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Using the Snapshot Feature

Dell™ PowerVault™ Modular Disk Storage Manager CLI Guide

  Using Host Servers to Create an Initial Snapshot Virtual Disk

  Creating a Snapshot Virtual Disk

  Changing Snapshot Virtual Disk Settings

  Stopping and Deleting a Snapshot Virtual Disk

This chapter describes how the Snapshot feature works, lists the snapshot script commands, and explains how to use the commands to create snapshot virtual disks. Additional information about the Snapshot feature and related definitions is available in the online help, the Installation Guide, the MD Storage Manager User's Guide, and the Owner's Manual.

The Snapshot feature creates a snapshot virtual disk that you can use as a backup of your data. A snapshot virtual disk is a logical point-in-time image of a standard virtual disk. Because it is not a physical copy, a snapshot virtual disk is created more quickly than a physical copy and requires less physical disk space. Typically, you create a snapshot virtual disk so that an application, such as a backup application, can access the snapshot virtual disk. The application reads the data while the source virtual disk remains online and user accessible. You can also create several snapshot virtual disks of a source virtual disk and write data to the snapshot virtual disks to perform testing and analysis.

NOTE: If you ordered Premium Features for the Snapshot Virtual Disks, you received a Premium Features Activation card shipped in the same box as your Dell PowerVault™ MD storage array. Follow the directions on the card to obtain a key file and to enable the feature. For more information, see "Premium Feature — Snapshot Virtual Disks" in the User's Guide.

Snapshot virtual disks allow you to perform the following tasks:

• Create a complete image of the data on a source virtual disk at a particular point in time.
  
  
• Use only a small amount of disk space.
  
  
• Provide quick, frequent, nondisruptive backups; or test new versions of a database system without affecting actual data.
  
  
• Provide for snapshot virtual disks to be read, written, and copied.
  
  
• Use the same availability characteristics of the source virtual disk (such as redundant array of independent disks (RAID) protection and redundant path failover).
  
  
• Map the snapshot virtual disk and make it accessible to any host on a storage area network. You can make snapshot data available to secondary hosts for read and write access by mapping the snapshot to the hosts.
  
  
• Create up to four snapshots per virtual disk.
  
  

NOTE: The maximum number of snapshot virtual disks is one-half of the total number of virtual disks supported by the RAID controller module.

• Increase the capacity of a snapshot virtual disk.
  
  

Table 4-1 lists the components that comprise a snapshot virtual disk and briefly describes what they do.

Component	Description
Source virtual disk	Standard virtual disk from which the snapshot is created
Snapshot virtual disk	Point-in-time image of a standard virtual disk
Snapshot repository virtual disk	Virtual disk that contains snapshot metadata and copy-on-write data for a particular snapshot virtual disk

Table 4-1 lists the snapshot virtual disk commands and brief descriptions of what the commands do.

Table 4-1. Snapshot Virtual Disk Commands

Using Host Servers to Create an Initial Snapshot Virtual Disk

NOTICE: Before using the Snapshot Virtual Disks Premium Feature in a Microsoft® Windows® clustered configuration, you must first map the snapshot virtual disk to the cluster node that owns the source virtual disk. This ensures that the cluster nodes correctly recognize the snapshot virtual disk. If you map the snapshot virtual disk to the node that does not own the source virtual disk before the snapshot enabling process is completed, the operating system may fail to correctly identify the snapshot virtual disk. This can result in data loss on the source virtual disk or an inaccessible snapshot. For details on mapping the snapshot virtual disk to the secondary node, refer to the Dell PowerEdge™ Cluster SE600W Systems Installation and Troubleshooting Guide on support.dell.com

NOTE: You can create concurrent snapshots of a source virtual disk on both the source disk group and on another disk group.

Before creating a Snapshot Virtual Disk, note the following:

• The following types of virtual disks are not valid source virtual disks: snapshot repository virtual disks, snapshot virtual disks, target virtual disks that are participating in a virtual disk copy.
  
