You should first understand what Allocation Unit Size (AUS) means. It is the smallest data block on the disk. Your actual data will be seperated to those units 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 still be saved in 4 units in the disk because as mentioned above 128KB is the smallest size of an allocation unit. 384KB will be allocated in 3 units and remaining 116KB will be allocated in another unit. You can observe this behaviour on file properties screen on Windows, what is your file size and how much space this file actually covers on the disk.
Best Answer: First of all DO NOT format your hard drive if you have data on it you need. You WILL loose all data on it. First of all I would back that data up somewhere else. Then when you connect it to your computer I would right click on it, select tools then click check drive for errors and select auto fix any errors.
And the operating system reads only that AUSd much data at a low level disk read operation. Those being said, using 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, last AU not being used fully. But, here's the trade-off, using large AUS, significantly again, improves the disk reading performance.
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The O.S can read more data at one read. Imagine, O.S makes 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, higher AUS will give you appreciable read performance 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. There are no significant storage overhead at high levels. Besides there is enough hrdw overhead since the actual physical sector size is 512Bytes. There is a part of file system formatting that records the cluster information, from how many sector this cluster is created, to the partition structure. The sector size emulation is a job of disk driver.
File system server should deal with logical organization (NTFS, FAT etc) at high level O.S ops, smallest unit reads/writes at low level O.S ops and disk driver itself must work back to back with controller(hardware) for low level hardware. – Apr 27 '12 at 3:33. N sized units, M number units, N.M capacity disk, 'what is the probability of hitting this unit?' And remember, disk has to be more precise in locating the beginnings of the units.
So, Random access performance is something bound with M^2/N. 4K units, 8 units, 32K capacity disk.
R.A bound with 64/4. 8K units, 4 units, same capacity, same disk.
R.A becomes 16/8. You wouldn't find an article about this kind of calculation, but believe me:) It is more job to 'randomly' locate a data using large unit sizes over small sizes – Apr 27 '12 at 3:50. 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. Default cluster sizes for NTFS The following table describes the default cluster sizes for NTFS.