Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.
The Question
SuperUser reader Andrew Keeton is curious about what exactly he’s supposed to put in the allocation section when formatting a drive. He writes:
While the default setting is usually the best choice for most users, let’s dig a little deeper.
What should I choose for the allocation unit size setting? The options 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?”
The Answers
SuperUser contributors Jonathan and Andrew offer some insight. Jonathan writes:
Andrew expands upon Jonathan’s answer with:
But again, nowadays hard drive capacity is getting higher and higher it makes small difference by choosing the right allocation size. Suggest you just keep the default.
Also keep in mind that the majority file are relatively small, larger files are large in size but small in units.
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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 smaller than 16TB).
To find out what the cluster size is on an existing disk:
fsutil fsinfo ntfsinfo X:
