Model

What retained snapshots cost

Compaction raises the rewrite fraction, and retention multiplies it.

storage \approx size \times (1 + f R)

$f$: Rewrite fraction; share of the table rewritten per day
$R$: Retention; days of snapshots kept

Recurring principle: Retention is paid by the live path

How it behaves

One input on the horizontal axis, the others held fixed.

light rewriting (5%/day)aggressive compaction (30%/day)

Data table

Storage ÷ live table size, by retention
Retention R (days)	series	Storage ÷ table size
0	light rewriting (5%/day)	1×
0.375	light rewriting (5%/day)	1.02×
0.75	light rewriting (5%/day)	1.04×
1.13	light rewriting (5%/day)	1.06×
1.5	light rewriting (5%/day)	1.07×
1.88	light rewriting (5%/day)	1.09×
2.25	light rewriting (5%/day)	1.11×
2.63	light rewriting (5%/day)	1.13×
3	light rewriting (5%/day)	1.15×
3.38	light rewriting (5%/day)	1.17×
3.75	light rewriting (5%/day)	1.19×
4.13	light rewriting (5%/day)	1.21×
4.5	light rewriting (5%/day)	1.23×
4.88	light rewriting (5%/day)	1.24×
5.25	light rewriting (5%/day)	1.26×
5.63	light rewriting (5%/day)	1.28×
6	light rewriting (5%/day)	1.3×
6.38	light rewriting (5%/day)	1.32×
6.75	light rewriting (5%/day)	1.34×
7.13	light rewriting (5%/day)	1.36×
7.5	light rewriting (5%/day)	1.38×
7.88	light rewriting (5%/day)	1.39×
8.25	light rewriting (5%/day)	1.41×
8.63	light rewriting (5%/day)	1.43×
9	light rewriting (5%/day)	1.45×
9.38	light rewriting (5%/day)	1.47×
9.75	light rewriting (5%/day)	1.49×
10.1	light rewriting (5%/day)	1.51×
10.5	light rewriting (5%/day)	1.52×
10.9	light rewriting (5%/day)	1.54×
11.3	light rewriting (5%/day)	1.56×
11.6	light rewriting (5%/day)	1.58×
12	light rewriting (5%/day)	1.6×
12.4	light rewriting (5%/day)	1.62×
12.8	light rewriting (5%/day)	1.64×
13.1	light rewriting (5%/day)	1.66×
13.5	light rewriting (5%/day)	1.68×
13.9	light rewriting (5%/day)	1.69×
14.3	light rewriting (5%/day)	1.71×
14.6	light rewriting (5%/day)	1.73×
15	light rewriting (5%/day)	1.75×
15.4	light rewriting (5%/day)	1.77×
15.8	light rewriting (5%/day)	1.79×
16.1	light rewriting (5%/day)	1.81×
16.5	light rewriting (5%/day)	1.83×
16.9	light rewriting (5%/day)	1.84×
17.3	light rewriting (5%/day)	1.86×
17.6	light rewriting (5%/day)	1.88×
18	light rewriting (5%/day)	1.9×
18.4	light rewriting (5%/day)	1.92×
18.8	light rewriting (5%/day)	1.94×
19.1	light rewriting (5%/day)	1.96×
19.5	light rewriting (5%/day)	1.98×
19.9	light rewriting (5%/day)	1.99×
20.3	light rewriting (5%/day)	2.01×
20.6	light rewriting (5%/day)	2.03×
21	light rewriting (5%/day)	2.05×
21.4	light rewriting (5%/day)	2.07×
21.8	light rewriting (5%/day)	2.09×
22.1	light rewriting (5%/day)	2.11×
22.5	light rewriting (5%/day)	2.13×
22.9	light rewriting (5%/day)	2.14×
23.3	light rewriting (5%/day)	2.16×
23.6	light rewriting (5%/day)	2.18×
24	light rewriting (5%/day)	2.2×
24.4	light rewriting (5%/day)	2.22×
24.8	light rewriting (5%/day)	2.24×
25.1	light rewriting (5%/day)	2.26×
25.5	light rewriting (5%/day)	2.28×
25.9	light rewriting (5%/day)	2.29×
26.3	light rewriting (5%/day)	2.31×
26.6	light rewriting (5%/day)	2.33×
27	light rewriting (5%/day)	2.35×
27.4	light rewriting (5%/day)	2.37×
27.8	light rewriting (5%/day)	2.39×
28.1	light rewriting (5%/day)	2.41×
28.5	light rewriting (5%/day)	2.42×
28.9	light rewriting (5%/day)	2.44×
29.3	light rewriting (5%/day)	2.46×
29.6	light rewriting (5%/day)	2.48×
30	light rewriting (5%/day)	2.5×
0	aggressive compaction (30%/day)	1×
0.375	aggressive compaction (30%/day)	1.11×
0.75	aggressive compaction (30%/day)	1.23×
1.13	aggressive compaction (30%/day)	1.34×
1.5	aggressive compaction (30%/day)	1.45×
1.88	aggressive compaction (30%/day)	1.56×
2.25	aggressive compaction (30%/day)	1.67×
2.63	aggressive compaction (30%/day)	1.79×
3	aggressive compaction (30%/day)	1.9×
