Overview

A duplicate image of a volume at a certain point in time is referred to as a snapshot. If you unintentionally update copy-source volume data, you can restore the data from a previously taken snapshot. The copy-source volume is referred to as the primary volume (P-VOL), and the copy-destination volume is referred to as the secondary volume (S-VOL). When the P-VOL is updated, only the data existing before updating in the update-target area is copied to the storage pool as data in snapshot volumes.

By storing only difference data between P-VOL and S-VOL, this scheme reduces capacity and allows a cost-efficient copy to be created. Furthermore, because data is shared between S-VOLs, less capacity is used even if multiple generations of snapshots are taken.

CAUTION:

You should back up any necessary data to another device (for example, via the host) with the following in mind. Also, monitor the used capacity of the storage pool to secure sufficient free space.
- The S-VOL uses the data in the P-VOL. Therefore, if a failure occurs in the P-VOL and the data of the P-VOL becomes unavailable, the data of the S-VOL also becomes unavailable.
- Difference data is stored in the storage pool. If the storage pool becomes unavailable due to a failure or the storage pool becoming full, data in snapshot volumes will be lost.
- If data in snapshot volumes is lost during restore (that is, during execution of an operation to restore P-VOL data from S-VOL data), data in the restore-destination volume (P-VOL) also becomes unavailable.
- You can take snapshots of a volume with data reduction enabled (P-VOL) to create snapshot volumes (S-VOLs). However, the data reduction function is not enabled for the S-VOLs. Consider that when managing storage pool capacity.
Snapshot volumes cannot be expanded. To expand volumes whose volumeType is "Snapshot" and whose snapshotAttribute is "P-VOL", delete all the snapshot volumes created from the applicable volumes. Volumes whose volumeType is "Snapshot" and whose snapshotAttribute is "P/S-VOL" or "S-VOL"cannot be expanded.

A snapshot configuration consisting of the copy source and copy destinations as follows is referred to as a snapshot tree. A different generation of the S-VOL is created by the time at which a snapshot is taken.

Another snapshot can be created by using an already created snapshot as the copy source. Another snapshot can be created by using an already created snapshot as the copy source. This snapshot configuration is called a cascade configuration. A cascade configuration can comprise of a maximum of 64 layers. In a cascade configuration, a volume can have both the P-VOL and S-VOL attributes. Such a volume is called a P/S-VOL.

A cascade configuration with a maximum of 64 layers can be created under the P-VOL layer. For example, the cascade configuration in the following figure has three layers.

The following table lists the operations on snapshots.

Operation	Description	Reference
Prepare to take a snapshot	Create a volume (S-VOL) for taking snapshots. When you prepare to take a snapshot, a volume (S-VOL) will be created. A snapshot cannot be taken without creating a volume (S-VOL).	Preparing to take a snapshot
Take a snapshot	Take a snapshot for the prepared S-VOL.	Taking a snapshot (CLI or REST API)
Delete snapshots	Delete snapshot volumes that are no longer needed.	Deleting snapshots (CLI or REST API)
Restore P-VOLs from a snapshot	Restore P-VOL data from S-VOL data.	Restoring P-VOLs from a snapshot (CLI or REST API)
Obtain P-VOL information for the volume	Obtain information about the P-VOL from which S-VOL was created.	Obtaining P-VOL information for the volume (CLI or REST API)
Obtain a list of S-VOL information for the volume	Obtain a list of S-VOL information created from P-VOL.	Obtaining a list of S-VOL information for the volume (CLI or REST API)
Modify settings of individual volumes	Edit the settings for the volume.	Modifying settings of individual volumes (CLI or REST API)

Snapshot specifications

The following table lists the specifications for snapshots.

