How does parpar decide the number of recovery slices? | parpar 如何决定恢复切片数？

[DOSforever]

When generating recovery slices using a percentage, is the number of slices rounded instead of being rounded up?

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使用百分比指定生成恢复切片，切片数不是运算结果向上取整，而是四舍五入取整？

[animetosho]

--recovery-slices is round to nearest.

[DOSforever]

In most cases, this might be true, but sometimes it rounds up.
For example, if a file is 131,794,350 bytes long, with a slice size set to 131,072 bytes and a redundancy of 1%:

131,794,350 / 131,072 = 1005.5110931396, so the entire file is divided into 1006 slices.
Number of recovery slices: 1006 * 0.01 = 10.06. If rounded normally, it should be 10, but in reality, ParPar produces 11.

Input data: 125.69 MiB (1006 slices from 1 file)
Recovery data: 1408 KiB (11 * 128 KiB slices)
Input pass(es): 1, processing 11 * 128 KiB chunks per pass

The actual recovery slice files generated also include 11 slices:

*.vol00+01.par2
*.vol01+02.par2
*.vol03+04.par2
*.vol07+04.par2

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可能大部分情况是这样，但有时候为什么是向上取整。
比如有个文件长度为 131794350 字节，切片大小设为 131072 字节，冗余度为 1% 。

131794350 / 131072 = 1005.5110931396 ，整个文件被切分为 1006 个切片。
恢复切片数 ：1006 * 0.01 = 10.06 ，如果是四舍五入，应该是 10 ，可实际 ParPar 产生的却是 11 。

Input data : 125.69 MiB (1006 slices from 1 file)
Recovery data : 1408 KiB (11 * 128 KiB slices)
Input pass(es) : 1, processing 11 * 128 KiB chunks per pass

实际产生的恢复切片文件也是包含了11个切片

*.vol00+01.par2
*.vol01+02.par2
*.vol03+04.par2
*.vol07+04.par2

[animetosho]

Oh, you're right.

By default, --recovery-slices sets --min-recovery-slices to the same value, which is always rounded up.
If you set --min-recovery-slices to a lower value than --recovery-slices, the latter will be rounded to nearest.

Thanks for the correction!

[DOSforever]

Thank you for clarifying. It's hard to understand this just by looking at the built-in help, so it's best to have a separate instruction document. Also, may I ask why it was designed this way and what the idea behind this option design is?

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感谢解惑。如果只看自带的帮助说明很难知道是这么回事，所以最好有单独的说明文档。
另请问为何要这样设计，选项这样设计的思想是什么？

[animetosho]

If you specify a ratio of recovery, the assumption is that you want at least that amount. For example, if you want 5%, it's presumed that you want to be able to recover up to 5% damaged blocks. Round to nearest might give you 4%, for example, which wouldn't be enough.
If you specify a lower --min-recovery-slices, you're saying that you're fine with fewer recovery slices, so it'll try to match the --recovery-slices value more accurately.

There's a bit of a decision that has to be made about how much detail is put into the help file. It should describe the options without providing too much unnecessary detail.
I suspect the exact rounding behaviour isn't something most users care about, and those that do have a particular need will either do their own calculation and manually supply the exact number, or can easily just test it out (like you have).

[DOSforever]

If you specify a lower --min-recovery-slices, you're saying that you're fine with fewer recovery slices, so it'll try to match the --recovery-slices value more accurately.

Oh, it seems that the meaning of the --min-recovery-slices option is exactly the opposite of what I understood. I originally thought it was to ensure at least a minimum number of recovery slices. For example, sometimes the slice size is too large and the redundancy percentage is very small, resulting in a very small number of calculated recovery slices, sometimes only 1. So I set --min-recovery-slices to 2 to ensure at least 2 recovery slices. Unexpectedly, using this option actually causes the final number of recovery slices to lean toward lower values.

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哦，看来这个 --min-recovery-slices 选项的意义和我理解的刚好相反，我原以为是至少保证最小恢复切片数值，比如，有时候切片尺寸过大，而冗余百分比很小，最后计算出的恢复切片数很小，甚至只有1，所以我设定了 --min-recovery-slices 为2，保证恢复切片数至少2个。 没想到使用了这个选项反而是让最终生成的恢复切片数向低值靠拢。

[animetosho]

Unexpectedly, using this option actually causes the final number of recovery slices to lean toward lower values.

If you don't want that, you can just swap the two around, e.g. --min-recovery-slices 1% --recovery-slices 2

--min-recovery-slices rounds up
--recovery-slices rounds to nearest, noting that by default, it sets the minimum and thus will round up

(similarly, setting --min-recovery-slices 0 will cause --recovery-slices to round to nearest)