3.38	aggressive compaction (30%/day)	2.01×
3.75	aggressive compaction (30%/day)	2.13×
4.13	aggressive compaction (30%/day)	2.24×
4.5	aggressive compaction (30%/day)	2.35×
4.88	aggressive compaction (30%/day)	2.46×
5.25	aggressive compaction (30%/day)	2.58×
5.63	aggressive compaction (30%/day)	2.69×
6	aggressive compaction (30%/day)	2.8×
6.38	aggressive compaction (30%/day)	2.91×
6.75	aggressive compaction (30%/day)	3.02×
7.13	aggressive compaction (30%/day)	3.14×
7.5	aggressive compaction (30%/day)	3.25×
7.88	aggressive compaction (30%/day)	3.36×
8.25	aggressive compaction (30%/day)	3.48×
8.63	aggressive compaction (30%/day)	3.59×
9	aggressive compaction (30%/day)	3.7×
9.38	aggressive compaction (30%/day)	3.81×
9.75	aggressive compaction (30%/day)	3.92×
10.1	aggressive compaction (30%/day)	4.04×
10.5	aggressive compaction (30%/day)	4.15×
10.9	aggressive compaction (30%/day)	4.26×
11.3	aggressive compaction (30%/day)	4.38×
11.6	aggressive compaction (30%/day)	4.49×
12	aggressive compaction (30%/day)	4.6×
12.4	aggressive compaction (30%/day)	4.71×
12.8	aggressive compaction (30%/day)	4.82×
13.1	aggressive compaction (30%/day)	4.94×
13.5	aggressive compaction (30%/day)	5.05×
13.9	aggressive compaction (30%/day)	5.16×
14.3	aggressive compaction (30%/day)	5.27×
14.6	aggressive compaction (30%/day)	5.39×
15	aggressive compaction (30%/day)	5.5×
15.4	aggressive compaction (30%/day)	5.61×
15.8	aggressive compaction (30%/day)	5.72×
16.1	aggressive compaction (30%/day)	5.84×
16.5	aggressive compaction (30%/day)	5.95×
16.9	aggressive compaction (30%/day)	6.06×
17.3	aggressive compaction (30%/day)	6.17×
17.6	aggressive compaction (30%/day)	6.29×
18	aggressive compaction (30%/day)	6.4×
18.4	aggressive compaction (30%/day)	6.51×
18.8	aggressive compaction (30%/day)	6.63×
19.1	aggressive compaction (30%/day)	6.74×
19.5	aggressive compaction (30%/day)	6.85×
19.9	aggressive compaction (30%/day)	6.96×
20.3	aggressive compaction (30%/day)	7.08×
20.6	aggressive compaction (30%/day)	7.19×
21	aggressive compaction (30%/day)	7.3×
21.4	aggressive compaction (30%/day)	7.41×
21.8	aggressive compaction (30%/day)	7.52×
22.1	aggressive compaction (30%/day)	7.64×
22.5	aggressive compaction (30%/day)	7.75×
22.9	aggressive compaction (30%/day)	7.86×
23.3	aggressive compaction (30%/day)	7.97×
23.6	aggressive compaction (30%/day)	8.09×
24	aggressive compaction (30%/day)	8.2×
24.4	aggressive compaction (30%/day)	8.31×
24.8	aggressive compaction (30%/day)	8.43×
25.1	aggressive compaction (30%/day)	8.54×
25.5	aggressive compaction (30%/day)	8.65×
25.9	aggressive compaction (30%/day)	8.76×
26.3	aggressive compaction (30%/day)	8.88×
26.6	aggressive compaction (30%/day)	8.99×
27	aggressive compaction (30%/day)	9.1×
27.4	aggressive compaction (30%/day)	9.21×
27.8	aggressive compaction (30%/day)	9.32×
28.1	aggressive compaction (30%/day)	9.44×
28.5	aggressive compaction (30%/day)	9.55×
28.9	aggressive compaction (30%/day)	9.66×
29.3	aggressive compaction (30%/day)	9.78×
29.6	aggressive compaction (30%/day)	9.89×
30	aggressive compaction (30%/day)	10×

Worked examples, recomputed

Every number the mechanisms quote for this relationship is recomputed from it, so none of them disagrees with it.

Where it appears

Each mechanism states it in its own names. The general symbol each name stands for is shown with it.

Table & Storage Maintenance
Who pays for the gap between the order data arrives in and the order it is read in: the writer, every reader, or a background job, and what is the break-even?
$size \times (1 + f \cdot R)$
Read it in place →
Metadata, Catalog & Lineage
What can a system prove about its data without reading the data, and how does it know that proof is still true?
$extra = size \times f \times R$
Read it in place →