NOTE: If you ordered this feature, you received a Premium Feature 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.
NOTE: The basic snapshot feature allows up to four snapshots to be present at the same time. If the enhanced snapshot premium feature was ordered, then up to eight snapshots may be present at the same time.
A snapshot virtual disk is a point-in-time image of a virtual disk in a storage array. It is not an actual virtual disk containing a copy of the original data; rather, it is a reference to the data that was contained on a virtual disk at a specific time. A snapshot virtual disk is the logical equivalent of a complete physical copy. However, you can create a snapshot virtual disk much faster than a physical copy, using less disk space.
The virtual disk on which the snapshot is based, called the source virtual disk, must be a standard virtual disk in your storage array. Typically, you create a snapshot so that an application, such as a backup application, can access the snapshot and read the data while the source virtual disk remains online and accessible.
NOTE: No I/O requests are permitted on the source virtual disk while the virtual disk snapshot is being created.
A snapshot repository virtual disk containing metadata and copy-on-write data is automatically created when a snapshot virtual disk is created. The only data stored in the snapshot repository virtual disk is that which has changed since the time of the snapshot.
After the snapshot repository virtual disk is created, I/O write requests to the source virtual disk resume. Before a data block on the source virtual disk is modified, however, the contents of the block to be modified are copied to the snapshot repository virtual disk for safekeeping. Because the snapshot repository virtual disk stores copies of the original data in those data blocks, further changes to those data blocks write only to the source virtual disk. The snapshot repository uses less disk space than a full physical copy, because the only data blocks that are stored in the snapshot repository virtual disk are those that have changed since the time of the snapshot.
When you create a snapshot virtual disk, you specify where to create the snapshot repository virtual disk, its capacity, and other parameters. You can disable or delete the snapshot virtual disk when you no longer need it, such as when the backup is complete. If you disable a snapshot virtual disk, you can re-create and reuse it the next time you perform a backup (see Re-creating Snapshot Virtual Disks for more information). If you delete a snapshot virtual disk, you also delete the associated snapshot repository virtual disk.
NOTE: Deleting a snapshot does not affect data on the source virtual disk.
The information that follows will better prepare users for using the snapshot virtual disk premium feature of the Dell PowerVault systems.
NOTE: The following host preparation sections also apply when using the snapshot feature through the CLI interface.
Creating a Snapshot Virtual Disk Using the
Simple Path
You can choose the simple path to create a snapshot virtual disk if the disk group of the source virtual disk has the required amount of free space. A snapshot repository virtual disk requires a minimum 8 MB of free capacity. The destination of a snapshot repository virtual disk is determined based on the free capacity available in the disk group.
If 8 MB of free capacity is not available in the disk group of the source virtual disk, the Create Snapshot Virtual Disks feature defaults to the advanced path (see Creating a Snapshot Virtual Disk Using the Advanced Path). In the advanced path option, you can choose to place the snapshot repository virtual disk in another disk group or you can use unconfigured capacity on the storage array to create a new disk group.
About the Simple Path
Using the simple path, you can specify the following parameters for your snapshot virtual disk:
Snapshot Virtual Disk Name — A user-specified name that helps you associate the snapshot virtual disk to its corresponding snapshot repository virtual disk and source virtual disk.
Snapshot Repository Virtual Disk Name — A user-specified name that helps you associate the snapshot repository virtual disk to its corresponding snapshot virtual disk and source virtual disk.
Using the simple path, the following defaults are used for the other parameters of a snapshot virtual disk:
Capacity Allocation — The snapshot repository virtual disk is created using free capacity on the same disk group where the source virtual disk resides.
Host-to-Virtual Disk Mapping — The default setting is Map now.
Percent Full — When the snapshot repository virtual disk reaches the specified repository full percentage level, the event is logged in the Main Event Log (MEL). The default snapshot repository full percentage level is 50 percent of the source virtual disk.
Snapshot Repository Virtual Disk Full Conditions — When the snapshot repository virtual disk becomes full, you are given a choice of failing write activity to the source virtual disk or failing the snapshot virtual disk.
Preparing Host Servers to Create the Snapshot Using the Simple Path
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.
Mapping the snapshot virtual disk to the node that does not own the source virtual disk before the Snapshot enabling process is completed can result in the operating system mis-identifying the snapshot virtual disk. This, in turn, 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 DellPowerEdge™ 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.
NOTE: Virtual Disk Copy is an Advanced (Premium) feature.
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.