  
• You cannot create a snapshot of a virtual disk that contains unreadable sectors.
  
  
• You must satisfy the requirements of your host operating system for creating snapshot virtual disks. Failure to meet the requirements of your host operating system results in an inaccurate point-in-time image of the source virtual disk or the target virtual disk in a virtual disk copy.
  
  

Creating a Snapshot Virtual Disk

The create snapshotVirtualDisk command provides three methods for defining the physical disks for your snapshot repository virtual disk:

• Define each physical disk for the snapshot repository virtual disk by enclosure ID and slot ID.
  
  
• Define a disk group in which the snapshot repository virtual disk resides. Optionally define the capacity of the repository virtual disk.
  
  
• Define the number of physical disks, but not specific physical disks, for the repository virtual disk.
  
  

When using the create snapshotVirtualDisk command to create a snapshot virtual disk, the standard virtual disk name for the source virtual disk is the minimum information required. When you provide only the standard virtual disk name, the storage management software provides default values for the other required property parameters for a snapshot virtual disk.

NOTE: In some cases, depending on the host operating system and any virtual disk manager software in use, the software prevents you from mapping the same host to both a source virtual disk and its associated snapshot virtual disk.

An error message appears in the command line when the utility cannot distinguish between the following:

• Source virtual disk and snapshot virtual disk (for example, if the snapshot virtual disk has been removed)
  
  
• Standard virtual disk and virtual disk copy (for example, if the virtual disk copy has been removed)
  
  

If you are running a Linux operating system, run the hot_add utility to register the snapshot virtual disk with the host operating system.

NOTE: The hot_add utility is not available for Windows.

Enabling the Snapshot Virtual Disk Feature

The first step in creating a snapshot virtual disk is to make sure the feature is enabled on the storage array. You need a feature key to enable the feature. The command for enabling the feature key file is:

enable storageArray feature file="filename"

where the file parameter is the complete file path and file name of a valid feature key file. Enclose the file path and file name in quotation marks (" "). Valid file names for feature key files usually end with .key extension.

Creating a Snapshot Virtual Disk with User-Assigned Physical Disks

Creating a snapshot virtual disk by assigning the physical disks allows you to choose from the available physical disks when defining your storage array configuration. When you choose the physical disks for your snapshot virtual disk, you automatically create a new disk group. You can specify which physical disks to use and the RAID level for the new disk group.

Preparing Host Servers to Create an Initial Snapshot Virtual Disk

NOTICE: Before you create a new point-in-time image of a source virtual disk, stop any data access (I/O) activity or suspend data transfer to the source virtual disk to ensure that you capture an accurate point-in-time image of the source virtual disk. Close all applications, including Windows Internet Explorer®, to make sure all I/O activity has stopped.

NOTE: Removing the drive letter of the associated virtual disk(s) in Windows or unmounting the virtual drive in Linux will help to guarantee a stable copy of the drive for the Snapshot.

Before creating a snapshot virtual disk, the server has to be in the proper state. To ensure that the host server is properly prepared to create a snapshot virtual disk, you can either use an application to carry out this task, or you can perform the following steps:

1. Stop all I/O activity to the source.
   
   
2. Using your Windows system, flush the cache to the source. At the host prompt, type
   
   

SMrepassist -f <filename-identifier>

and press <Enter>. See "SMrepassist Utility" in the User's Guide for more information.

3. Remove the drive letter(s) of the source in Windows or unmount the virtual drive(s) in Linux to help guarantee a stable copy of the drive for the Snapshot. If this is not done, the snapshot operation will report that it has completed successfully, but the snapshot data will not be updated properly.
   
   

NOTE: Verify that the virtual disk has a status of Optimal or Disabled by clicking the Summary tab and then clicking the Disk Groups & Virtual Disks link.

4. Follow any additional instructions for your operating system. Failure to follow these additional instructions can create unusable snapshot virtual disks.
   
   

NOTE: If your operating system requires additional instructions, you can find those instructions in your operating system documentation.

