This article provides some guidance for those planning on using the new 'alternating' backup feature that appeared in PresSTORE 4.2. The aim of this new option within backup plans, is to ease the setup of backup workflows where tapes are switched on/off-site on a regular basis. 

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Take this example backup plan. 

We've have two backup events, one FULL and the other INC. FULL runs on Fridays and INC's on the other weekdays. Notice however that there are two pools assigned to each event: Bkup_Pool_A and Bkup_Pool_B. Also notice that both the FULL and INC events have both pools assigned.

When one of these events runs within this backup plan, it will check the available volumes in the library. If any volumes in the A pool are found, PresSTORE will determine that these are the volumes available on that day and set about writing A volumes for that run of the backup. Same will apply for FULL backups on Friday or INC backups on the other weekdays. 

However, if no A volumes are available in the library, the backup events allow for the second B pool to be used. Note that the order of the pools as defined in the backup event window is significant and the pools will be used in the specified order. If A volumes are available then no B volumes will be written in the above example.

The described configuration allows for the customer to alternate between two pools of volumes without having to deliberately schedule which pool to use on specific days. Therefore if the customer should leave the A volumes in the library for two days, they will be written on both days without the backup failing with an error on the second day.

When calculating what to save in a given run of the backup, PresSTORE takes into account which pool it is writting (in this example A or B) and saves all data required to keep that pool updated with all data necessary to keep it independent of the other pool(s). So when an INC backup runs to the B pool, all data that has changed since the last time that pool was used is saved to the volumes.

Up to three pools can be added to a backup event.

Potential problems

Take the following backup plan: In this plan we are attempting to save FULL backups to a different pool to the INC backups. The intention is that the FULL backups all go to the Bkup_Pool_Full pool while the INC backups are written to alternating A and B pools, which ever pool is available will be written.

This configuration will not work as expected because when using alternating pools, they have to contain both the FULL and INC backups. When using alternating pools, it's not possible to use different pools for FULL and INC backup events. The result of running the above plan would be that when the INC event runs, it will again save all of the data (like a FULL backup) to each of the A and B pools when it first sees then. Only when each of these pools contains a FULL backup can the INC run to these pools.

Using two backup plans

Take the backup plan configuration in the image below. This is not using the alternating backup feature, it has two separate backup events to write INC backups to pool A on Mon/Wed and pool B on Tue/Thu. The planned workflow here is that the customer would keep the Bkup_Pool_Full volumes offsite during the week and only bring them on-site on Fridays. They would also alternative the A and B incremental pools on/off-site during the week so the required volumes are available on the correct days.

The problem with this configuration is that, because all of these backup events exist in the same backup plan, the A and B pools will not be maintained independently of each other. This mean that when an INC backup runs to Pool A on a Wednesday, it will only save the data saved since the previous INC backup on Tuesday. Therefore if all of the B tapes are lost due to some disaster scenario, the customer will not be able to perform a complete restore, as some of their INC data is stored on the lost tapes.

In order to implement such a backup workflow, the configuration needs to be split into two backup plans. These two backup plans will then operate independently of each other and each will contain a complete set of the customers data independently of the other. This workflow is more complex to configure because two backup plans are required. In this case it might be worth considering converting to the alternating backup workflow to make the configuration simpler, and the handling easier for the customer.

Backup with Alternating Pools - definition from the Archiware Manual (available within the product help menu)

Starting with PresSTORE 4.2, the backup module allows to define alternating backup pools. Setting up alternating backup pools defines a backup plan than writes up to three independent backup data sets. PresSTORE will then automatically select the first of the pools that it finds available at backup time and adds all data that is missing in that pool. So a plan can be defined that dynamically reacts on the tapes that are made available and saves the data missing in that pool.

To setup a plan with alternating pools just define a backup schedule where as many pools are defined as backup sets shall be created. Up to three pools can be added. The definition of the backup pool is done in the Backup Event, see section 7.7.1, Creating a Backup Plan. In case multiple backup events shall be defined, you may want to define the same pools in all backup events.

This mechanism can be used in conjunction with the progressive setup with automatic recycling as well as with a normal full / incremental backup schedule.

Note that the use of alternating pools in one Backup Event is different to defining multiple Backup Events with a separate media pools each. The later will not create independent backup sets but write one complete set of all data across multiple pools .