Is it best to reformat the hard drive to exFAT using 512kb chunk, or smaller or bigger chunks?

I can reformat a brand new 2TB WD Passport drive to exFAT, with choice of many "Allocation Unit Size":

128kb 256kb 512kb 1024kb 4096kb 16384kb 32768kb 

which one is best if this drive is mainly used for recording HDTV programs using Media Center on Windows 7? thanks.

This is related to question: Is it best to reformat our external Hard drive to exFAT for compatibility with Mac?

5 Answers

You should first understand what

Allocation Unit Size (AUS)

means.

It is the smallest data block on the disk. Your actual data will be separated into units of that size while saving to the disk. For example, if you have a file sized 512KB and you have 128KB allocation unit size, your file will be saved in 4 units in the disk (512KB/128KB).

If your file's size is 500KB and you have 128KB AUS, your file will still be saved in 4 units on the disk because as mentioned above 128KB is the smallest size of an allocation unit. 384KB will be allocated in 3 units, the remaining 116KB will be allocated in a final unit, and 12KB of that unit will be empty. You can observe this behaviour on the file properties dialog on Windows; what your file size is and how much space this file actually covers on the disk are two different concepts. The operating system reads only the allocation unit size worth of data at a low level disk read operation.

That being said, using a large AUS significantly reduces the free space utilization due to not using the last allocation unit completely. And as a side effect, the number of files to store on the disk is reduced due to same problem: the last AU not being used fully. But here's the trade-off: using a large AUS significantly improves the disk reading performance. The O.S. can read more data at one read. Imagine if the O.S. need to make only a couple of disk reads to completely read a GB sized file!

Using small AUS improves the free space utilization but reduces the disk read performance. Think using large AUS in reverse, same category problems and improvements, but in reverse...

So, what is the conclusion here? If you will store large (I mean large!) files on the disk, a higher AUS will give an appreciable read performance boost while reducing the file count and free space

Which AUS you should use? This depends on how much your average file size is. Also you can compute the free space utilization according to your file sizes.

Given that HD recordings are large files, a large allocation unit (16384 or 32768 KB) will give better performance. The impact of slack space (space wasted due to allocation units not used fully--files are stored in allocation units which must be used as whole units) will be limited with a small number of files. On the other hand, if you have many smaller files, use a smaller allocation unit to reduce wasted space.

You can safely use 4K allocation unit for exFAT. Even if you have thousands of small files you won't waste a lot of space. In case of default 128KB allocation unit for e.g. 64GB usb stick, 1024 files of 4K bytes will occupy 128MB instead of 4MB, since every file requires at least one allocation unit.

If you use your disk mostly for audio and video files use a larger allocation unit.

FAT32 is not an option for disks larger than 32GB so choose whatever Windows allows.

Basically, the larger the files you intend on keeping the larger each allocation unit size you may want in use - but not too big or too small! I think DragonLord explained it pretty well.

So if wasted space bugs you then maybe you might want to think about using a different file system. Something like EXT4 perhaps. Problem there is Microsoft OS's (Windows, really) don't work too well with anything other than FAT (vFAT, FAT32, etc.) or NTFS. And if you ever end up with files larger than 4Gig you may end up cursing any FAT type system you may be using. Therefore, I would recommend using the NTFS file system with the recommended allocation unit size (I believe that's 4K). That way, if you end up with files larger than 4Gig you will still be able to store your monster files at least until you can break them up or transcode them into something smaller. (I assume we're talking about huge multimedia files which is why I bring up "transcoding" since I seem to always find ways to make files smaller when I transcode, especially if they were recorded using MCE.)

About the only reason I can see for using FAT (vFAT, FAT32, FAT16, etc.) is so that other operating systems can read/write files on the storage device. FAT is about as universally accepted as it gets. Otherwise, I don't recommend using FAT (unless the device's capacity is 4Gig or less) - use NTFS at least for Windows. You can always make another partition with a different file system even if it's on the same physical drive. Hope it helps.

As Wikipedia says:

To provide improvement in the allocation of cluster storage for a new file, Microsoft incorporated a method to pre-allocate contiguous clusters and bypass the use of updating the FAT table.

So basically you could choose 4KB or smaller allocation unit with exFAT and be safe when writing bigger files, like HD video material.