If you want to use a snapshot regularly, such as for backups, use the Disable Snapshot and Re-create Snapshot options to reuse the snapshot. Disabling and re-creating snapshots preserves the existing virtual disk-to-host mappings to the snapshot virtual disk.

After your server has been prepared, see Creating the Initial Snapshot Virtual Disk.

Creating the Initial Snapshot Virtual Disk

After first preparing the host server(s) as specified in the preceding procedure, use the following examples to make a virtual disk snapshot.

The following syntax is the general form of the command to create a snapshot virtual disk:

create snapshotVirtualDisk sourceVirtualDisk="sourceVirtualDiskName" [repositoryRAIDLevel=(0 | 1 | 5) (repositoryPhysicalDisks=(enclosureID0,slotID0 ... enclosureIDn,slotIDn)
userLabel="snapshotVirtualDiskName" warningThresholdPercent=percentValue repositoryPercentOfSource=percentValue repositoryUserLabel="repositoryName" repositoryFullPolicy=(failSourceWrites | failSnapShot)] [enclosureLossProtect=(TRUE | FALSE)]

NOTE: Use one or all of the optional parameters as needed to help define your configuration. You do not, however, need to use any optional parameters.

Refer to steps 1 through 4 in the preceding section, Preparing Host Servers to Create an Initial Snapshot Virtual Disk. The following example shows a command in which users assign the physical disks:

client>smcli 123.45.67.89 -c "create snapshotVirtualDisk sourceVirtualDisk=\"Mars_Spirit_4\" repositoryRAIDLevel=5 repositoryPhysicalDisks=(1,1 1,2 1,3 1,4 1,5);"

The command in this example creates a new snapshot of the source virtual disk Mars_Spirit_4. The snapshot repository virtual disk consists of five physical disks that form a new disk group. The new disk group has a RAID level of 5. This command also takes a snapshot of the source virtual disk, starting the copy-on-write operation.

Refer to steps 1 through 4 in the preceding section, Preparing Host Servers to Create an Initial Snapshot Virtual Disk. The following example is the script file version of the command:

create snapshotVirtualDisk sourceVirtualDisk="Mars_Spirit_4" repositoryRAIDLevel=5 repositoryPhysicalDisks=(1,1 1,2 1,3 1,4 1,5);

A minimal version of this command might look like the following example:

client>smcli 123.45.67.89 -c "create snapshotVirtualDisk sourceVirtualDisk=\"Mars_Spirit_4\";"

The command in this example creates a new snapshot for the source virtual disk Mars_Spirit_4. The repository virtual disk is created in the same disk group as the source virtual disk, which means that the repository virtual disk has the same RAID level as the source virtual disk. This command starts the copy-on-write operation.

Refer to steps 1 through 4 in the preceding section, Preparing Host Servers to Create an Initial Snapshot Virtual Disk. The following example is the script file version of the command:

create snapshotVirtualDisk sourceVirtualDisk="Mars_Spirit_4";

Creating a Snapshot Virtual Disk with Software-Assigned Physical Disks

This version of the create snapshotVirtualDisk command lets you choose an existing disk group in which to place the snapshot repository virtual disk. The storage management software determines which physical disks to use. You can also define how much space to assign to the repository virtual disk. Because you are using an existing disk group, the RAID level for the snapshot virtual disk defaults to the RAID level of the disk group in which you place it. You cannot define the RAID level for the snapshot virtual disk. The general syntax for this command is:

create snapshotVirtualDisk sourceVirtualDisk="sourceVirtualDiskName" [repositoryDiskGroup=diskGroupNumber freeCapacityArea=freeCapacityIndexNumber userLabel="snapshotVirtualDiskName" warningThresholdPercent=percentValue repositoryPercentOfSource=percentValue repositoryUserLabel="repositoryName" repositoryFullPolicy=(failSourceWrites | failSnapShot)] [enclosureLossProtect=(TRUE | FALSE)]

NOTE: Use one or all of the optional parameters as needed to define your configuration. It is not necessary to use any optional parameters.

