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Snapshot data is reported in two ways, logical snapshot capacity and physical snapshot capacity.
Logical snapshot capacity is based on delta count for each snapshot and based on the full 128 KB track size. Data that is shared across multiple snapshots will be reported by each snapshot. Therefore, these values cannot be used to determine actual backend snapshot usage. The following Solutions Enabler commands and Unisphere displays report logical snapshot capacity.
The values in the following SnapVX outputs are defined as follows:
Snapshot Dev Size: The provisioned size of the source device at the time the snapshot was created.
Total Deltas: Data that has been written by the application since the snapshot was created.
Non-Shared: Data that is unique to the specific snapshot and not shared by any other snapshots.
The following shows the logical snapshot capacity per snapshot:
The fields in the demand report are defined as follows:
Total Snapshot Capacity: The combined subscribed size of all existing snapshots.
Modified %: Percentage of Total Snapshot Capacity written by the application since the snapshots were created.
NonShared Snapshot Capacity: Capacity of the Modified % that is not shared by multiple snapshots.
Shared Snapshot Capacity: Capacity of the Modified % that is shared by multiple snapshots.
System demand report:
The Efficiency Dashboard in Unisphere provides a graph of snapshot usage as shown in the following figure:
Physical snapshot capacity is the actual backend capacity that is snapshot data. Physical snapshot capacity may be significantly less than logical snapshot capacity due to efficiency of features such as shared allocations and Data Reduction.
The storage group demand report displays physical snapshot capacity of each SG in the specified SRP, along with a sum of the combined snapshot usage. The fields are defined as follows:
Note: The following output only displays Storage Groups that have an explicit Service Level or Workload.
Physical snapshot capacity per SG:
Physical snapshot capacity per TDEV is available in the symcfg -sid XYZ list -srp -detail -v command as shown in the following example. The output also provides a sum of the combined snapshot usage across the TDEVs in the SRP.
The fields in the output are defined as follows:
Thin Device Snapshot
----------- ---------------------------------------------
Sym Subs Allocated Used Allocated Used
Dev Emul (GB) (GB) (%) (GB) (GB) (GB)
----- ---- ------- ---------- ---- --------- ----
0020D FBA 10240.0 224.5 2 49.7 1.3 1.2
0020E FBA 10240.0 214.6 2 41.8 3.6 3.1
0020F FBA 10240.0 214.5 2 41.8 3.4 2.9
00210 FBA 10240.0 215.6 2 41.8 1.8 1.6
------- ---------- ----- --------- ----
Total 40960.0 869.2 2 175.1 10.1 8.8
Linked target capacity usage aspects:
Linking targets in nocopy mode is the recommended best practice because they provide same performance as full copy linked targets while using less backend resources. Using full copy mode in PowerMax systems with Data Reduction enabled is equal to a nocopy target due to deduplication.
Some use cases require dedicated linked target volumes. For example, when presenting separate copies to multiple users at the same time. Linked targets can also be shared across source volumes in some use cases. For example, if a set of linked targets is used only to verify the validity of snapshot data from a Storage Group, once the data is verified, the targets can be unlinked and then linked to the next Storage Group for the same purpose. Limiting the number of devices in a system whenever possible reduces usage of system resources and simplifies management.
During nocopy link and relink operations, all existing data on the target is returned to the pool. Therefore, it is not recommended to deallocate a target after an unlink operation if the target will be linked again soon. Deallocating the target in this case will not cause any harm, although a device is not available for use while the operation is running. But it is an unnecessary step since the subsequent link operation will deallocate the device anyway.
However, if there are no plans to use the target again soon, then users should consider performing a free –all operation on the target to deallocate any exclusive allocations and remove the device from any shared allocations.
Note: Extreme care must be taken when using the free –all command. See Unlinking for more information. The free –all command is available with the symdev, symsg, symdg, and symcg commands.
Linked target capacity usage monitoring:
Nocopy linked targets typically have some combination of exclusive and shared data. Shared data is created during the define process. Exclusive data is created by host writes. Operations such as snapshot termination, and deallocation, freeing, or host writes to other targets can cause shared data to become exclusive data.
Solutions Enabler command symcfg –sid <sid> list –tdev –detail reports total and exclusive allocations for individual TDEVs. The output includes a flag to indicate the presence of shared tracks.
Exclusive allocated tracks per TDEV: