Dell EMC VMAX3 SnapVX Local Replication


We have been familiar with TimeFinder Mirror, Clone & Snapshot technologies of Symmetrix VMAX & Engunity OS.  TimeFinder SnapVX, usually called SnapVX, was introduced with the VMAX3 with New HYPERMAX OS version 5977 . SnapVX delivers instant and storage-consistent point-in-time replicas of host devices that can be used for purposes such as the creation of gold copies, patch testing, reporting test/dev environments, backup and recovery, data warehouse refreshes, or any other process that requires parallel access to or preservation of the primary storage devices.
SnapVX uses Redirect-On-Write (ROW) technology, which is new to TimeFinder. When an update is made to a source track, this update is asynchronously written to a new location in the SRP, and the original data is preserved for the snapshot. Pointers are used to make sure that each copy of the track is used by the correct data. SNAPVX snapshots do not require target volumes and in nocopy mode they will only use extra space when the source volume is changed. A single source volume can have up to 256 snapshots, and these snapshots also save space by sharing point-in-time (PIT) tracks, or ‘snapshot deltas’. The snapshot deltas are stored in the SRP alongside the source volume and each snapshot has a set of pointers that reference the set of snapshot deltas that are needed to preserve a PIT image.
SnapVX is simplest when your storage has been designed with an application per storage group, you can still use device groups or files if you want but VMAX3 supports 64K storage groups, that is enough for one per application in most environments and means only managing a single entity for each application for provisioning as well as local replication and remote replication. You can snap multiple applications together using a cascaded storage group containing all of the child storage groups for each application. SnapVX snapshots are consistent by design so no need to specify any additional flags to obtain a point in time image of a live system.
TimeFinder  SnapVX Features:

  • Target-less Snapshots: Highly efficient mechanism for taking periodic point-in-time copies of source data without the need for target devices.
  • Linked Snapshot: A snapshot can be linked to a target volume to provide access to point-in-time data to a host.
  • Copy mode linked targets : This provides full volume copies of the point-in-time data of the source volumes similar to full clone copy.
  • Up to 256 snapshots per source volume
  • Up to 1024 target volumes can be linked to the snapshots of a source volume
  • Cascading snapshots from targets is allowed once the target is fully defined or copied
  • No limitation on the amount of cascading of snapshots and targets as long as the 256 snapshot count and 1024 target volume count are not exceeded
  • Cascaded targets can be restored from snapshots as long as they are in Copy mode and are fully copied
  • One-to-many snapshot deltas where changed data is written to an SRP only once but can be used by all snapshots taken before that point in time. This is the same space efficient technique used in Snap VP.
  • Redirect-on-write: Writes to a source that has snapshots will be accepted asynchronously and placed in a new location in the SRP. The source volume will point to this data while the snapshots will continue to point to the original data.
  • Asynchronous Copy-on-First-Write: In certain situations, to prevent a write from being directed to a lower storage tier, a source volume’s original data may instead be copied to a lower tier to allow the new source data to reside in the higher tier.
  • Storage group snapshots: Rather than using a device file or composite group, users can now specify the creation of a snapshot for an entire storage group of one or more hosts.

Snapshot Generation and Time to Live:
Generation Number:  Each time we create snapshot it is reference with the generation number. If the same snapshot name is used then the most recent designated generation numbers will be 0 while prior one is incremented by 1 and so on. Terminating a snapshot will result in reassignment of generation numbers.
Time-to-Live : Time-to-Live (TTL) can be used to automatically terminate a snapshot at a set time. This can be specified at the time of snapshot creation or can be modified later. HYPERMAX OS will terminate the snapshot at the set time. If a snapshot has linked targets, it will not be terminated. It will be terminated only when the last target is unlinked. TTL can be set as a specific date or as a number of days from creation time.
SnapVX Linked targets can have one of two modes:
Nocopy Mode: Does not copy data to the linked target volume but still makes the point-in-time accessible via pointers to the snapshot. The point-in-time image will not be available after the target is unlinked because some target data may no longer be associated with the point-in-time1
Copy Mode: Copies all relevant tracks from the snapshot’s point-in-time to the linked target volume to create a complete copy of the point-in-time that will remain available after the target is unlinked.

TimeFinder SnapVX Operation Steps:

1. Identify storage group for snapshot
symsg –sid xx list | grep –i <SG_NAME>    #Unix Management
symsg –sid xx list | findstr <SG_NAME>    #Windows Management

2. Create Snapshot
Example: Storage group Name : snapvx_source ,  snapshot name daily_snap, and expiration 7 days.
symsnapvx -sid xx -nop -sg snapvx_source establish -name daily_snap -ttl -delta 7
–delta is used to specify the number of days for expiration from the time the snapshot was created.
“Snapshot names are case sensitive, can be up to 32 characters long and can contain underscores '_' and hyphens '-'.” 
3. Display Snapshot details
symsnapvx -sid xx -sg snapvx_source list –detail

4. Display a summary of the state of snapshot
symsnapvx list -sid xx -sg snapvx_source -summary –tb
5. Display SnapVX Snapshots And Capacity Consumed
symcfg list –srp –demand –type sg
6. Link Snapshot
symsnapvx -sid xx -sg snapvx_source -snapshot_name daily_snap link -lnsg snapvx_target
To access this snapshot, you need to link a host mapped target volume to the snapshot data.
-copy “Specifies copy mode for link operation which performs a background copy to the target device.
SnapVX uses Copy mode to create full-volume copies of the data bycopying it to the target device's SRP.
If Copy mode is not specified when linking the snapshot, the NoCopy link is the default”
Other TimeFinder SnapVX Operation commands:
# Create a Copy mode link
symsnapvx -sid 001 -sg snapvx_source -lnsg snapvx_target -snapshot_name daily_snap -gen 6 link -copy
#Display Linked Status
symsnapvx list -sid xx -sg snapvx_source  -linked
#Rename snapshot
symsnapvx –g snapvx_source   -snapshot_name  rename daily_snap –name hourly_snap
7. Relink Snapshot
symsnapvx -sid xx -sg snapvx_source -snapshot_name daily_snap -lnsg snapvx_target -gen 6 relink
# To relink in Copy mode
symsnapvx -sid xx -sg snapvx_source -snapshot_name daily_snap -lnsg snapvx_target -gen 6 relink -copy

8.Unlink Snapshot :
symsnapvx -sid xx -sg snapvx_source -snapshot_name daily_snap link -lnsg snapvx_target -gen 6 unlink
This will permanently remove the snapshot relationship

9. Restore Snapshot
You can restore your original source volume from a snapshot. Restores work by copying back the differential data.
symsnapvx -sid xx -sg snapvx_source -snapshot_name daily_snap -gen 6 restore
10 Terminate Snapshot
# Verify that generation 6 of snapshot is in restored state
symsnapvx -sid 038 -sg snapvx_source -snapshot_name daily_snap -gen 6 terminate -restored
# Terminate generation 5 snapshot
symsnapvx -sid 001 -sg snapvx_source -snapshot_name daily_snap -gen 6 terminate


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