Line data Source code
1 : use crate::{SegmentMethod, SegmentSizeResult, SizeResult, StorageModel};
2 :
3 : //
4 : // *-g--*---D--->
5 : // /
6 : // /
7 : // / *---b----*-B--->
8 : // / /
9 : // / /
10 : // -----*--e---*-----f----* C
11 : // E \
12 : // \
13 : // *--a---*---A-->
14 : //
15 : // If A and B need to be retained, is it cheaper to store
16 : // snapshot at C+a+b, or snapshots at A and B ?
17 : //
18 : // If D also needs to be retained, which is cheaper:
19 : //
20 : // 1. E+g+e+f+a+b
21 : // 2. D+C+a+b
22 : // 3. D+A+B
23 :
24 : /// `Segment` which has had its size calculated.
25 : #[derive(Clone, Debug)]
26 : struct SegmentSize {
27 : method: SegmentMethod,
28 :
29 : // calculated size of this subtree, using this method
30 : accum_size: u64,
31 :
32 : seg_id: usize,
33 : children: Vec<SegmentSize>,
34 : }
35 :
36 : struct SizeAlternatives {
37 : /// cheapest alternative if parent is available.
38 : incremental: SegmentSize,
39 :
40 : /// cheapest alternative if parent node is not available
41 : non_incremental: Option<SegmentSize>,
42 : }
43 :
44 : impl StorageModel {
45 10 : pub fn calculate(&self) -> SizeResult {
46 10 : // Build adjacency list. 'child_list' is indexed by segment id. Each entry
47 10 : // contains a list of all child segments of the segment.
48 10 : let mut roots: Vec<usize> = Vec::new();
49 10 : let mut child_list: Vec<Vec<usize>> = Vec::new();
50 10 : child_list.resize(self.segments.len(), Vec::new());
51 :
52 85 : for (seg_id, seg) in self.segments.iter().enumerate() {
53 85 : if let Some(parent_id) = seg.parent {
54 75 : child_list[parent_id].push(seg_id);
55 75 : } else {
56 10 : roots.push(seg_id);
57 10 : }
58 : }
59 :
60 10 : let mut segment_results = Vec::new();
61 10 : segment_results.resize(
62 10 : self.segments.len(),
63 10 : SegmentSizeResult {
64 10 : method: SegmentMethod::Skipped,
65 10 : accum_size: 0,
66 10 : },
67 10 : );
68 10 :
69 10 : let mut total_size = 0;
70 20 : for root in roots {
71 10 : if let Some(selected) = self.size_here(root, &child_list).non_incremental {
72 10 : StorageModel::fill_selected_sizes(&selected, &mut segment_results);
73 10 : total_size += selected.accum_size;
74 10 : } else {
75 0 : // Couldn't find any way to get this root. Error?
76 0 : }
77 : }
78 :
79 10 : SizeResult {
80 10 : total_size,
81 10 : segments: segment_results,
82 10 : }
83 10 : }
84 :
85 85 : fn fill_selected_sizes(selected: &SegmentSize, result: &mut Vec<SegmentSizeResult>) {
86 85 : result[selected.seg_id] = SegmentSizeResult {
87 85 : method: selected.method,
88 85 : accum_size: selected.accum_size,
89 85 : };
90 : // recurse to children
91 85 : for child in selected.children.iter() {
92 75 : StorageModel::fill_selected_sizes(child, result);
93 75 : }
94 85 : }
95 :
96 : //
97 : // This is the core of the sizing calculation.
98 : //
99 : // This is a recursive function, that for each Segment calculates the best way
100 : // to reach all the Segments that are marked as needed in this subtree, under two
101 : // different conditions:
102 : // a) when the parent of this segment is available (as a snaphot or through WAL), and
103 : // b) when the parent of this segment is not available.