The following example is a command in which software assigns the physical disks:

client>smcli 123.45.67.89 -c "create snapshotVirtualDisk sourceVirtualDisk=\"Mars_Spirit_4\" repositoryDiskGroup=2 freeCapacityArea=2;"

The command in this example creates a new snapshot repository virtual disk in disk group 2. The source virtual disk is Mars_Spirit_4. The size of the snapshot repository is 4 GB. This command also takes a snapshot of the source virtual disk, which starts the copy-on-write operation.

Define the capacity of a snapshot repository virtual disk as any percentage of the size of the source virtual disk. A value of 20 percent is a good compromise between size and speed. In the previous example, the size of the snapshot repository is set to 4 GB. The underlying assumption is that the source virtual disk size is 20 GB (0.2 x 20 GB = 4 GB).

The following example is the script file version of the command:

create snapshotVirtualDisk sourceVirtualDisk="Mars_Spirit_4" repositoryDiskGroup=2 freeCapacityArea=2;

Creating a Snapshot Virtual Disk by Specifying a Number of Physical Disks

With this version of the create snapshotVirtualDisk command, you must specify the number of physical disks and the RAID level for the snapshot repository virtual disk. This version of the create snapshotVirtualDisk command creates a new disk group. You must have physical disks in the storage array that are not assigned to a disk group for this command to work:

create snapshotVirtualDisk sourceVirtualDisk="sourceVirtualDiskName" [repositoryRAIDLevel=(0 | 1 | 5 | 6) repositoryPhysicalDiskCount=numberOfPhysicalDisks physicalDiskType=(SAS | SATA) userLabel="snapshotVirtualDiskName" warningThresholdPercent=percentValue repositoryPercentOfSource=percentValue repositoryUserLabel="repositoryName" repositoryFullPolicy=(failSourceWrites | failSnapShot)] [enclosureLossProtect=(TRUE | FALSE)]

NOTE: Use one or all optional parameters as needed to define your configuration. It is not necessary to use any optional parameters.

The following example is a command in which users specify the number of physical disks:

client>smcli 123.45.67.89 -c "create snapshotVirtualDisk sourceVirtualDisk=\"Mars_Spirit_4\" repositoryRAIDLevel=5 repositoryPhysicalDiskCount=3;"

The command in this example creates a new snapshot repository virtual disk that consists of three physical disks. The three physical disks comprise a new disk group with a RAID level of 5. This command also takes a snapshot of the source virtual disk, which starts the copy-on-write operation.

The following example is the script file version of the command:

create snapshotVirtualDisk sourceVirtualDisk="Mars_Spirit_4" repositoryRAIDLevel=5 repositoryPhysicalDiskCount=3;

User-Defined Parameters

Parameters for the create snapshotVirtualDisk command enable you to define the snapshot virtual disk to suit the requirements of your storage array. Table 4-2 lists the parameters and descriptions of what the parameters do.

Table 4-2. Snapshot Virtual Disk Parameters 

The following example of the create snapshotVirtualDisk command includes user-defined parameters:

client>smcli 123.45.67.89 -c "create snapshotVirtualDisk sourceVirtualDisk=\"Mars_Spirit_4\" repositoryRAIDLevel=5 repositoryPhysicalDiskCount=5 physicalDiskType=SAS userLabel=\"Mars_Spirit_4_snap1\" repositoryUserLabel=\"Mars_Spirit_4_rep1\" warningThresholdPercent=75 repositoryPercentOfSource=40 repositoryFullPolicy=failSnapShot;"

The following example is the script file version of the command:

create snapshotVirtualDisk sourceVirtualDisk="Mars_Spirit_4" repositoryRAIDLevel=5 repositoryPhysicalDiskCount=5 physicalDiskType=SAS userLabel="Mars_Spirit_4_snap1" repositoryUserLabel="Mars_Spirit_4_rep1" warningThresholdPercent=75 repositoryPercentOfSource=40 repositoryFullPolicy=failSnapShot;

NOTE: In the previous examples, the names for the snapshot virtual disk and repository virtual disk are defined by the user. If you do not choose to create names for the snapshot virtual disks or the repository virtual disks, the RAID controller modules provide default names. (See Names of Snapshot Virtual Disks and Repository Virtual Disks for an explanation of naming conventions.)

