Replicating Storage Units Using Snapshot

What is Snapshot?

Snapshot is a method of making a virtual, point-in-time copy of a storage unit. EVM provides snapshot as a simple, single-click operation. All of the underlying StorageWorks CLI commands that are required for snapshot have been automated and incorporated into EVM.

When you specify the snapshot method in a job, EVM will automatically make duplicates, or BCV units, and mount them on a computer with the mount points that you specify. The BCV units are immediately available for a variety of purposes. You can also have the EVM job leave the BCV units unmounted, for example, as nearline backup.

 

Which ACS Versions Support Snapshot with EVM?

Version 8.5P and 8.5S.

 

When I Create a Job, I Don't See Snapshot as a Choice. Why?

Snapshot functionality is provided by the StorageWorks ACS. If the version of ACS on your subsystem does not support it, snapshot will not appear as a choice in EVM.

 

How Long Does a Snapshot Take?

Snapshot only requires a matter of seconds, no matter how large the source unit or what type it is.

 

If it's Virtual, Can I Write to a Snapshot BCV Unit?

Yes. A BCV unit that is created using snapshot is functionally equivalent to a physical disk with both read and write capability.

 

After I Make a Snapshot BCV Unit, Can I Delete the Original Unit?

No. A snapshot always relies, at least in part, on the source unit for data. If the source unit is deleted, its associated BCV unit will become unusable. A snapshot BCV can be thought of as a disposable or "throw away" copy.

 

How Does EVM Replicate a Storage Unit with Snapshot?

For each unit being replicated, EVM automatically determines the correct procedures, then searches the StorageWorks subsystem and selects free disks for the BCV unit. EVM then instructs the subsystem controller to perform the snapshot.

For example, assume that D1 is the source unit in an EVM job. When the job is run, EVM will select free disks and the controller will write pointers (a bitmap) to its cache memory. Within seconds, a new unit, D4, will appear as the BCV of D1. EVM can then mount D4 as specified in the job.

This happens in just seconds because there is no copying of data. As long as unit D4 is retained (usually not long term) the controller will manage reads and writes as appropriate. All writes to D4 are directed to the BCV disks. Reads of D4 are routed to D1 for data that has not changed since the snapshot was taken, or to the BCV disks for data that has changed since the snapshot was taken.

 

Can I Take Snapshots of RAID 3/5 Units?

Yes. The parity data in RAID 3/5 units is maintained when using the snapshot method.

 

Can I Make Multiple Snapshots of the Same Unit?

No. You can only have one snapshot at a time of a given unit.

 

What is the maximum number of snapshots I can have on one subsystem?

Four. You can have up to four simultaneous snapshots per subsystem.

 

Can I Make Snapshots of BCV Units?

No. EVM tracks the BCV units that it creates with each job and will not allow you to select one as a source for another job. Also, the ACS does not support taking a snapshot of a unit that is, itself, a snapshot of another unit.