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 host 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:
Stop all I/O activity to the source.
Using your Windows system, flush the cache to the source. At the host
prompt, type
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 Disk Groups & Virtual Disks.
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.
Creating the Snapshot Using the Simple Path
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.
After first preparing the host server(s) as specified in the preceding procedure, complete the following steps to create a virtual disk snapshot using the simple path:
In MD Storage Manager, click the Configure tab, and then click Create
Snapshot Virtual Disks.
The Additional Instructions dialog appears; click Close in this dialog to
continue.
Click the plus sign (+) to the left of the disk group to expand it, then click
the virtual disk from which you want to create a snapshot.
Click Next.
A No Capacity Exists warning appears if there is not enough space in the disk group of the source virtual disk to create the snapshot.
On the Create Snapshot Virtual Disks – Select Path screen, select the
Simple path.
NOTE: A snapshot repository virtual disk requires 8 MB of free space. If the required free space is not available in the disk group of the source virtual disk, the Create Snapshot Virtual Disks feature defaults to the advanced path.
Click Next.
Type a name for the snapshot in the Snapshot virtual disk name text box.
Type a name for the snapshot repository virtual disk in the Snapshot
repository virtual disk name text box.
Click Next.
Choose whether to map the virtual disk to a host or host group now or
later:
To map now, select Map now, select a host or host group by clicking it, and then assign a LUN.
To map later, select Map later.
Click Finish to create the snapshot virtual disk and the associated
snapshot repository virtual disk.
After you have created one or more snapshot virtual disks, mount or
reassign a drive letter of the source virtual disk.
If needed, assign host-to-virtual disk mapping between the snapshot
virtual disk and the host operating system that accesses it.
NOTE: In some cases, depending on the host type 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.
If you are using a Linux-based system, run the hot_add utility to register
the snapshot virtual disk with the host operating system.
NOTE: The hot_add utility is not needed for Windows.
Creating a Snapshot Virtual Disk Using the
Advanced Path
About the Advanced Path
Use the advanced path to choose whether to place the snapshot repository virtual disk on free capacity or unconfigured capacity and to change the snapshot repository virtual disk parameters. You can select the advanced path regardless of whether you use free capacity or unconfigured capacity for the snapshot virtual disk.
Using the advanced path, you can specify the following parameters for your snapshot virtual disk:
Snapshot Virtual Disk Name — A user-specified name that helps you associate the snapshot virtual disk to its corresponding snapshot repository virtual disk and source virtual disk.
Snapshot Repository Virtual Disk Name — A user-specified name that helps you associate the snapshot repository virtual disk to its corresponding snapshot virtual disk and source virtual disk.
Capacity Allocation — This parameter allows you to choose where to create the snapshot repository virtual disk. You can allocate capacity by using one of the following methods:
Use free capacity on the same disk group where the source virtual disk resides.
Use free capacity on another disk group.
Use unconfigured capacity and create a new disk group for the snapshot repository virtual disk.
Dell recommends placing the snapshot repository virtual disk within the disk group of the source virtual disk. This ensures that if drives associated with the disk group are moved to another storage array, all the virtual disks associated with the snapshot virtual disk remain in the same group.
Percent Full — When the snapshot repository virtual disk reaches the user-specified repository full percentage level, the event is logged in the Major Event Log (MEL). The default snapshot repository full percentage level is 50% of the source virtual disk.
Snapshot Repository Virtual Disk Full Conditions — You can choose whether to fail writes to the source virtual disk or fail the snapshot virtual disk when the snapshot repository virtual disk becomes full.
Host-to-Virtual Disk Mapping — Choose whether to map the snapshot virtual disk to a host or host group now or to map the snapshot virtual disk later. The default setting is Map later.
Preparing Host Servers to Create the Snapshot Using the Advanced Path
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.
Mapping the snapshot virtual disk to the node that does not own the source virtual disk before the Snapshot enabling process is completed can result in the operating system mis-identifying the snapshot virtual disk. This, in turn, 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 DellPowerEdge™ Cluster SE600W Systems Installation and Troubleshooting Guide on support.dell.com
The destination of a snapshot repository virtual disk is determined based on the free capacity available in the disk group. A snapshot repository virtual disk requires a minimum 8 MB of free capacity. You can choose your preferred creation path—simple or advanced—if the disk group of the source virtual disk has the required amount of free space.