Names of Snapshot Virtual Disks and Repository Virtual Disks

The names of snapshot virtual disks and repository virtual disks can be any combination of alphanumeric characters, hyphens, and underscores. The maximum length of the virtual disk names is 30 characters. You must enclose the name in quotation marks. The character string cannot contain a new line. Make sure that you use unique names or the RAID controller module firmware returns an error.

One technique for naming the snapshot virtual disk and the repository virtual disk is to add a hyphenated suffix to the original name of the source virtual disk. The suffix distinguishes between the snapshot virtual disk and the repository virtual disk. For example, if you have a source virtual disk with a name Engineering Data, the snapshot virtual disk can have a name Engineering Data-S1. The repository virtual disk can have a name of Engineering Data-R1.

If you do not choose a unique name for the either the snapshot virtual disk or repository virtual disk, the RAID controller modules create a default name by using the name of the source virtual disk. For example, if the name of the source virtual disk is aaa and it does not have a snapshot virtual disk, then the default name is aaa-1. If the source virtual disk already has n – 1 number of snapshot virtual disks, then the default name is aaa-n. Similarly, if the name of the source virtual disk is aaa and it does not have a repository virtual disk, then the default repository virtual disk name is aaa-R1. If the source virtual disk already has n – 1 number of repository virtual disks, then the default name is aaa-Rn.

In the examples from the previous section, the user-defined name of the snapshot virtual disk was Mars_Spirit_4_snap1. The user-defined name of the repository virtual disk was Mars_Spirit_4_rep1. The default name provided by the RAID controller module for the snapshot virtual disk would be Mars_Spirit_4-1. The default name provided by the RAID controller module for the repository virtual disk would be Mars_Spirit_4-R1.

Changing Snapshot Virtual Disk Settings

The set (snapshot) virtualDisk command enables you to change the property settings for a snapshot virtual disk. Using this command, you can change the following parameters:

• Name of the snapshot virtual disk
  
  
• Warning threshold percent
  
  
• Repository full policy
  
  

The following example shows the command to change the name of a snapshot virtual disk:

client>smcli 123.45.67.89 -c "set virtualDisk [\"Mars_Spirit_4-1\"] userLabel=\"Mars_Odyssey_3-2\";"

The following example is the script file version of the command:

set virtualDisk ["Mars_Spirit_4-1"] userLabel="Mars_Odyssey_3-2";

When you change the warning threshold percent and repository full policy, you can apply the changes to one or several snapshot virtual disks. The following example uses the set (snapshot) virtualDisk command to change these properties on more than one snapshot virtual disk:

client>smcli 123.45.67.89 -c "set virtualDisks [\"Mars_Spirit_4-1\" \"Mars_Spirit_4-2\"
\"Mars_Spirit_4-3\"] warningThresholdPercent=50 repositoryFullPolicy=failSourceWrites;"

The following example is the script file version of the command:

set virtualDisks ["Mars_Spirit_4-1" "Mars_Spirit_4-2" "Mars_Spirit_4-3"] warningThresholdPercent=50 repositoryFullPolicy=failSourceWrites;

Stopping and Deleting a Snapshot Virtual Disk

When you create a snapshot virtual disk, copy-on-write immediately starts running. As long as a snapshot virtual disk is enabled, storage array performance is affected by the copy-on-write operations to the associated snapshot repository virtual disk. If you no longer want copy-on-write operations to run, you can use the stop snapshot virtualDisk command to stop the copy-on-write operations. When you stop a snapshot virtual disk, the snapshot virtual disk and the repository virtual disk are still defined for the source virtual disk; only copy-on-write has stopped. The following example stops a snapshot virtual disk:

client>smcli 123.45.67.89 -c "stop snapshot virtualDisks [\"Mars_Spirit_4-2\" \"Mars_Spirit_4-3\"];"

The following example is the script file version of the command:

stop snapshot virtualDisks ["Mars_Spirit_4-2" "Mars_Spirit_4-3"];

When you stop the copy-on-write operations for a specific snapshot virtual disk, only that snapshot virtual disk is disabled. All other snapshot virtual disks remain in operation.

