In BE 2012, when you use fixed disk, the
storage that would be created is called Disk Storage. When you store a
backup set in a Disk Storage, you can specify the retention period for
the backup set. The management of this backup set is done through a
mechanism called Data Lifecycle Management (DLM). DLM is explained here
https://www-secure.symantec.com/connect/blogs/data-lifecycle-management-be-2012
Note that in addition to your specified retention
period, DLM will not groom the last copy of the latest recovery point
chain and the grooming process takes place every 4 hours or when a low
disk space condition is encountered. This makes it difficult to
visualise what is on your disk and visualising what is on your disk is
critical when you are trying to squeeze as much as possible onto your
disk.
You can turn off the last copy feature of DLM using the procedure in this document
This is needed if you want to delete the last backup set of a server that has been de-comissioned.
In this article, I would use some example/scenario to illustrate how DLM works and when backup sets are deleted.
Minimum Retention Period, Only DLM At Work
The retention period for the backup set is 1
hour. This is so that the backup sets can be deleted as soon as
possible and they are only retained because of the other criteria
specified by DLM.
Full + Incremental Backups
We have a full backup (Fx) running every Sunday
and incremental backups (Incrxx) on Monday to Friday. All jobs starts
at 1 a.m. and finshes at 2 a.m. The jobs are as follows:-
| Week | Sun | Mon | Tue | Wed | Thu | Fri | Sat |
|---|---|---|---|---|---|---|---|
| 1 | F1 | Incr11 | Incr12 | Incr13 | Incr14 | Incr15 | |
| 2 | F2 | Incr21 | Incr22 | Incr23 | Incr24 | Incr25 | |
| 3 | F3 | Incr31 | Incr32 | Incr33 | Incr34 | Incr35 |
Even though the retention period for the backup
set created by full backup job F1 expires at 3 a.m. on Sunday, it is not
groomed even when incremental backup job Incr11 runs. This is because
backup set F1 is the last recovery point of the server.
Likewise, backup set Incr11 is not groomed when
job Incr12 runs, even though its protection expires at 3 a.m. on Monday.
This is because Incr11 is the last recovery point of the server.
Backup set F1 is also not groomed because backup set Incr11 depends on
it. So it continues, after job Incr15 has completed, you would have
backup sets F1, Incr11, Incr12, Incr13, Incr14 and Incr15 on your disk.
When job F2 runs, all the backup sets from Week 1
are still not groomed because Incr15 is the last recovery point of the
server and it depends on the rest of the backup sets. After job F2 has
completed, it is now the last recovery point of the server and it is not
dependent on the backup sets from Week 1, so these can be groomed. For
a maximum of 4 hours until the next grooming cycle, you would have
backup sets F1, Incr11, Incr12, Incr13, Incr14, Incr15 and F2 on your
disk.
In this scenario, your disk must have adequate space to hold 2 full backups plus 1 week of incremental backups.
Full + Differential Backups
We have a full backup (Fx) running every Sunday
and differential backups (Diffxx) on Monday to Friday. All jobs starts
at 1 a.m. and finshes at 2 a.m. The jobs are as follows:-
| Week | Sun | Mon | Tue | Wed | Thu | Fri | Sat |
|---|---|---|---|---|---|---|---|
| 1 | F1 | DIff11 | DIff12 | DIff13 | DIff14 | DIff15 | |
| 2 | F2 | DIff21 | DIff22 | DIff23 | DIff24 | DIff25 | |
| 3 | F3 | DIff31 | DIff32 | DIff33 | DIff34 | DIff35 |
Like the incremental jobs case discussed earlier,
when job Diff12 runs, backup sets F1 and Diff11 are still not groomed
even though their overwrite protection has expired. This is because
backup set Diff11 is the last recovery point of the server and it is
dependent on backup set F1.
After job Diff12 has completed, backup set Diff12
is the last recovery point of the server and it is only dependent on
backup set F1. It is not dependent on backup set Diff11, so backup set
Diff11 can be groomed. Until the next groom cycle which can be as long
as 4 hous away, you would have backup sets F1, Diff11 and Diff12 on your
disk. When job Diff13 runs, you should only have backup sets F1 and
Diff12 on your disk.