Item	Specifications
Number of S-VOLs (per P-VOL)	1,023
Maximum number of simultaneous operations of volume operations and snapshot operations	Approx. 20 If you perform more than 20 operations at the same time, operations might not be performed successfully due to a timeout. Volume operations see volume creation, deletion, expansion, and setting edition. Snapshot operations see preparation to take snapshots, taking, deleting and restoring snapshots. The operation target also includes volumes with data reduction enabled. A single operation handling multiple volumes is counted as one operation. For example, a volume creation operation with 20 or more volumes specified is counted as one operation. Completion of the job means completion of the operation. When the job state is "Succeeded", the operation is complete.
Snapshot volume name	Number of characters: 1 to 32 Characters that can be used: Numbers (0 to 9), uppercase alphabet (A to Z), lowercase alphabet (a to z), symbols (- , . : @ _)
Maximum number of layers in a cascade configuration	64

Snapshot-specific information obtained from volume information

You can obtain the following snapshot-specific information by obtaining a list of snapshot information.

Type of information	Description
snapshotStatus	Snapshot status indication: Normal, Empty (empty status without snapshot meta data), Deleting, Restoring, Preparing (to take snapshots), Prepared, or Error An ordinary volume is indicated by "-". This is a temporary status when creating or deleting S-VOLs. When you create S-VOLs, the status transitions to Preparing after a while, and the applicable volumes are deleted when you delete S-VOLs. You can also obtain snapshotStatus from a list of volume information.
snapshotProgressRate [%]	Progress rate of snapshot operation. Indicated only when snapshotStatus is Preparing, Restoring or Deleting. In the case of other statuses, null is output. snapshotProgressRate is not displayed at the same time as the match rate (snapshotConcordanceRate [%]). You can also obtain snapshotProgressRate [%] from volume information.
snapshotTimestamp	Time when the snapshot is taken. This is displayed when snapshotStatus is "Normal." In the case of other statuses, null is output. You can also obtain snapshotTimestamp from volume information.
snapshotType	"Snapshot" is displayed for a snapshot volume. You can also obtain snapshotType from volume information.
snapshotConcordanceRate [%]	Match rate [%] between this volume and the comparison-target volume. When this volume is the latest generation S-VOL, the volume to be compared will be the P-VOL. When this volume is not the latest generation S-VOL, the volume to be compared will be an S-VOL of a newer generation. For example, if the P-VOL is written in a multi-generation configuration (1:N configuration for a P-VOL and S-VOL), only the latest generation of S-VOL will have a lower match rate. If the S-VOL is written, the match rate will decrease only for the S-VOL of the written generation. The match rate is an approximate value and might have a large margin of error. Differential data indicated by the match rate is managed in units of 256 KiB. When restoring and deleting snapshots, the larger the amount of data difference between P-VOL and S-VOL is, the longer the process will take. This information can be used to estimate the processing time for snapshot restoration and deletion in advance. You can also obtain snapshotConcordanceRate [%] from volume information. (Bare metal) For details about the approximate processing time for snapshot operations, see Processing time for snapshot operations (Bare metal) in this document.
isWrittenInSvol	This information shows whether the host has written to the S-VOL. This is displayed when snapshotStatus is "Normal." In the case of other statuses, null is output. You can also obtain isWrittenInSvol from volume information.

Status transition (snapshot operation and snapshotStatus)

The snapshot Status transits depending on the snapshot operation.

Note:

It might take about one minute for snapshotStatus to transit. To confirm the transition of snapshotStatus, wait about one minute and then check again.

Tip:

Whether the volume is a snapshot volume can be found in snapshotAttribute of volume information. "P-VOL", "P/S-VOL", or "S-VOL" is indicated. An ordinary volume is indicated by "-".

Also, the number of S-VOLs in the P-VOL or P/S-VOL can be found in numberOfSnapshots in volume information.

Whether the P-VOL and S-VOL can be written or read

The P-VOL is readable/writable regardless of snapshotStatus of the copy-destination volume.

The S-VOL is readable/writable only when snapshotStatus is "Normal."

