Re: Copying Mac MFS disks on an Apple II

[Greg Buchner <apple22@mn.rr.com>]

In article <acdalv4olv4h1ajrgdhr0njafd5gv2tum6@4ax.com>,
 Jeff Blakeney <CUTjblakeney@syndicomm.com> wrote:

> On Tue, 02 Sep 2003 13:01:00 GMT, "M. Pender" <mpender@hotmail.com>
> wrote:
> 
> >IIRC Mac HFS disk blocks are not the same size as ProDos disk blocks; there
> >are a few extra bytes thrown.  I doubt that the 512 byte ProDos blocks will
> >be very appealing to the Mac.
> 
> ProDOS and HFS both used fixed block sizes of 512 byte blocks.  HFS+
> supports variable block sizes but I'm almost positive the smallest
> size they can be is 512 bytes and they must contain a number of bytes
> that is a power of 2 (ie. 512, 1024, 2048).

(I hope I've got all of this right...)

Actually it's slightly more complicated.  You have to deal with physical 
blocks (the actual blocks on the hard drive, which are almost always 512 
bytes) and logical blocks, which can vary in multiples of 512 bytes 
(unless the phsysical blocks on the HD are different).

ProDOS supports only 512 byte logical blocks so they map 1:1 to the HD, 
65536 blocks to the volume.

HFS supports logical block sizes that can be many multiples of 512 
bytes, but only supports 65536 blocks on a volume (you can end up with 
logical block sizes of over 100K or a lot more on a large volume).  
Note: This can waste a lot of space on a large HD, a 1 byte file and a 
90K file could end up taking the same amount of logical blocks on an HFS 
volume.

HFS+ also supports logical block sizes that can be many multiples of 512 
bytes, but is usually limited to 512, 1024, 2048 or 4096 bytes.  I think 
it can have 2^32 (maybe more) blocks so large drives can be supported 
with small enough logical blocks sizes to not waste space.

Greg B.

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