If 8 MB of free capacity is not available in the disk group of the source virtual disk, the Create Snapshot Virtual Disks feature defaults to the advanced path (see Creating a Snapshot Virtual Disk Using the Advanced Path). In the advanced path option, you can choose to place the snapshot repository virtual disk in another disk group or you can use unconfigured capacity on the storage array to create a new disk group.
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.
NOTE: Virtual Disk Copy is an Advanced (Premium) feature.
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.
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 host server has to be in the proper state. Perform the following steps to prepare your host server:
Stop all I/O activity to the source.
Using your Windows system, flush the cache to the source. At the host
prompt, type
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 Disk Groups & Virtual Disks.
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.
Creating the Snapshot Using the Advanced Path
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.
After first preparing the host server(s) as specified in the preceding procedure, complete the following steps to create a virtual disk snapshot using the advanced path:
In MD Storage Manager, click the Configure tab, and then click Create
Snapshot Virtual Disks.
The Additional Instructions dialog appears; click Close in this dialog to
continue.
Click the plus sign (+) to the left of the disk group to expand it, then click
the virtual disk from which you want to create a snapshot.
Click Next.
A No Capacity Exists warning appears if there is not enough space in the disk group of the source virtual disk to create the snapshot.
On the Create Snapshot Virtual Disks – Select Path screen, select the
Advanced path.
NOTE: A snapshot repository virtual disk requires 8 MB of free space. If the required free space is not available in the disk group of the source virtual disk, the Create Snapshot Virtual Disks feature defaults to the advanced path.
Click Next.
Type a name for the snapshot in the Snapshot virtual disk name text box.
Type a name for the snapshot repository virtual disk in the Snapshot
repository virtual disk name text box.
Click Next.
Choose whether to create the snapshot virtual disk from unconfigured
capacity or free capacity.
To create the snapshot virtual disk from unconfigured capacity:
Select Unconfigured capacity, then click Next.
On the Create Snapshot Virtual Disks – Specify Capacity screen,
choose a RAID level, then click Next.
To create the snapshot virtual disk from free capacity:
Select Free capacity.
Select a free capacity node, then click Next.
On the Create Snapshot Virtual Disks – Specify Repository Virtual Disk
Capacity screen, enter how much space you want to allocate for the
snapshot repository virtual disk in the Specify capacity box, then click
Next.
In the Notify me when the repository disk becomes x% full box, specify at
what percentage MD Storage Manager should consider the snapshot
repository virtual disk to be full.
Specify what should happen if the snapshot repository virtual disk
becomes full.
Select Fail the snapshot virtual disk to leave the source disk available.
Select Fail writes to the source virtual disk to leave the snapshot virtual disk available and stop data from writing to the source virtual disk.
Choose whether to map the virtual disk to a host or host group now or
later.
To map now, select Map now, select a host or host group by clicking it, then assign a LUN.
To map later, select Map later.
Click Finish to create the snapshot virtual disk and the associated
snapshot repository virtual disk.
After you have created one or more snapshot virtual disks, mount or
reassign a drive letter of the source virtual disk.
If needed, assign host-to-virtual disk mapping between the snapshot
virtual disk and the host operating system that accesses it.
NOTE: In some cases, depending on the host type 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.
If you are using a Linux-based system, run the hot_add utility to register
the snapshot virtual disk with the host operating system.
NOTE: The hot_add utility is not needed for Windows.
Specifying Snapshot Virtual Disk Names
Choose a name that helps you associate the snapshot virtual disk and snapshot repository virtual disk with its corresponding source virtual disk. The following information is useful when naming virtual disks:
By default, the snapshot name is shown in the Snapshot virtual disk name field as:
<source-virtual disk-name>—<sequence-number>
wheresequence-number is the chronological number of the snapshot relative to the source virtual disk.
The default name for the associated snapshot repository virtual disk that is shown in the Snapshot repository virtual disk field is:
<source-virtual disk-name>—R<sequence-number>
For example, if you are creating the first snapshot virtual disk for a source virtual disk called Accounting, the default snapshot virtual disk is Accounting-1, and the associated snapshot repository virtual disk default name is Accounting-R1. The default name of the next snapshot virtual disk you create based on Accounting is Accounting-2, with the corresponding snapshot repository virtual disk named as Accounting-R2 by default.
Whether you use the software-supplied sequence number that (by default) populates the Snapshot virtual disk name or the Snapshot repository virtual disk name field, the next default name for a snapshot or snapshot repository virtual disk still uses the sequence number determined by the software. For example, if you give the first snapshot of source virtual disk Accounting the name Accounting-8, and do not use the software-supplied sequence number of 1, the default name for the next snapshot of Accounting is still Accounting-2.