Whether or not the operations can be performed for each snapshotStatus

The snapshot tree configuration and whether or not the operations can be performed for each snapshotStatus are as follows:

Note:

It might take about one minute for snapshotStatus to transit. To confirm the transition of snapshotStatus, wait about one minute and then check again.

■ Non-cascade configuration

"Snapshot operation for A" in the table indicates preparation for taking a snapshot of volume A or taking a snapshot of volume A. To prepare for taking a snapshot of volume B under volume A or take a snapshot of volume B under volume A, see "Snapshot operation for B" in "■ Cascade configuration."

snapshotType	snapshotStatus of A	Snapshot operation for A
snapshotType	snapshotStatus of A	Prepare and take a snapshot (PrepareAndFinalize)	Prepare	Take a snapshot (Finalize)	Restore	Delete
None	None (A does not exist)	〇	〇	-	-	-
Snapshot	Empty	-	-	×	×	× ^*
	Preparing	-	-	×	×	×
	Prepared	-	-	〇	△	〇
	Normal	-	-	△	〇	〇
	Restoring	-	-	×	×	×
	Error	-	-	×	×	〇
	Deleting	-	-	×	×	△
〇: Succeeds △: Succeeds but nothing is performed (The job ends normally without performing processing.) ×: Does not succeed -: The operation is not available * If removal processing is interrupted because the storage cluster has stopped, the processing can be completed by retrying the operation.

■ Cascade configuration

You can take a snapshot of volume B only if the volume is operable.

snapshotType	snapshotStatus of A	Snapshot operation for B
snapshotType	snapshotStatus of A	Prepare and take a snapshot (PrepareAndFinalize)	Prepare	Take a snapshot (Finalize)	Restore	Delete
Snapshot	Empty	×	×	-	-	-
	Preparing	×	×	-	-	-
	Prepared	×	〇	×	△	〇
	Normal	〇	〇	〇	〇	〇
	Restoring	×	×	×	△	〇
	Error	×	×	×	×	〇
	Deleting	×	×	-	-	-
〇: Succeeds △: Succeeds but nothing is performed (The job ends normally without performing processing.) ×: Does not succeed -: The operation is not available

You can take a snapshot of volume A only if the volume is operable.

snapshotType	snapshotStatus of B	Snapshot operation for A
snapshotType	snapshotStatus of B	Prepare and take a snapshot (PrepareAndFinalize)	Prepare	Take a snapshot (Finalize)	Restore	Delete
Snapshot	Empty	-	-	〇	〇	×
	Preparing	-	-	〇	〇	×
	Prepared	-	-	〇	〇	×
	Normal	-	-	△	×	×
	Restoring	-	-	△	×	×
	Error	-	-	△	×	×
	Deleting	-	-	△	×	×
〇: Succeeds △: Succeeds but nothing is performed (The job ends normally without performing processing.) ×: Does not succeed -: The operation is not available

■ When sharing the copy-source VOL among multiple snapshots

You can take a snapshot of volume B only if the volume is operable.

snapshotType	snapshotStatus of A	Snapshot operation for B
snapshotType	snapshotStatus of A	Prepare and take a snapshot (PrepareAndFinalize)	Prepare	Take a snapshot (Finalize)	Restore	Delete
Snapshot	Empty	〇	〇	〇	〇	〇
	Preparing	〇	〇	〇	〇	〇
	Prepared	〇	〇	〇	〇	〇
	Normal	〇	〇	〇	〇	〇
	Restoring	×	〇	×	×	〇
	Error	×	×	〇	〇	〇
	Deleting	×	×	〇	〇	〇
〇: Succeeds △: Succeeds but nothing is performed (The job ends normally without performing processing.) ×: Does not succeed

■ QoS settings of snapshots

QoS settings can be applied to a P-VOL, S-VOL, and P/S-VOL.

For details about the QoS function, see About the Quality of Service (QoS) function.

Note:

Snapshots do not inherit QoS settings of the copy-source volumes.