EMC - VNX2 - Snapshot

1.Usefull Link

EMC VNX – Snapshots

VNX Snapshots Overview and Provisioning

Using VNX Snapshots on Windows

Using VNX Snapshots on Linux

Performance Issue with VNX Snapshots

2. Overview

VNX Snapshot is a storage system-based software application that allows the user to create snapshots of pool-based LUNs. In fact, VNX Snapshots can only be used with pool LUNs. A snapshot is a virtual point-in-time copy of a LUN and takes only seconds to create. VNX Snapshots use a very different internal mechanism to that used by SnapView snapshots, though both are pointer-based. VNX Snapshot data may be in the original Primary LUN space or may have been written to a different location in the Pool. 

As a result of the Relocate on First Write (ROW) technology used, VNX Snapshots use appreciably less additional space than a full copy would use. A VNX Snapshot will use appreciably less space than that occupied by its Primary LUN, and will make more efficient use of space than SnapView Snapshots. 

An enabler gives the user access to VNX Snapshots, while a separate enabler allows the use of SnapView Snapshot and Clone technology. These two methods of making point-in-time copies are independent, and have limits which are independent of each other. 

They can coexist on the same storage system, and even on the same Pool LUNs. Note that VNX Snapshots cannot be used on Classic LUNs. Management of VNX Snapshots is performed through Unisphere or Navisphere Secure CLI. A host-based utility, SnapCLI, can perform a subset of the VNX Snapshot management operations, and will be discussed later.

The Primary LUN is the production LUN that is replicated. This is the LUN that is in use by the application (and the production host) and it is not visible to secondary hosts. When a snapshot is attached to a snapshot mount point, it is made available to a secondary host.
A Snapshot is the VNX Snapshot equivalent of the SnapView session.

• A Snapshot Mount Point is the VNX Snapshots equivalent of the SnapView Snapshot – a virtual LUN that is used to make the replica visible to a secondary host. The SMP is associated with the primary LUN, and can be used for snapshots of that LUN only.

• Consistency Groups allow primary LUNs or Snapshot Mount Points to be grouped together persistently. Operations can be performed on the group as a single object

VNX Snapshots address limitations of copy on first write (COFW) SnapView Snapshots. The VNX Snapshot technology is redirect on write (ROW or ROFW). VNX Snapshots are limited to Pool-based LUNs (i.e. not Classic LUNs). Up to 256 writeable VNX Snapshots can be associated with any Primary LUN, though only 255 are user visible. Because the VNX Snapshot uses pointers rather than a full copy of the LUN, it is space-efficient, and can be created almost instantaneously. The ROW mechanism does not use a read from the Primary LUN as part of its operation, and thus eliminates the most costly (in performance terms) part of the process.

• A Reserved LUN Pool is not required for VNX Snapshots - VNX Snapshots use space from the same Pool as their Primary LUN. Management options allow limits to be placed on the amount of space used for VNX Snapshots in a Pool.

• VNX Snapshots allow replicas of replicas; this includes Snapshots of VNX Snapshots, Snapshots of attached VNX Snapshot Mount Points, and Snapshots of VNX Snapshot Consistency Groups. VNX Snapshots can coexist with SnapView snapshots and clones, and are supported by RecoverPoint.

• If all VNX Snapshots are removed from a Thick LUN, the driver will detect this and begin the defragmentation process. This converts Thick LUN slices back to contiguous 256 MB addresses. The process runs in the background and can take a significant amount of time. The user can not disable this conversion process directly, however, it can be prevented by keeping at least one VNX Snapshot of the Thick LUN.

A VNX Snapshot Mount Point (SMP) is a container that holds SCSI attributes
•WWN
•Name
•Storage Group LUN ID, etc

The VNX Snapshot Consistency Group allows Snapshots to be taken at the same point in time on multiple Primary LUNs. If individual Snapshots were made of the Primary LUNs, it is possible that updates to one or more Primary LUNs could take place between the time of the Snapshot on the first Primary LUN and the time of the Snapshot on the last Primary LUN. This causes inconsistency in the Snapshot data for the set of LUNs. The user can ensure consistency by quiescing the application but this is unacceptable in many environments.

• A Consistency Group can have a Snapshot taken of it, and can have members added or removed. Restore operations can only be performed on Groups that have the same members as the Snapshot. This may require modifying Group membership prior to a restore.

• When a Snapshot is made of a Group, updates to all members are held until the operation completes. This has the same effect as a quiesce of the I/O to the members, but is performed on the storage system rather than on the host.

• VNX Snapshot Set – a group of all Snapshots from all LUNs in a Consistency Group. For simplifications, is referred to as CG Snap throughout the material. VNX Snapshot Family – a group of Snaps from the same Primary LUN