104 : //
105 85 : fn size_here(&self, seg_id: usize, child_list: &Vec<Vec<usize>>) -> SizeAlternatives {
106 85 : let seg = &self.segments[seg_id];
107 85 : // First figure out the best way to get each child
108 85 : let mut children = Vec::new();
109 85 : for child_id in &child_list[seg_id] {
110 75 : children.push(self.size_here(*child_id, child_list))
111 : }
112 :
113 : // Method 1. If this node is not needed, we can skip it as long as we
114 : // take snapshots later in each sub-tree
115 85 : let snapshot_later = if !seg.needed {
116 62 : let mut snapshot_later = SegmentSize {
117 62 : seg_id,
118 62 : method: SegmentMethod::Skipped,
119 62 : accum_size: 0,
120 62 : children: Vec::new(),
121 62 : };
122 62 :
123 62 : let mut possible = true;
124 64 : for child in children.iter() {
125 64 : if let Some(non_incremental) = &child.non_incremental {
126 46 : snapshot_later.accum_size += non_incremental.accum_size;
127 46 : snapshot_later.children.push(non_incremental.clone())
128 : } else {
129 18 : possible = false;
130 18 : break;
131 : }
132 : }
133 62 : if possible {
134 44 : Some(snapshot_later)
135 : } else {
136 18 : None
137 : }
138 : } else {
139 23 : None
140 : };
141 :
142 : // Method 2. Get a snapshot here. This assumed to be possible, if the 'size' of
143 : // this Segment was given.
144 85 : let snapshot_here = if !seg.needed || seg.parent.is_none() {
145 62 : if let Some(snapshot_size) = seg.size {
146 54 : let mut snapshot_here = SegmentSize {
147 54 : seg_id,
148 54 : method: SegmentMethod::SnapshotHere,
149 54 : accum_size: snapshot_size,
150 54 : children: Vec::new(),
151 54 : };
152 63 : for child in children.iter() {
153 63 : snapshot_here.accum_size += child.incremental.accum_size;
154 63 : snapshot_here.children.push(child.incremental.clone())
155 : }
156 54 : Some(snapshot_here)
157 : } else {
158 8 : None
159 : }
160 : } else {
161 23 : None
162 : };
163 :
164 : // Method 3. Use WAL to get here from parent
165 85 : let wal_here = {
166 85 : let mut wal_here = SegmentSize {
167 85 : seg_id,
168 85 : method: SegmentMethod::Wal,
169 85 : accum_size: if let Some(parent_id) = seg.parent {
170 75 : seg.lsn - self.segments[parent_id].lsn
171 : } else {
172 10 : 0
173 : },
174 85 : children: Vec::new(),
175 : };
176 160 : for child in children {
177 75 : wal_here.accum_size += child.incremental.accum_size;
178 75 : wal_here.children.push(child.incremental)
179 : }
180 85 : wal_here
181 85 : };
182 85 :
183 85 : // If the parent is not available, what's the cheapest method involving
184 85 : // a snapshot here or later?
185 85 : let mut cheapest_non_incremental: Option<SegmentSize> = None;
186 85 : if let Some(snapshot_here) = snapshot_here {
187 54 : cheapest_non_incremental = Some(snapshot_here);
188 54 : }
189 85 : if let Some(snapshot_later) = snapshot_later {
190 : // Use <=, to prefer skipping if the size is equal
191 44 : if let Some(parent) = &cheapest_non_incremental {
192 36 : if snapshot_later.accum_size <= parent.accum_size {
193 34 : cheapest_non_incremental = Some(snapshot_later);
194 34 : }
195 8 : } else {
196 8 : cheapest_non_incremental = Some(snapshot_later);
197 8 : }
198 41 : }
199 :
200 : // And what's the cheapest method, if the parent is available?
201 85 : let cheapest_incremental = if let Some(cheapest_non_incremental) = &cheapest_non_incremental
202 : {
203 : // Is it cheaper to use a snapshot here or later, anyway?
204 : // Use <, to prefer Wal over snapshot if the cost is the same
205 62 : if wal_here.accum_size < cheapest_non_incremental.accum_size {
206 39 : wal_here
207 : } else {
208 23 : cheapest_non_incremental.clone()
209 : }
210 : } else {
211 23 : wal_here
212 : };
213 :
214 85 : SizeAlternatives {
215 85 : incremental: cheapest_incremental,
216 85 : non_incremental: cheapest_non_incremental,
217 85 : }
218 85 : }
219 : }
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