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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 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?"

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Defaults are almost always good. –  Mike Christiansen Oct 2 '12 at 6:31

5 Answers 5

up vote 14 down vote accepted

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 NFTS. If you have lots of small files, then it's a good idea to keep the allocation size small 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 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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+1, also several programs like scandisk or defrag utilities have trouble with non standard cluster sizes. –  hyperslug Aug 28 '09 at 22:23
    
I ended up going with the default (thanks for the advice hyperslug). I figure that any small performance gains I might get from tweaking the allocation unit size would be dwarfed by the USB transfer speed. –  Andrew Keeton Aug 29 '09 at 17:11

In terms of space efficiency, smaller 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 small 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 your photos, music and videos are stored (files typically 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:
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Never defrag a SSD –  Andrew Dec 30 '13 at 16:13
    
To clarify, i was referring to if file defragmentation and access times slow down with ssd. –  sunk818 Dec 31 '13 at 18:03
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Some discussion here: superuser.com/questions/97071/do-ssds-suffer-from-fragmentation –  Andrew Jan 1 at 14:29

I am setting up four Seagate ST3000DM001 3TB drives in a RAID 10 configuration. After reading up on how to format it for the Allocation Unit Size, I found this article: General Guidelines for Improving Operating System Performance.

When configuring drive arrays and logical drives within your hardware drive controller, ensure you match the controller stripe size with the allocation unit size that the volumes will be formatted with. This will ensure disk read and write performance is optimal and offer better overall server performance.

I am using the ASUS P8Z77-V DELUXE motherboard's built-in RAID capability from Intel chipset. Since the Default Stripe size for RAID 10 on the motherboard is 64KB, I am formatting my RAID array using the "64 kilobytes" option in Windows.

Anyway, I hope this information I found helps for other people researching for this topic and add more information to it.

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It can make a difference.

A file can only be a multiple of whatever you select, so a finer grained format will mean you will waste less space on files - but make your allocation table bigger.

If you're going to have thousands of files under 64k it's worth considering a lower size unit. However, if you're going to fill the drive with huge, hundred megabyte videos, then a higher unit size is more applicable.

If you don't know, just go with the default.

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Just be aware the larger cluster size you use, the better performance you will have. But the efficiency decreases.

If you have a 3TB disk and store thousands of pictures, you would want a smaller cluster size for space efficiency, but if you make it too small you will take a severe performance hit.

For any disk 500gb or bigger, I recommend 8k cluster size or larger.

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