[Info-vax] Backup/physical question

[John E. Malmberg]

On 3/15/2015 9:47 PM, JF Mezei wrote:
> FYI:
>
> It appears that bad blocks on diskette in a backup/physical do not
> result in the block being filled with 0s.

Of course not.  Bad blocks can not be read because they fail integrity 
checks.

You have to have low-level software to tell the controller to read raw 
data with out checking the integrity of the blocks.  VMS does not expose 
that API at the driver level.

> Also, when the output of backup/physical are sent to a saveset, and then
> back to a device, backup knows that blocks in the original were not OK:
>
> $ backup/physical/log dua2.save/save lda1:
> %BACKUP-E-BADDATA, error in block 1020 of LDA1: detected during save
> %BACKUP-S-CREATED, created LDA1:
>
> However, as with direct device to device, the suspect block is filled
> with what appears to be data, it is not all zeroed out.

As expected.

The badblock feature is also used to make dual format volumes, for 
example ISO-9660, where the ISO-9960 blocks show up as bad for VMS.

> Neat:
>
> Copyed a diskette from VMS to a .dmg file. FTP file to Mac. Opened it as
> a disk. Turns out it was an MS-DOS FAT disk !!!

No it is not.

> It did have different geometry.
> 		FAT	HFS
> blocks: 	1440	2880
> cylinders:	80	80
> sectors/trk	9	18
> tracks/cyl	2	2

That shows 368 Kbytes for the FAT formatted disk.  I do not know the 
sector size the HFS formatted media.

If HFS is using 512 byte sectors, then it should be 737 Kbytes.

> Initially figured it would be single sided, but the tracks/cyl indicates
> 2. So I guess there was low density and high density which change the
> number of sectors per cyulinder.

Geometry reports may be lies.  Especially for hard drives.

For backward compatibility with software that has limitations the size 
of the field for cylinders or sectors / track, the driver may just 
calculate new ones that meet ancient restrictions.  It may not even be 
looking at the actual geometry reported by the device controller.

> BTW, I have now had 2 diskettes that were read without any errors and
> the process is far faster.

Based on the total size seen, you have a single sided double density floppy.

VMS uses the same sector size and count as MS-DOS, so you can use 
pre-formatted 720K and 1.2 M floppy disks on it.

Regards,
-John
wb8tyw at qsl.network