[Info-vax] OT: For MAC Lovers Only :-)

[glen herrmannsfeldt]

Stephen Hoffman <seaohveh at hoffmanlabs.invalid> wrote:
> On 2012-10-16 20:40:44 +0000, JF Mezei said:

(snip)
>> Is it fair to state that sector = block in most cases ?

> That depends on your definition of "most"...

> A VMS block is 512 bytes.

> Some disks commonly found on VMS use 2048-byte sectors (eg: optical), 
> and various new (magnetic) disks are increasingly using 4096-byte 
> sectors.

Yes. At the hardware level, block and sector are pretty
much interchangable, but not at the software or OS level.

> You just knew there was going to be a link, right?  
> <http://labs.hoffmanlabs.com/node/284>

> There are also cases (on VMS) where one block is comprised of two or 
> four sectors, too.  (I know of two reasonably well-known devices where 
> the sectors were smaller than a VMS block.  There may be more.  I'll 
> leave it to the reader to figure out which devices...)

Yes. Sometimes the OS works with a different size for
efficiency reasons. Now, there is also the cluster, which is
(often) more than one block/sector grouped together to make
allocation more efficient. 

The 512 byte sector is still pretty popular for floppies
and hard disks, but CD-ROMs use 2048 bytes. Some drives
can read 2048 byte sectors of the CD, and return 512 byte
blocks to the requesting system. 

So, it goes both ways. The software might work with a block
that is bigger or smaller than the hardware sector.

-- glen