Storage Media Rotation Scheme using BackupChain
I want to back up my files to two different USB drives. Can I do that using BackupChain?
Delta compression requires information about the previous
copy of each file, which is inferred from the target folder.
This means that when you rotate backup drives you end
up with two separate and independent file version histories. Note
that BackupChain needs only a fraction of the previous delta
file (about 100kb or so) to create the next incremental backup
file.
Assuming you set up a daily storage drive rotation with two drives,
BackupChain will create a full delta and then an incremental
delta on each drive but it will be from a different day, always
two days apart.
If you rotate the drives every night and you run the backup only
once each night, the backups will be incremental and refer to
the last time they ran on the same drive.
The file history would look as follows:
| USB Drive #1 | USB Drive #2 | ||
| Connected on Mon / Wed / Fri / Sun | Connected on Tue / Thu / Sat | ||
| Full delta generated on Monday | Full delta generated on Tuesday | ||
| First incremental delta is created on Wednesday and compared to Monday | First incremental delta is generated on Thursday and compared to Tuesday's delta | ||
| Second incremental delta is created on Friday and compare to Wednesday's delta | Second incremental delta is created on Sat and compared to Thursday's. |
Each drive's history is valid in isolation but they are actually
2 days apart from each cycle. BC's "memory" of what has happened
is inferred from looking at the drive's or target folder's
content. This technique is also useful when you want to rotate
storage drives with plain file copy (with or without
compression) as well.
This mechanism has several benefits:
1. It ensures any "gaps" are always filled
2. You can add an additional drive to the loop as needed at any time.
Gaps can occur if you delete a file in the file store
or if one of your storage hard drives fails and needs to be
replaced.
Now, if you had seven drives, it would take seven days until you
start seeing incremental deltas and each increment would be
seven days apart because BackupChain would compare the current image to
what it was seven days ago.
The closer the delta comparisons are in time the greater the
storage savings because fewer changes occur inside files in
shorter time spans.
This is why we recommend to use two or three drives in rotation. The
best result in terms of storage savings is of course with just
one drive.
The
One Alternative to Rotation
Another way
is to use a third temporary step and have the
deltas stored in a permanently connected drive. Then back up that
temporary folder with a plain file copy task in BackupChain (no compression, keep
only one version) to an external drive. In this scenario you can rotate
with as many drives as you like and still have maximum storage
savings and have identical copies on each drive. The
disadvantage of this recovery strategy is you need an additional
temporary storage location.