forked from TrueCloudLab/restic
5d4568d393
This writes files by using (*os.File).Truncate, which resolves to the
truncate system call on Unix.
Compared to the naive loop,
for _, b := range p {
if b != 0 {
return false
}
}
the optimized allZero is about 10× faster:
name old time/op new time/op delta
AllZero-8 1.09ms ± 1% 0.09ms ± 1% -92.10% (p=0.000 n=10+10)
name old speed new speed delta
AllZero-8 3.84GB/s ± 1% 48.59GB/s ± 1% +1166.51% (p=0.000 n=10+10)
60 lines
1.3 KiB
Go
60 lines
1.3 KiB
Go
//go:build !windows
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// +build !windows
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package restorer
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import "bytes"
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// WriteAt writes p to f.File at offset. It tries to do a sparse write
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// and updates f.size.
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func (f *partialFile) WriteAt(p []byte, offset int64) (n int, err error) {
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n = len(p)
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end := offset + int64(n)
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// Skip the longest all-zero prefix of p.
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// If it's long enough, we can punch a hole in the file.
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skipped := zeroPrefixLen(p)
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p = p[skipped:]
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offset += int64(skipped)
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switch {
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case len(p) == 0 && end > f.size:
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// We need to do a Truncate, as WriteAt with length-0 input
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// doesn't actually extend the file.
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err = f.Truncate(end)
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if err != nil {
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return 0, err
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}
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case len(p) == 0:
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// All zeros, file already big enough. A previous WriteAt or
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// Truncate will have produced the zeros in f.File.
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default:
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n, err = f.File.WriteAt(p, offset)
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}
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end = offset + int64(n)
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if end > f.size {
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f.size = end
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}
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return n, err
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}
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// zeroPrefixLen returns the length of the longest all-zero prefix of p.
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func zeroPrefixLen(p []byte) (n int) {
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// First skip 1kB-sized blocks, for speed.
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var zeros [1024]byte
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for len(p) >= len(zeros) && bytes.Equal(p[:len(zeros)], zeros[:]) {
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p = p[len(zeros):]
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n += len(zeros)
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}
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for len(p) > 0 && p[0] == 0 {
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p = p[1:]
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n++
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}
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return n
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}
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