# Storage See also: [How to manage storage](https://documentation.ubuntu.com/juju/3.6/howto/manage-storage.md#manage-storage) In Juju, **storage** refers to a data volume that is provided by a [cloud](https://documentation.ubuntu.com/juju/3.6/reference/cloud.md#cloud). Depending on how things are set up during deployment, the data volume can be machine-dependent (e.g., a directory on disk tied to the machine which goes away if the unit is destroyed) or machine-independent (i.e., it can outlive a machine and be reattached to another machine). Most storage can be dynamically added to, and removed from, a unit. However, by their nature, some types of storage cannot be dynamically managed (see [maas](https://documentation.ubuntu.com/juju/3.6/reference/cloud/list-of-supported-clouds/maas.md#storage-provider-maas)). Also, certain cloud providers may impose restrictions when attaching storage (see [ebs](https://documentation.ubuntu.com/juju/3.6/reference/cloud/list-of-supported-clouds/amazon-ec2.md#storage-provider-ebs)). ## Storage directive In Juju, a **storage directive** is a collection of storage specifications that can be used to dictate how storage is allocated when provisioning storage for an application. This directive has the form ```text [=, , >] ``` where - `` is the storage name as defined in the charm (see more: [Charmcraft | ``](https://documentation.ubuntu.com/charmcraft/stable/reference/files/charmcraft-yaml-file/#storage)); - `` is a pre-defined [storage pool](#storage-pool); - `` is the storage volume count; - `` is the size of each storage volume. The order of the arguments does not actually matter – they are identified based on a regex (pool names must start with a letter and sizes must end with a unit suffix). If at least one storage directive component is specified, the following default values come into effect: * ``: the default storage pool. * ``: the minimum number required by the charm, or ‘1’ if the storage is optional * ``: determined from the charm’s minimum storage size, or 1GiB if the charm does not specify a minimum In the absence of any explicit storage directive, the storage will be put on the root filesystem (`rootfs`). ## Storage pool See also: [How to manage storage pools](https://documentation.ubuntu.com/juju/3.6/howto/manage-storage-pools.md#manage-storage-pools) A **storage pool** is a mechanism for administrators to define sources of storage that they will use to satisfy application storage requirements. A single pool might be used for storage from units of many different applications - it is a resource from which different stores may be drawn. A pool describes [storage provider](#storage-provider)-specific parameters for creating storage, such as performance (e.g. IOPS), media type (e.g. magnetic vs. SSD), or durability. For many providers, there will be a shared resource where storage can be requested (e.g. for Amazon EC2, `ebs`). Creating pools there maps provider specific settings into named resources that can be used during deployment. Pools defined at the model level are easily reused across applications. Pool creation requires a pool name, the provider type and attributes for configuration as space-separated pairs, e.g. tags, size, path, etc. ## Storage provider In Juju, a **storage provider** refers to the technology used to make storage available to a charm. ### List of storage providers There are three storage providers you can use with all clouds: `loop`, `rootfs`, and `tmpfs`. In addition, for some clouds there are also cloud-specific providers. #### Cloud-specific storage providers Many clouds provide additional storage providers beyond the generic ones. For the cloud-specific storage providers available on your cloud, see [List of supported clouds](https://documentation.ubuntu.com/juju/3.6/reference/cloud/list-of-supported-clouds.md#list-of-supported-clouds) > `` > Storage. #### `loop` See also: [Wikipedia | Loop device](https://en.wikipedia.org/wiki/Loop_device) Block-type. Creates a file on the unit’s root filesystem, associates a loop device with it. The loop device is provided to the charm. #### NOTE Loop devices require extra configuration to be used within LXD. See more: [lxd](https://documentation.ubuntu.com/juju/3.6/reference/cloud/list-of-supported-clouds/lxd.md#storage-provider-lxd). #### `rootfs` See also: [The Linux Kernel Archives | ramfs, rootfs and initramfs](https://www.kernel.org/doc/Documentation/filesystems/ramfs-rootfs-initramfs.txt) Filesystem-type. Creates a sub-directory on the unit’s root filesystem for the unit/charmed operator to use. #### `tmpfs` See also: [Wikipedia | Tmpfs](https://en.wikipedia.org/wiki/Tmpfs) Filesystem-type. Creates a temporary file storage facility that appears as a mounted file system but is stored in volatile memory.