# Maximize number of backups that fit on backup drive

This is an intrinsically "practical" question, but it leads to a well-defined mathematical problem. Let me start with the practical part:

I regularly back up my data. My backup strategy are differential backups, where the first backup is a full backup containing all files and subsequent backups are either full backups or differential backups, containing only data that has been added/changed since the last full backup.1

My backup storage is limited to $$C$$. I use the term "snapshot" to refer to the data from either a full backup or a differential backup.

Question: After how many differential backups should I make a new full backup to maximize the number of snapshots I can squeeze in the storage capacity $$C$$?

Intuition: A full backup takes up more space than a differential backup, but the more data has been added since the last full backup, the larger becomes each additional differential backup. Therefore, there is a trade-off between an additional large full backup vs. many larger differential backups.

I tried to formalize my problem as follows:

Denote data that is newly added between $$t-1$$ and $$t$$ as $$s(t)$$ and let $$s(0) = d_0$$ (some constant, size of initial data). Ignore deletes, i.e. total data at time $$\tilde{t}$$ is $$f(\tilde{t})=\sum_{t=0}^{\tilde{t}} s(t)$$, which equals the size of a full backup at this time. The size of a differential backup ($$d$$) at time $$\tilde{t}$$ depends on the time of the last full backup $$b$$: $$d(\tilde{t},b) = \sum_{t=b+1}^{\tilde{t}} s(t)$$.

With 1 full backup and only differential backups afterwards, I am not running out of storage capacity $$C$$ as long as $$T$$ is small enough2 such that $$f(0) + \sum_{\tilde{t}=1}^T d(\tilde{t}, 0) \leq C.$$

With 2 full backups, each followed by differential backups, $$T_1$$ must be small enough such that $$f(0) + \sum_{\tilde{t}=1}^{T_0} \Big[ d(\tilde{t}, 0) \Big] + f(T_0+1) + \sum_{\tilde{t}=T_0+2}^{T_1} d(\tilde{t}, T_0+1) \leq C.$$

I think the pattern becomes clear now:3 with $$N$$ full backups, $$T_{N-1}$$ must be small enough such that $$f(0) + \sum_{\tilde{t}=1}^{T_0} \Big[ d(\tilde{t}, 0) \Big] + \sum_{n=1}^{N-1} \Big[ f(T_{n-1}+1) + \sum_{\tilde{t}=T_{n-1}+2}^{T_n} d(\tilde{t}, T_{n-1}+1) \Big] \leq C. \tag{1} \label{eq:cond}$$

This leads to the ultimate question:

Which $$N$$ maximizes the largest $$T_{N-1}$$ for which equation $$(\ref{eq:cond})$$ holds?

I am not sure how to approach a solution to this problem, nor am I sure if further assumptions are needed to make progress. Maybe it is helpful to assume that the amount of new data in each period $$s(t)$$ is constant ($$s(t) = \bar{s} ~ \forall ~ t$$)?

Plugging my definitions of $$f(\tilde{t})$$ and $$d(\tilde{t}, b)$$ into equation $$(\ref{eq:cond})$$ yields: $$d_0 + \Big[ \sum_{\tilde{t}=1}^{T_0} \sum_{t=1}^{\tilde{t}} s(t) \Big] + \sum_{n=1}^{N-1} \Bigg[ \Big[ \sum_{t=0}^{T_{n-1}+1} s(t) \Big] + \Big[ \sum_{\tilde{t}=T_{n-1}+2}^{T_n} \sum_{t=T_{n-1}+2}^{\tilde{t}} s(t) \Big] \Bigg] \leq C, \tag{1'} \label{eq:cond-plugged-in}$$ but I do not know how to proceed from here.

Any help, either on tackling equation $$(\ref{eq:cond-plugged-in})$$ or completely different strategies to solve the problem are highly appreciated!

1 Note that I am not referring to incremental backups, where each backup contains only the data since the previous incremental backup. Differential backups always contain all data since the last full backup.

2 I assume that $$C$$ is large enough for at least 1 full and 1 differential backup.

3 To be more explicit, the conditions for 3 and 4 full backups are $$f(0) + \sum_{\tilde{t}=1}^{T_0} d(\tilde{t}, 0) + f(T_0+1) + \sum_{\tilde{t}=T_0+2}^{T_1} d(\tilde{t}, T_0+1) + f(T_1+1) + \sum_{\tilde{t}=T_1+2}^{T_2} d(\tilde{t}, T_1+1) \leq C,$$

$$f(0) + \sum_{\tilde{t}=1}^{T_0} d(\tilde{t}, 0) + f(T_0+1) + \sum_{\tilde{t}=T_0+2}^{T_1} d(\tilde{t}, T_0+1) + f(T_1+1) + \sum_{\tilde{t}=T_1+2}^{T_2} d(\tilde{t}, T_1+1) + f(T_2+1) + \sum_{\tilde{t}=T_2+2}^{T_3} d(\tilde{t}, T_2+1) \leq C.$$

