TimeFinder/Snap requires specially configured device types in Symmetrix, so-called virtual devices and save devices:
If, after a snap unit has been activated, data is modified on the original unit (or on the snap unit), the original data is written to a save device. A group of save devices is called a save pool.
The virtual device contains the references to the original data. The virtual device of the Symmetrix is the snap unit which can be accessed by the server.
Virtual devices
A virtual device consists of track pointers. The pointers refer to the physical devices which contain the snap session’s original data. In the case of unmodified data (tracks) the pointers point to the original unit, and in the case of modified data to the save device involved. As virtual devices only contain the relevant pointers, they only need a minimal volume capacity.
After being assigned with /START-SNAP-SESSION and activated with /ACTIVATE-SNAP, the snap unit is immediately available to the application. All the pointers initially point to the original unit. The Copy-On-First-Write process begins: When the first write access takes place, the track concerned is copied to a free track on a save device and the pointer is switched to this.
Via the virtual device, the application which is working with a snap unit has a complete view of the data which is provided via the original unit’s relevant pointers or by the save device.
The virtual devices must be configured in Symmetrix. Only these virtual devices can be used as snap units.
Virtual devices must match the original unit in terms of their configured size and, if required, meta type and meta stripe size. They must be generated during hardware generation of the BS2000 server using IOGEN (/390 servers) or in X2000 (x86 servers). For information on generation, see section "Snap units (Symmetrix)".
When the snap session is terminated (= cancelation of the snap pair), the pointers are deleted again and the memory area on the save device is released.
Save devices
Save devices save all original data which was modified on the original unit (and also on the snap unit) in the course of a snap session.
Save devices are internal devices which are specially configured in Symmetrix by a qualified technician which provide storage space to accommodate the old contents of modified tracks. Save devices cannot be accessed by the server. You are recommended to back up save devices using RAID procedures. A sufficient number of save devices of sufficient size must be configured and enabled.
Save pools
Save devices are grouped on Symmetrix in so-called save pools. Each save pool is available in its entirety to the user for his/her snap session(s).
By default there is a default pool. Additionally, further named save pools can be configured and have save devices assigned.
A save pool is selected when a snap session is started. Via the SHC-OSD parameter file the PRESELECTED-SAVE-POOL operand enables a save pool name to be preset, and this name is then selected by default.
The available capacity of a save pool is determined by the size and number of its save devices and is consequently limited. The required capacity must therefore be planned carefully in advance.
The required capacity depends on the following factors:
Data volume of the applications for which snap sessions are to be generated
Number of snap units per original unit
Change volume on the original unit (and the associated snap units)
CAUTION!
When a save pool is full (overflow), the existing snap sessions can no longer be continued. They can only be terminated. The data modified on the snap units is lost. The SHC-OSD functions for monitoring the save pools should therefore be used.
Setting up multiple save pools permits partitioning here, for example by having the snap sessions of different applications which operate with different pubsets assigned to different save pools. As a result, overflows of save pools only affect the snap sessions of the pools involved and consequently the application(s) operating with this pool.
Monitoring the save pools
SHC-OSD offers monitoring of the occupancy level of the save pool for Symmetrix.
The /SHOW-SYMMETRIX-CONFIGURATION command with the INFORMATION=*SAVE-POOLS operand (see "SHOW-SYMMETRIX-CONFIGURATION Displays the configuration (Symmetrix/VMAX3)") provides information on the save pools, including their current occupancy levels.
In addition, the messages NDE0055 and NDE0056 (which you can answer) on the console show when the limit value for the occupancy level of a save pool has been exceeded.
The limit value for the occupancy level of a save pool is set in the SHC-OSD parameter file using the SAVE-POOL-SATURATION operand (default value: 80%). The SNAP-SAVE-POOL-SAT operand in the /MODIFY-SHC-PROCESSING command modifies the limit value during ongoing operation. The current value can be inquired with /SHOW-SHC-PROCESSING.
When the limit value is reached, administrative intervention enables capacity to be provided again in the save pool. Old snap sessions which use the same save pool and whose data has been saved can be terminated (/STOP-SNAP-SESSION). Alternately, the save pool in the Symmetrix configuration can be extended. When sufficient space is once more available in the save pool, this is indicated by the messages NDE0051/NDE0052 on the console. Unanswered messages NDE0055/NDE0056 are then answered automatically.
Limiting the number of snap units / snap sessions
Currently a maximum of 127 snap units can be configured for an original unit in Symmetrix.
In SHC-OSD the use of 127 snap units is preset (this can be checked with /SHOW-SHC-PROCESSING). If a only maximum of 15 snap units are to be supported, this must be set with /MODIFY-SHC-PROCESSING MULTI-SNAP=*DISABLE.
The maximum number of clone and snap session active at any one time is 16. If the system is operating with a maximum of 127 snap units, all snap units count as only one snap session. If the system is operating with a maximum of 15 snap units, the first one counts as two snap sessions and each additional snap unit as one. The first snap session counts twice.
The maximum value for pubsets may be reduced when the “Snapsets” function (see the “Introduction to System Administration” manual [5]) is used because a specific number of sessions can be reserved for Snapsets.
Generating virtual devices
Virtual devices in Symmetrix (see "Snap units (Symmetrix)") are by default in the NOT-READY status and are only in the READY status while they are being activated as snap units and can then only be seen and accessed by the server.
All BS2000 devices which are connected over Fibre Channel to x86 servers must be configured in X2000:
On x86 servers all the BS2000 devices which are planned to be used as snap units should be generated in advance in X2000. Devices in the READY or NOT-READY status can be configured. “Overconfiguration“, i.e. configuration of BS2000 devices which are not (yet) available, is also possible.