In addition to asking for the file system you’d like to use, disk formatting tools will also ask for an “Allocation unit size”. What does this mean and what value should you select?

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問題

超級使用者讀者Andrew Keeton很好奇，格式化驅動器時，他到底應該在分配部分放些什麼。他寫道：

I’m formatting a 1TB external hard drive as NTFS. This drive is mainly meant for storing media such as music and video.

What should I choose for the allocation unit size setting? The opti*** range from 512 bytes to 64K. Are there any guidelines that I might apply to other drive types? Should I stop poking around and just leave it at “default?”

雖然預設設定通常是大多數使用者的最佳選擇，但讓我們深入研究一下。

答案

超級使用者貢獻者Jonathan和Andrew提供了一些見解。喬納森寫道：

If you are a “Standard User” by Microsoft’s definition, you should keep the default 4096 bytes. Basically, the allocation unit size is the block size on your hard drive when it formats NTFS. If you have lots of **all files, then it’s a good idea to keep the allocation size **all so your harddrive space won’t be wasted. If you have lots of large files, keeping it higher will increase the system performance by having less blocks to seek.

But again, nowadays hard drive capacity is getting higher and higher it makes **all difference by choosing the right allocation size. Suggest you just keep the default.

Also keep in mind that the majority file are relatively **all, larger files are large in size but **all in units.

安德魯進一步闡述了喬納森的回答：

In terms of space efficiency, **aller allocation unit sizes perform better. The average space wasted per file will be half the chosen AUS. So 4K wastes 2K per file and 64K wastes 32K. However, as Jonathon points out, modern drives are massive and a little wasted space is not worth fussing over and this shouldn’t be a determining factor (unless you are on a **all SSD).

Compare 4K vs 64K average case waste (32K-2K = 30K), for 10,000 files that only comes out to 300,000KB or around 300MB.

Instead think about how the OS uses space. Let’s say you have a 3K file which needs to grow 2K. With a 4K AUS the data needs to be split over two blocks – and they may not be together so you get fragmentation. With a 64K AUS there are a lot fewer blocks to keep track of and less fragmentation. 16x the block size means 1/16th the number of blocks to keep track of.

For a media disk where you photos, music and videos are stored, every file is at least 1MB I use the biggest AUS. For a windows boot partition I use the Windows default (which is 4K for any NTFS drive **aller than 16TB).

To find out what the cluster size is on an existing disk:

fsutil fsinfo ntfsinfo X:

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