Filesystem overhead of FFS

rare_j · 16 March 2009, 17:06

hello
I was wondering if anyone knows how to calculate what the fliesystem overhead is of fastfilesystem for any given volume or partition size.

What I mean is when I format a hard disk of say 100mb, even when it is empty it says there has been 20K or whatever used. So how can I calculate in advance how many bytes will be consumed by the filesystem before I've written any files to the disk?

If anyone knows the formula please share!

Thanks

thomas · 16 March 2009, 18:16

There are two blocks reserved in the beginning, the root block in the midle of the parititon and one bit in a bitmap block for each block on the partition. A bitmap block can hold something like 488 bytes with 8 bits each, making 3904 bits per bitmap block. For 100 MB with a block size of 512 bytes, you'd need bits for 204800 blocks. With 3904 bits per bitmap block, you'd need 53 bitmap blocks.

So there is a total of 56 occupied blocks on an empty 100 MB partition which take about 28 KB.

A more detailed description on the internal structures of FastFileSystem can be found on the Amiga Developer CD.

rare_j · 16 March 2009, 19:00

That's exactly the information I need thomas, thanks.

16 March 2009, 17:06	#1
rare_j Zone Friend Join Date: Apr 2005 Location: London Posts: 1,176	Filesystem overhead of FFS hello I was wondering if anyone knows how to calculate what the fliesystem overhead is of fastfilesystem for any given volume or partition size. What I mean is when I format a hard disk of say 100mb, even when it is empty it says there has been 20K or whatever used. So how can I calculate in advance how many bytes will be consumed by the filesystem before I've written any files to the disk? If anyone knows the formula please share! Thanks

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16 March 2009, 18:16	#2
thomas Registered User Join Date: Jan 2002 Location: Germany Posts: 6,985	There are two blocks reserved in the beginning, the root block in the midle of the parititon and one bit in a bitmap block for each block on the partition. A bitmap block can hold something like 488 bytes with 8 bits each, making 3904 bits per bitmap block. For 100 MB with a block size of 512 bytes, you'd need bits for 204800 blocks. With 3904 bits per bitmap block, you'd need 53 bitmap blocks. So there is a total of 56 occupied blocks on an empty 100 MB partition which take about 28 KB. A more detailed description on the internal structures of FastFileSystem can be found on the Amiga Developer CD.

16 March 2009, 19:00	#3
rare_j Zone Friend Join Date: Apr 2005 Location: London Posts: 1,176	That's exactly the information I need thomas, thanks.

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