Volumes and snapshots

Volumes are the basic service of the StorPool storage system. A volume always have a name, a global ID and a certain size. It can be read from and written to. It could be attached to hosts as read-only or read-write block device under the /dev/storpool directory (available as well at /dev/storpool-byid).

The volume name is a string consisting of one or more of the allowed characters - upper and lower latin letters (a-z,A-Z), numbers (0-9) and the delimiter dot (.), colon (:), dash (-) and underscore (_).

Creating a volume

Deleting a volume

Renaming a volume

Resizing a volume

Snapshots

Snapshots are read-only point-in-time images of volumes. They are created once and cannot be changed. They can be attached to hosts as read-only block devices under /dev/storpool.

All volumes and snapshots share the same name-space. Names of volumes and snapshots are unique within a StorPool cluster. This diagram illustrates the relationship between a snapshot and a volume. Volume vol1 is based on snapshot snap1. vol1 contains only the changes since snap1 was taken. In the common case this is a small amount of data. Arrows indicate a child-parent relationship. Each volume or snapshot may have exactly one parent which it is based upon. Writes to vol1 are recorded within the volume. Reads from vol1 may be served by vol1 or by its parent snapshot - snap1, depending on whether vol1 contains changed data for the read request or not.

Snapshots and volumes are completely independent. Each snapshot may have many children (volumes and snapshots). Volumes cannot have children.

snap1 contains a full image. snap2 contains only the changes since snap1 was taken. vol1 and vol2 contain only the changes since snap2 was taken.

Creating a snapshot of a volume

There is a volume named vol1.

After the first snapshot the state of vol1 is recorded in a new snapshot named snap1. vol1 does not occupy any space now, but will record any new writes which come in after the creation of the snapshot. Reads from vol1 may fall through to snap1.

Then the state of vol1 is recorded in a new snapshot named snap2. snap2 contains the changes between the moment snap1 was taken and the moment snap2 was taken. snap2’s parent is the original parent of vol1.

Creating a volume based on an existing snapshot (a.k.a. clone)

Before the creation of vol1 there is a snapshot named snap1.

A new volume, named vol1 is created. vol1 is based on snap1. The newly created volume does not occupy any space initially. Reads from the vol1 may fall through to snap1 or to snap1’s parents (if any).

Deleting a snapshot

vol1 and vol2 are based on snap1. snap1 is based on snap0. snap1 contains the changes between the moment snap0 was taken and when snap1 was taken. vol1 and vol2 contain the changes since the moment snap1 was taken.

After the deletion, vol1 and vol2 are based on snap1’s original parent (if any). In the example they are now based on snap0. When deleting a snapshot, the changes contained therein will not be propagated to its children and StorPool will keep the snap1 in deleting state to prevent from an explosion of disk space usage.

Rebase to null (a.k.a. promote)

vol1 is based on snap1. snap1 is in turn based on snap0. snap1 contains the changes between the moment snap0 was taken and when snap1 was taken. vol1 contains the changes since the moment snap1 was taken.

After promotion vol1 is not based on a snapshot. vol1 now contains all data, not just the changes since snap1 was taken. Any relation between snap1 and snap0 is unaffected.

Rebase

vol1 is based on snap1. snap1 is in turn based on snap0. snap1 contains the changes between the moment snap0 was taken and when snap1 was taken. vol1 contains the changes since the moment snap1 was taken.

After the rebase operation vol1 is based on snap0. vol1 now contains all changes since snap0 was taken, not just since snap1. snap1 is unchanged.

Example use of snapshots

This is a semi-realistic example of how volumes and snapshots may be used. There is a snapshot called base.centos7. This snapshot contains a base CentOS 7 VM image, which was prepared carefully by the service provider. There are 3 customers with 4 virtual machines each. All virtual machine images are based on CentOS 7, but may contain custom data, which is unique to each VM.

This example shows another typical use of snapshots - for restore points back in time for this volume. There is one base image for Centos 7, three snapshot restore points and one live volume cust123.v.1

More information

StorPool provides a tool for generating a visual representation of the volumes and snapshots. For details, see StorPool tree.
You can manage volumes and snapshots using the command line interface. For details, see Volumes and Snapshots.