ACRONIS BACKUP AND RECOVERY 10 ADVANCED SERVER SBS EDITION - UPDATE 3 User Manual page 363

A new level 3 differential backup has not yet been created, so the backup of day five is still stored.
Since it depends on the full backup of day one, that backup is available as well. This enables us to go
as far back as 11 days, which is the best-case scenario.
The following day, however, a new third-level differential backup is created, and the old full backup is
deleted.
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This gives us only a four day recovery interval, which turns out to be the worst-case scenario.
On day 14, the interval is five days. It increases on subsequent days before decreasing again, and so
on.
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The roll-back period shows how many days we are guaranteed to have even in the worst case. For a
four-level scheme, it is four days.
7.3.7.6 Custom backup scheme
At a glance

Custom schedule and conditions for backups of each type
 Custom schedule and retention rules
Parameters
Parameter
Full backup
Incremental
Differential
Clean up archive
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Meaning
Specifies on what schedule and under which conditions to perform a full backup.
For example, the full backup can be set up to run every Sunday at 1:00 AM as
soon as all users are logged off.
Specifies on what schedule and under which conditions to perform an
incremental backup.
If the archive contains no backups at the time of the task run, a full backup is
created instead of the incremental backup.
Specifies on what schedule and under which conditions to perform a differential
backup.
If the archive contains no full backups at the time of the task run, a full backup is
created instead of the differential backup.
Specifies how to get rid of old backups: either to apply retention rules (p. 38)
regularly or clean up the archive during a backup when the destination location
runs out of space.
By default, the retention rules are not specified, which means older backups will
not be deleted automatically.
Using retention rules
Specify the retention rules and when to apply them.
This setting is recommended for backup destinations such as shared folders or
centralized vaults.
When there is insufficient space while backing up
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