• This is a crosspost of this math.SE question; see OR chat.
– CL.
Mar 7 at 11:03
• What kind of answer are you looking for? A closed form expression? An algorithm solving your problem? (In which case dynamic programming gives an easy pseudo-polynomial time solution) Mar 7 at 19:41
• The assertion that $f(\tilde{t})$ is the size of a full backup at time $\tilde{t}$ ignores not only deletions but also overwriting of old data with updated data. It is correct that a full backup will use no more than $f(\tilde{t})$ space. Mar 7 at 21:49
• After making a full backup, would you not delete all previous backups (full and differential)? Mar 7 at 21:54
• @Tassle Actually, either would be great. I didn't expect a closed-form solution to be possible at all, but seeing one would be fascinating. However, an algorithm would also help - anything that tackles the underlying practical problem is highly appreciated.
– CL.
Mar 8 at 8:02

Here is an easy dynamic programming solution which runs in $$O(n^2)$$ time, where $$n$$ is the number of updates to the data.

Create an array of partial sums $$p$$ of size $$n$$, such that $$p[j]=\sum_{i=0}^{j}s(i)$$ for all $$0\leq j \leq n-1$$.

For $$0\leq k \leq t \leq n$$ let $$m[k][t]$$ denote the minimum amount of storage required to store the first $$k+1$$ snapshots, knowing that the last full backup happened at snapshot $$t$$.

We have:

• $$m = s(0)$$
• $$m[i][i] = \min_{0\leq t \leq i-1}\{m[i-1][t]\}+p[i]$$ for all $$0
• $$m[i][t] = m[i-1][t] + (p[i]-p[t])$$ for all $$0\leq t

These relations let you compute the whole matrix $$m$$ in $$O(n^2)$$ time. If you then let $$m'[i] = \min_{0\leq t \leq i}\{m[i][t]\}$$ denote the minimum amount of storage required to store the first $$i+1$$ snapshots for all $$0\leq i \leq n-1$$, the maximum number of snapshots you can store is one more than the maximum $$i$$ such that $$m'[i]\leq C$$.

Note that you can also compute $$m'$$ as you are computing $$m$$ and stop whenever you find that $$m'[i] > C$$. You can also easily adapt this approach to handle deletes.

Now to actually find out when you should do a full backup and when you should do a differential backup, there are two main approaches (essentially doing the same thing). One involves storing some additional information while computing $$m'$$, the other a sort of backtracking to figure out which choices led to the values of $$m$$ and $$m'$$ we end up with.

I prefer the former, and it goes like this:

• Let $$q[i]$$ denote the last time before $$i$$ we have to do a full backup in order to do store the first $$i+1$$ backups using $$m'[i]$$ storage. In other words, $$q[i]=t$$, where $$0\leq t \leq i$$ is the value which minimizes $$m[i][t]$$. You can compute $$q$$ while computing $$m'$$.
• To store the $$i+1$$ first snapshots with the least amount of storage, you need to do your last full backup at time $$t=q[i]$$ and minimize the storage required for the first $$t$$ snapshots. To minimize the storage of the first $$t$$ snapshots you need to do your last full backup at time $$t'=q[t-1]$$ and minimize the storage required for the first $$t'$$ snapshots. To minimize the storage of the first $$t'$$ snapshots you need to do your last full backup at time $$t''=q[t'-1]$$... And so on until you reach $$0$$, where you have no choice but to do a full backup.

The values $$t,t',t'',t'''\ldots$$ you will reach in this process tell you exactly when you have to do a full backup in order to store the first $$i+1$$ snapshots using $$m'[i]$$ storage (which is the minimum possible).

• Thanks, that looks great! I'll look into this as soon as possible.
– CL.
Mar 9 at 8:51
• Thank you again for your answer! I now found the time to … try to understand it. There's one thing that I cannot get my mind around: once I know the maximum number of snapshots that can be stored, how do I know what frequency of full backups is needed to reach this maximum? Is this something I can see from $m$? Sorry for my naiveté; I'm new to OR.
– CL.
Mar 14 at 17:23
• By the way, I tried to implement the algorithm suggested in this answer in R and my current approach is gist.github.com/ClaudiusL/ed53fc4fd55c1b8dfa4212bd699625bb
– CL.
Mar 14 at 17:26
• @CL. You are welcome! I have edited my answer to explain how to do that. Note that none of this is specific to OR, dynamic programming is a widely used technique in algorithms design generally. I encourage you to google the subject if you are interested in algorithms! The general principle is very simple once the first difficulties are overcome, and it is very useful. Mar 14 at 22:03
• Thanks a lot for your effort (and patience), Tassle!
– CL.
Mar 28 at 19:41