When job F2 runs, backup sets F1 and Diff15 is
still on your disk because Diff15 is the last recovery point of the
server and it depends on backup set F1. After job F2 completes, backup
set F2 is now the last recovery point of the server and it is not
dependent on either backup sets F1 or Diff15, so they can be groomed.
Until the next grooming cycle, you would have backup sets F1, Diff15
and F2 on your disk.
In this case, the maximum disk space that you need to cater for is 2 full backups and the size of a Friday differential backup.DLM + A 2-Weeks Retention Period
Full + Incremental Backups
We have a full backup (Fx) running every Sunday
and incremental backups (Incrxx) on Monday to Friday. All jobs starts
at 1 a.m. and finshes at 2 a.m. The jobs are as follows:-
| Week | Sun | Mon | Tue | Wed | Thu | Fri | Sat |
|---|---|---|---|---|---|---|---|
| 1 | F1 | Incr11 | Incr12 | Incr13 | Incr14 | Incr15 | |
| 2 | F2 | Incr21 | Incr22 | Incr23 | Incr24 | Incr25 | |
| 3 | F3 | Incr31 | Incr32 | Incr33 | Incr34 | Incr35 |
In this scenario, you need to have enough disk space for 3 full backups and 15 incremental backups.
Full + Differential Backups
As before, we have a full backup (Fx) running
every Sunday and differential backups (Diffxx) on Monday to Friday. All
jobs starts at 1 a.m. and finshes at 2 a.m. The jobs are as follows:-
| Week | Sun | Mon | Tue | Wed | Thu | Fri | Sat |
|---|---|---|---|---|---|---|---|
| 1 | F1 | DIff11 | DIff12 | DIff13 | DIff14 | DIff15 | |
| 2 | F2 | DIff21 | DIff22 | DIff23 | DIff24 | DIff25 | |
| 3 | F3 | DIff31 | DIff32 | DIff33 | DIff34 | DIff35 |
As with the differential backups case discussed
earlier, once job F2 completes, backup set F2 is the last recovery point
of the server, the chain of backup sets from Week 1 are no longer
required under the DLM rule and can be groomed. However, these are
retained because they are still protected by the their retention
periods. Backup set Diff15 will only expire at 2 a.m. on the Friday of
Week 3. However, backup set Diff15 is only dependent on backup set F1.
Thus by Friday on Week 3, backup sets Diff11, Diff12, Diff13 and
Diff14 would have expired and would have been groomed.
In this scenario, there should be sufficient disk
space for hold the backup sets for Weeks 2 & 3, plus backup sets F1
and Diff15.
Is DLM an unnecessary complication?
You may think so if you got lost in some of my
scenarios above, but this is not so. On the contrary, DLM is another
layer of protection against setting inappropriate retention periods
which do not adequately protect a backup chain. For example, the
retention period for differential backup are set so short that by the
end of the week, the Monday's differential backup set has be overwritten
and the subsequent differential backups are rendered useless. DLM will
protect against this happening.
If you are trying to squeeze every bit of space on
your disk, then you have to sit down with a pen and paper and figure
out like above what backup sets are retain on your disk. Otherwise, get
a disk with lots of space.
It does not mean that in all cases, DLM needs more
disk space. In some situation, there is actually a savings. Take the
first case above (Full backup on Sunday and incremental backups on
Monday to Friday), as we have seen, with just DLM, the backup sets F1,
Incr11, Incr12, Incr13, Incr14 and Incr15 can be groomed after backup
job F2 runs and the disk space occupied by them would be freed. Without
DLM, we would have to use retention period to prevent them from being
overwritten. At the minimum, you would have to retain them for 1 week.
After backup job F2 runs, only backup set F1 can be overwritten.
Backup sets Incr11, Incr12, Incr13, Incr14, Incr15 would still be
protected and cannot be overwritten, although they are useless without
backup set F1.
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