Re-creating the Snapshot Virtual Disk

To restart a copy-on-write operation, use the recreate snapshot virtualDisk command. This command starts a fresh copy-on-write operation using an existing snapshot virtual disk. When you restart a snapshot virtual disk, the snapshot virtual disk must have either an Optimal or a Disabled state. The following conditions then occur:

• All copy-on-write data previously on the snapshot repository virtual disk is deleted.
  
  
• Snapshot virtual disk and snapshot repository virtual disk parameters remain the same as the previously disabled snapshot virtual disk and snapshot repository virtual disk. You can also change the userLabel, warningThresholdPercent, and repositoryFullPolicy parameters when you restart the snapshot virtual disk.
  
  
• The original names for the snapshot repository virtual disk are retained.
  
  

Preparing Host Servers to Re-create a Snapshot Virtual Disk

NOTICE: Before you create a new point-in-time image of a source virtual disk, stop any data access (I/O) activity or suspend data transfer to the source virtual disk and snapshot virtual disk to ensure that you capture an accurate point-in-time image of the source virtual disk. Close all applications, including Windows Internet Explorer, to make sure all I/O activity has stopped.

NOTE: Removing the drive letter of the associated virtual disk in Windows or unmounting the virtual drive in Linux will help to guarantee a stable copy of the drive for the Snapshot.

Before re-creating a snapshot virtual disk, both the server and the associated virtual disk you are re-creating have to be in the proper state. To ensure that the host server is properly prepared to re-create a snapshot virtual disk, you can either use an application to carry out this task, or you can perform the following steps:

1. Stop all I/O activity to the source and snapshot virtual disk (if mounted).
   
   
2. Using your Windows system, flush the cache to both the source and the snapshot virtual disk (if mounted). At the host prompt, type
   
   

SMrepassist -f <filename-identifier>

and press <Enter>. See "SMrepassist Utility" in the User's Guide for more information.

3. Remove the drive letter(s) of the source and (if mounted) snapshot virtual disk in Windows or unmount the virtual drive(s) in Linux to help guarantee a stable copy of the drive for the Snapshot. If this is not done, the snapshot operation will report that it has completed successfully, but the snapshot data will not be updated properly.
   
   
4. Follow any additional instructions for your operating system. Failure to follow these additional instructions can create unusable snapshot virtual disks.
   
   

NOTE: If your operating system requires additional instructions, you can find those instructions in your operating system documentation.

After your server has been prepared, see Re-creating the Snapshot Virtual Disk to re-create the snapshot virtual disk.

Re-creating a Snapshot Virtual Disk

After first preparing the host server(s) as specified in the preceding procedure, use the following examples to re-create a virtual disk snapshot.

Refer to steps 1 through 4 in the preceding section, Preparing Host Servers to Re-create a Snapshot Virtual Disk. The following example shows the command to restart a snapshot virtual disk:

client>smcli 123.45.67.89 -c "recreate snapshot virtualDisks [\"Mars_Spirit_4-2\" \"Mars_Spirit_4-3\"];"

Refer to steps 1 through 4 in the preceding section, Preparing Host Servers to Re-create a Snapshot Virtual Disk. The following example is the script file version of the command:

recreate snapshot virtualDisks ["Mars_Spirit_4-2" "Mars_Spirit_4-3"];

If you do not intend to use a snapshot virtual disk again, you can delete the snapshot virtual disk using the delete virtualDisk command. When you delete a snapshot virtual disk, the associated snapshot repository virtual disk is also deleted.

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