Last week I have written a short post about introduction to MirrorView. This week I would like to write a little bit more about MirrorView configuration, terminology and usage. Let’s start with terminology
VNX MirrorView Key Terminology
Primary Image – LUN containing production data and the contents of which is replicated to secondary image.
Secondary Image – LUN containing a mirror of the primary image LUN residing on a different VNX (secondary site)
Image condition – Provides additional information about the status of updates for a secondary image.
State – Remote mirror states and image states.
Consistency Group – Set of mirrors that are managed as a single entity and whose secondary images always remains in a consistent and recoverable state with the primary image and each other.
Consistency Group State – Indicated the current state of the consistency group.
Fracture – Condition in which I/O is not mirrored to the secondary image. Manually initiated by the administrator or by the system when it determines the secondary image is unreachable.
Promote – changes an image’s role from secondary to primary.
Basic MirrorView Configuration
MirrorView allows for a large amount of topologies and configuration. The primary and secondary images must have the same server-visible capacity (user capacity), because they are allowed to reverse roles for fail-over and fail-back (see Promote definition above). But Primary Image and Secondary Image can reside on different RAID configuration. In order to use MirrorView, the software need to be loaded on both (primary and secondary) VNX arrays. Secondary LUNs are not accessible to hosts during the mirroring. Bi-directional mirroring (VNX array can be both primary and secondary site) is supported, as long as the primary and secondary images within mirror reside on different storage systems.
Consistency Groups allow all LUNs that are belonging to a give application to be treated as a single entity and managed as a whole. This helps to ensure that the remote images are consistent. As a result, the remote images are always re-startable copies of the local images. When a mirror is part of a Consistency Groups, most operations on individual members are prohibited (for example fracture, or synchronize can only be executed for the Consistency Group).
Site Level Fan-In
MirrorView supports 4:1 Fan-In ratio. It means that one VNX array can be a destination (secondary site) for 4 (different) primary VNX arrays. It’s a common configuration when remote VNX Array is used for consolidated backups, simplified failover or consolidated remote processing activities. The 4:1 Fan-In ratio is applicable to both MirrorView/S and MirrorView/A
LUN Level Fan-Out
Fan-out mirroring may be used to replicate data from one primary LUN to up-to-two secondary LUNs residing on different arrays. (MirrorView/S 1:2 Fan-out ratio). This configuration enables administrator to synchronously mirror one primary image to two different secondary images. In case of MirrorView/A, one primary image can be mirrored only to single secondary image (MirroView/A 1:1 Fan-out ratio).
MirrorView ports are automatically assigned when the system is initialized. All MirrorView traffic goes through one dedicated port of each connection type (FC or iSCSI) per Storage Processor. For VNX that have FC and iSCSI systems, one FC port and one iSCSI are available for MirrorView traffic (per SP).
A path must exist between the MirrorView ports of SP-A of the primary and SP-A of a secondary system. Same relationship must exist for Storage Processor B. MirrorView ports may be shared with host I/O, but that might cause performance issues.
Mirrored Image States
Once an image has been mirrored, the image may be in one of three availability states:
- Inactive – inactive mirrored states means that the Administrator has stopped mirroring.
- Active – an active status is considered a normal state, where all I/Os are allowed on the image.
- Attention – this state indicated that something has happened to the mirrored image and action by an Admin is required.
In terms of the mirrored image consistency and relationship with the source image, MirrorView contains five data states
- Out-of-Sync – means that a full sync is in order
- In-Sync – indicate that the primary and secondary contains identical data and the operation is in progress
- Rolling Back – the act of returning a primary to a predefined point-in-time.
- Consistent – the mirroring has been stopped and a write intent or fracture log is needed to continue the mirroring
- Synchronizing – the operation of synchronizing is in progress.
MirrorView Common Operations
Synchronization is a copy operation MirrorView performs during newly-created mirrors or to reestablish existing mirrors after an interruption. Initial synchronization is used to create a baseline copy of the primary image to the secondary. Primary images remain online during the sync process and until the synchronization is complete, the secondary image is unusable.
A secondary image is promoted to the role of primary when it is necessary to run production applications at the disaster recovery site. A promotion can only occur if the secondary image is in the consistent or synchronized state.
A fracture stops MirrorView replication from the primary image to the secondary mirror. Administrative fractures are usually initiated to suspend replication, as opposed to a system fracture which is initiated by the MirrorView software. A system fracture typically means a communication failure between the primary and secondary systems.
With MirrorView/S writes continue to the primary image but are not replicated to the secondary during a fracture. Replication can resume when the user issues a synchronize command.
With MirrorView/A the current updates stop during a fracture and no futher updates will start until a synchronize request is issued.