The next available sequence number is based on the number of existing snapshots of a source virtual disk. If you delete a snapshot virtual disk, its sequence number becomes available again.
You must choose a unique name for the snapshot virtual disk and the snapshot repository virtual disks, or an error message is displayed.
Names are limited to 30 characters. After you reach this limit in either the Snapshot virtual disk name or the Snapshot repository virtual disk name fields, you can no longer type in the field. If the source virtual disk is 30 characters, the default names for the snapshot and its associated snapshot repository virtual disk use the source virtual disk name truncated enough to add the sequence string. For example, for Host Software Engineering Group GR-1, the default snapshot name is Host Software Engineering GR-1, and the default repository name would be Host Software Engineering GR-R1.
Snapshot Repository Capacity
If you receive a warning that the capacity for the snapshot repository virtual disk is approaching its threshold, you can increase the capacity of a snapshot repository virtual disk by using one of the following methods:
Use the free capacity available on the disk group of the snapshot repository virtual disk.
Add unconfigured capacity to the disk group of the snapshot repository virtual disk. Use this option when no free capacity exists on the disk group.
You cannot increase the storage capacity of a snapshot repository virtual disk if the snapshot repository virtual disk has any one of the following conditions:
The virtual disk has one or more hot spare drives in use.
The virtual disk has a status other than Optimal.
Any virtual disk in the disk group is in any state of modification.
The controller that has ownership of this virtual disk is currently adding capacity to another virtual disk. Each controller can add capacity to only one virtual disk at a time.
No free capacity exists in the disk group.
No unconfigured capacity is available to add to the disk group.
To expand the snapshot repository virtual disk from MD Storage Manager:
Click the Modify tab, then click Modify snapshot virtual disks.
Click Expand Snapshot Repository.
Click the snapshot repository virtual disk you want to expand.
If necessary, you can add free capacity to the volume group by adding an
unassigned drive. To add an unassigned drive:
Click Add Drives.
Select the capacity to add from the drop-down menu.
Click Add.
Enter the amount by which you want to expand the snapshot repository
virtual disk in the Increase capacity by field.
Click Finish to expand the capacity of the snapshot repository virtual disk.
Re-creating Snapshot Virtual Disks
Before re-creating a snapshot virtual disk, refer to the following guidelines.
Disabling a Snapshot Virtual Disk
Disable a snapshot virtual disk if one of the following conditions exists:
You do not need the snapshot now.
You intend to re-create the snapshot at a later time and want to retain the associated snapshot repository virtual disk so that you do not need to create it again.
You want to maximize storage array performance by stopping copy-on-write activity to the snapshot repository virtual disk.
The SMdevices utility displays the snapshot virtual disk in its output, even after the snapshot virtual disk is disabled.
To disable a snapshot virtual disk:
Click the Modify tab, then click Modify snapshot virtual disks.
Click Disable Snapshot Virtual Disks.
Highlight the snapshot virtual disk to be disabled and click Disable
beneath the list.
In the Confirm Disable Snapshot Virtual Disk dialog box, type yes and
then click OK.
The snapshot virtual disk is disabled. The associated snapshot repository virtual disk does not change status, but copy-on-write activity to the disabled snapshot virtual disk stops until the snapshot virtual disk is re-created.
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 host server and the associated virtual disk you are re-creating have to be in the proper state. Perform the following steps to prepare your host server and virtual disk:
Stop all I/O activity to the source and snapshot virtual disk (if mounted).
Using your Windows system, flush the cache to both the source and the
snapshot virtual disk (if mounted). At the host prompt, type
Click the Summary tab, then click Disk Groups & Virtual Disks to ensure
that the snapshot virtual disk is in Optimal or Disabled status.
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.
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.
Re-creating a Snapshot Virtual Disk
After first preparing the host server(s) as specified in the preceding procedure, re-create a snapshot virtual disk using the following steps.
NOTICE: This action invalidates the current snapshot.
Click the Modify tab, then click Modify snapshot virtual disks.
Click Re-create Snapshot Virtual Disks.
Highlight the snapshot virtual disk to re-create and click Re-Create
beneath the list.
In the Confirm Snapshot Virtual Disk Re-Creation dialog box, type yes
and then click OK.
Re-creating a snapshot repository virtual disk uses the previously configured snapshot name and parameters.
