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chore: remove weak hashing which does not pull its weight (#10005)

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We've had weak/rolling hashing in the code for quite a while. It was a
popular request for a while, based on the belief that rsync does this
and we should too. However, the benefit is quite small; we save on
average about 0.8% of transferred blocks over the population as a whole:

<img width="974" alt="Screenshot 2025-03-28 at 17 09 02"
src="https://github.com/user-attachments/assets/bbe10dea-f85e-4043-9823-7cef1220b4a2"
/>

This would be fine if the cost was comparably low, however the downside
of attempting rolling hash matching is that we (by default) do a
complete file read on the destination in order to look for matches
before we starting pulling blocks for the file. For any larger file this
means a sometimes long, I/O-intensive pause before the file starts
syncing, for usually no benefit.

I propose we simply rip off the bandaid and save the effort.
This commit is contained in:
Jakob Borg
2025-03-29 13:21:10 +01:00
committed by GitHub
parent 1a25ae32ca
commit b1c8f88a44
38 changed files with 288 additions and 1030 deletions
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lib/model/model_test.go
+4 -4
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@@ -222,7 +222,7 @@ func BenchmarkRequestOut(b *testing.B) {
b.ResetTimer()
for i := 0; i < b.N; i++ {
data, err := m.RequestGlobal(context.Background(), device1, "default", files[i%n].Name, 0, 0, 32, nil, 0, false)
data, err := m.RequestGlobal(context.Background(), device1, "default", files[i%n].Name, 0, 0, 32, nil, false)
if err != nil {
b.Error(err)
}
@@ -2819,9 +2819,9 @@ func TestIssue5002(t *testing.T) {
}
blockSize := int32(file.BlockSize())
m.recheckFile(protocol.LocalDeviceID, "default", "foo", file.Size-int64(blockSize), []byte{1, 2, 3, 4}, 0)
m.recheckFile(protocol.LocalDeviceID, "default", "foo", file.Size, []byte{1, 2, 3, 4}, 0) // panic
m.recheckFile(protocol.LocalDeviceID, "default", "foo", file.Size+int64(blockSize), []byte{1, 2, 3, 4}, 0)
m.recheckFile(protocol.LocalDeviceID, "default", "foo", file.Size-int64(blockSize), []byte{1, 2, 3, 4})
m.recheckFile(protocol.LocalDeviceID, "default", "foo", file.Size, []byte{1, 2, 3, 4}) // panic
m.recheckFile(protocol.LocalDeviceID, "default", "foo", file.Size+int64(blockSize), []byte{1, 2, 3, 4})
}
func TestParentOfUnignored(t *testing.T) {