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

Question

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?

Answer 1

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.

Answer 2

For filesystems with tons of small files, use 8 KiB cluster sizes. For filesystems with only large files, like media, use 128 KiB clusters. If not sure, use 8 KiB cluster sizes. There is negligible speed improvement for clusters larger than 128 KiB (see the top-left plot in the group of plots below), but the possibility for huuuuuge disk usage if you go to larger clusters.

Example: for my case, with tons of small files (just over 1 million files, comprising 74 GB):

1. 8 KiB cluster size --> 82 GB of storage spaced used up for my 74 GB of data.
   1. 338 MB write speed
2. 128 KiB cluster size --> 194 GB of storage space used up for my same 74 GB of data
   1. 390 MB write speed
3. 32 MiB cluster size --> 32768 GB (32.8 TB) (nope! not an error) of storage space used up for my 74 GB of data.
   1. 428 MB write speed

Microsoft's default values for cluster size max out at 128 KiB too. See the table at the end of my answer, from Microsoft.

Study these plots I meticulously made. The top-left plot trends apply regardless of how many small files you have, as I did those tests by rsyncing over a 5.3 GB file to the exFAT external SSD, but the other three plots are exacerbated by the quantity of tiny files I have. The log-linear trend in the bottom-right applies to everyone, but its slope, and therefore the y-axis values, depends on how many small files you have.

I have a full writeup on it on my website here if interested: https://gabrielstaples.com/exfat-clusters/

Full Python matplotlib/numpy plotting code is here: https://github.com/ElectricRCAircraftGuy/eRCaGuy_hello_world/blob/master/stack_exchange/format_exFAT_PLOTS.py

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I'm not answering the OPs question about "recording HDTV programs". I'm answering for people with lots of small files, including doing a full disk backup, as they will certainly land on this page too.

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I'd choose the smallest allocation unit possible if you have a lot of small files. This avoids wasted space for tiny files. Ex: use 4 KiB allocation unit size on exFAT instead of 128 KiB.

I just backed up 74 GB of data from an Apple APFS filesystem onto an external SSD with an exFAT filesystem with 128 KB allocation unit sizes (the default apparently when formatting to exFAT using Gnome Disks in Linux Ubuntu), and on the external drive, the formerly-74 GB of data on the APFS filesystem now takes up a whopping 194 GB on the exFAT filesystem! That's nuts! That's 2.62x more space taken, for nothing!

It's because I have thousands and thousands of tiny files that are, for instance, only a couple hundred bytes. On the old drive with the APFS filesystem, those would take up a single 512 byte cluster, and on the new external drive with the exFAT filesystem, those same 90 byte files take up a whopping 128 KB cluster, which is 128KiB * 1024 bytes/KiB / 512 bytes = 256 times larger storage space required on the exFAT drive for tiny files. Reducing that 128 KiB allocation unit to only 4 KiB, for instance, would take up 128/4 = 32x less space!

See also

1. My answer: Unix & Linux: Create and format exFAT partition from Linux - I show the commands for both Linux Ubuntu 20.04 with mkexfatfs, and Linux Ubuntu 22.04 with mkfs.exfat.

2. My answer on how to determine the cluster size of any filesystem you have: Server Fault: How to find the cluster size of any filesystem, whether NTFS, Apple APFS, ext4, ext3, FAT, exFAT, etc.

3. [REALLY USEFUL] Support.Microsoft.com: Default cluster size for NTFS, FAT, and exFAT:

   Default cluster sizes for exFAT

   The following table describes the default cluster sizes for exFAT.

   Volume size	Windows 7, Windows Server 2008 R2, Windows Server 2008, Windows Vista, Windows Server 2003, Windows XP
   7 MB–256 MB	4 KB
   256 MB–32 GB	32 KB
   32 GB–256 TB	128 KB
   > 256 TB	Not supported

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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.