Help: Reformatting 2nd HDD (Allocation Unit Size?)

Deyvid · 03-20-2009, 12:12 AM

Hi

I have a few questions about reformatting a 2nd HDD which is in an external HDD enclosure.

So I have this 40gb HDD in the external enclosure. The HDD was previously used as a primary HDD in a different computer, and has WindowsXP on it. I took all the files I wanted off of it, and am ready to reformat it and use it a secondary HDD (in the external enclosure).

In Windows Explorer, I right-click on the drive letter (in this case drive J:) and click Format..., when the dialog comes up, Capacity shows 37.2GB, File System shows "NTFS (Default)", my only question is about the Allocation Unit Size, which shows "4096 bytes" by default when the dialog pops up.

The available options for the Allocation Unit Size are:

Default allocation size
512 bytes
1024 bytes
2048 bytes
4096 bytes (highlighted by default)
8192 bytes
16 kilobytes
32 kilobytes
64 kilobytes

Which option should I select for Allocation Unit Size?

What is Allocation Unit Size?

**raptor_pa** · 03-20-2009, 05:35 AM

Allocation unit size is the size of the clusters - a cluster is the smallest portion of a disk that Windows acan use to store a file. In other words in any given cluster, there can be information about one and only one file. If you look at the windows properties tab of a file, you will see two size values - size and size on disk. the size of the file will be how many bytes are actually the file. The size on diskis the total of bytes in all clusters occupied by that file. The difference between them is known as 'file slack' - basically dead space. So it sounds like the bet bet would be to pick the smallest allocation unit size, to save space. The downside is the smaller the allocation unit, the slower the performance. A good compromise between access speed and saving space seems to be the default 4096 alocation unit size.

Deyvid · 03-20-2009, 04:48 PM

Thanks for the info and explanation. That helped solve my dilemma about which size to choose.

I will go with the default 4096 bytes option.

03-20-2009, 12:12 AM	#1 (permalink)
Deyvid Registered User Join Date: Jun 2007 Location: California Posts: 60 OS: Windows Vista Home Basic (SP1) My System	Help: Reformatting 2nd HDD (Allocation Unit Size?) Hi I have a few questions about reformatting a 2nd HDD which is in an external HDD enclosure. So I have this 40gb HDD in the external enclosure. The HDD was previously used as a primary HDD in a different computer, and has WindowsXP on it. I took all the files I wanted off of it, and am ready to reformat it and use it a secondary HDD (in the external enclosure). In Windows Explorer, I right-click on the drive letter (in this case drive J:) and click Format..., when the dialog comes up, Capacity shows 37.2GB, File System shows "NTFS (Default)", my only question is about the Allocation Unit Size, which shows "4096 bytes" by default when the dialog pops up. The available options for the Allocation Unit Size are: Default allocation size 512 bytes 1024 bytes 2048 bytes 4096 bytes (highlighted by default) 8192 bytes 16 kilobytes 32 kilobytes 64 kilobytes Which option should I select for Allocation Unit Size? What is Allocation Unit Size?

03-20-2009, 05:35 AM	#2 (permalink)
raptor_pa Mod Hardware Team Join Date: Dec 2008 Location: Central PA Posts: 4,915 OS: XP	Re: Help: Reformatting 2nd HDD (Allocation Unit Size?) Allocation unit size is the size of the clusters - a cluster is the smallest portion of a disk that Windows acan use to store a file. In other words in any given cluster, there can be information about one and only one file. If you look at the windows properties tab of a file, you will see two size values - size and size on disk. the size of the file will be how many bytes are actually the file. The size on diskis the total of bytes in all clusters occupied by that file. The difference between them is known as 'file slack' - basically dead space. So it sounds like the bet bet would be to pick the smallest allocation unit size, to save space. The downside is the smaller the allocation unit, the slower the performance. A good compromise between access speed and saving space seems to be the default 4096 alocation unit size. __________________

03-20-2009, 04:48 PM	#3 (permalink)
Deyvid Registered User Join Date: Jun 2007 Location: California Posts: 60 OS: Windows Vista Home Basic (SP1) My System	Re: Help: Reformatting 2nd HDD (Allocation Unit Size?) Thanks for the info and explanation. That helped solve my dilemma about which size to choose. I will go with the default 4096 bytes option.