will behave in the same way as logical volume 70from which snapshot is created. There is no limit of clones and snapshots that may be created as long as there is enough space on 71logical volume store. Snapshots are read only. Clones may be created only from snapshots or read only logical volumes. 72 73A snapshot can be removed only if there is a single clone on top of it. The relation chain will be updated accordingly. 74The cluster map of clone and snapshot will be merged and entries for unallocated clusters in the clone will be updated with 75addresses from the snapshot cluster map. The entire operation modifies metadata only - no data is copied during this process. 76 77### Inflation {#lvol_inflation} 78 79Blobs can be inflated to copy data from backing devices (e.g. snapshots) and allocate all remaining clusters. As a result of this 80operation all dependencies for the blob are removed. 81 82 83 84### Decoupling {#lvol_decoupling} 85 86Blobs can be decoupled from their parent blob by copying data from backing devices (e.g. snapshots) for all allocated clusters. 87Remaining unallocated clusters are kept thin provisioned. 88Note: When decouple is performed, only single dependency is removed. To remove all dependencies in a chain of blobs depending 89on each other, multiple calls need to be issued. 90 91## Configuring Logical Volumes 92 93There is no static configuration available for logical volumes. All configuration is done trough RPC. Information about 94logical volumes is kept on block devices. 95 96## RPC overview {#lvol_rpc} 97 98RPC regarding lvolstore: 99 100```bash 101bdev_lvol_create_lvstore [-h] [-c CLUSTER_SZ] bdev_name lvs_name 102 Constructs lvolstore on specified bdev with specified name. During 103 construction bdev is unmapped at initialization and all data is 104 erased. Then original bdev is claimed by 105 SPDK, but no additional spdk bdevs are created. 106 Returns uuid of created lvolstore. 107 Optional parameters: 108 -h show help 109 -c CLUSTER_SZ Specifies the size of cluster. By default its 4MiB. 110 --clear-method specify data region clear method "none", "unmap" (default), "write_zeroes" 111bdev_lvol_delete_lvstore [-h] [-u UUID] [-l LVS_NAME] 112 Destroy lvolstore on specified bdev. Removes lvolstore along with lvols on 113 it. User can identify lvol store by UUID or its name. Note that destroying 114 lvolstore requires using this call, while deleting single lvol requires 115 using bdev_lvol_delete rpc call. 116 optional arguments: 117 -h, --help show help 118bdev_lvol_get_lvstores [-h] [-u UUID] [-l LVS_NAME] 119 Display current logical volume store list 120 optional arguments: 121 -h, --help show help 122 -u UUID, --uuid UUID show details of specified lvol store 123 -l LVS_NAME, --lvs_name LVS_NAME show details of specified lvol store 124bdev_lvol_rename_lvstore [-h] old_name new_name 125 Change logical volume store name 126 optional arguments: 127 -h, --help show this help message and exit 128``` 129 130RPC regarding lvol and spdk bdev: 131 132```bash 133bdev_lvol_create [-h] [-u UUID] [-l LVS_NAME] [-t] [-c CLEAR_METHOD] lvol_name size 134 Creates lvol with specified size and name on lvolstore specified by its uuid 135 or name. Then constructs spdk bdev on top of that lvol and presents it as spdk bdev. 136 User may use -t switch to create thin provisioned lvol. 137 Returns the name of new spdk bdev 138 optional arguments: 139 -h, --help show help 140 -c, --clear-method specify data clusters clear method "none", "unmap" (default), "write_zeroes" 141bdev_get_bdevs [-h] [-b NAME] 142 User can view created bdevs using this call including those created on top of lvols. 143 optional arguments: 144 -h, --help show help 145 -b NAME, --name NAME Name of the block device. Example: Nvme0n1 146bdev_lvol_delete [-h] bdev_name 147 Deletes a logical volume previously created by bdev_lvol_create. 148 optional arguments: 149 -h, --help show help 150bdev_lvol_snapshot [-h] lvol_name snapshot_name 151 Create a snapshot with snapshot_name of a given lvol bdev. 152 optional arguments: 153 -h, --help show help 154bdev_lvol_clone [-h] snapshot_name clone_name 155 Create a clone with clone_name of a given lvol snapshot. 156 optional arguments: 157 -h, --help show help 158bdev_lvol_rename [-h] old_name new_name 159 Change lvol bdev name 160 optional arguments: 161 -h, --help show help 162bdev_lvol_resize [-h] name size 163 Resize existing lvol bdev 164 optional arguments: 165 -h, --help show help 166bdev_lvol_set_read_only [-h] name 167 Mark lvol bdev as read only 168 optional arguments: 169 -h, --help show help 170bdev_lvol_inflate [-h] name 171 Inflate lvol bdev 172 optional arguments: 173 -h, --help show help 174bdev_lvol_decouple_parent [-h] name 175 Decouple parent of a logical volume 176 optional arguments: 177 -h, --help show help 178``` 179
