* Rotation. Your backup rotation will also impact the size of the disk-based backup with deduplication system you need. If you are doing rolling full backups each night, then you need a larger system than if you are doing incremental backups on files during the week and then a weekend full backup. Rotation schemes are usually:
Database and email
Full backup on Monday, Tuesday, Wednesday, Thursday, weekend
Incrementals forever or optimized synthetics -- this copies only changed files each night, and there is no weekend full
Incrementals -- Copies changed files each night, full backup of all files on the weekend
Differentials -- Copies files each night that have changed since the last full backup, full backup of all files on the weekend
Rolling fulls -- Breaks total full backup into a subset and backs up a portion of the full backup each night (e.g., if the full backup is 30TB, then back up 10TB each night and keep rotating on a three-day schedule)
Because the backup rotation scheme you use changes how much data is being sent to the disk-based backup with deduplication system, this also impacts the system size you require.
* Cross protection. Two scenarios:
Sizing Scenario 1: You are backing up data at Site A and replicating to Site B for disaster recovery. For example, if Site A is 10TB and Site B is just for disaster recovery, then a system that can handle 10TB at Site A and 10TB at Site B is required.
Sizing Scenario 2: However, if backup data is kept at both Site A (e.g., 10TB) and at Site B (e.g., 6TB), and the data from Site A is being replicated to Site B while the data from Site B is being cross-replicated to Site A, then a larger system on both sides is required.
Bottom line for sizing a system
In summary, dozens of possible scenarios impact the sizing of a system, including:
How much data is in your full backup? What percentage of the data is compressed (including media files), encrypted, database, unstructured?
What is the required retention period in weeks/months on-site?
What is the required retention period in weeks/months off-site?
What is the nightly backup rotation?
Is data being replicated one way only or backed up from multiple sites and cross-replicated?
Other considerations unique to your environment
When working with a vendor, ensure it offers a tool that allows it to calculate the exact size of the system you need based on all of the above.
It is very common to see an organization acquire a disk backup with deduplication system and, in a few short months, have it fill up because the system was undersize, retention was longer, the rotation scheme put more data into the system, the deduplication method had a low deduplication ratio, or the data types were such that they could not deduplicate well.
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