[Info-vax] BACKUP/INIT

[jbriggs444]

On Feb 18, 8:37 am, Jan-Erik Soderholm <jan-erik.soderh... at telia.com>
wrote:
> jbriggs444 wrote 2011-02-18 14:20:
>
>
>
>
>
> > On Feb 18, 4:51 am, hel... at astro.multiCLOTHESvax.de (Phillip Helbig---
> > undress to reply) wrote:
> >> I just did INIT/SYS several times on a 1-GB disk (RZ26L).  I kept
> >> getting a cluster size of 16, even though I didn't specify this.  This
> >> is with VMS 7.3-2.
>
> >> In the old days, I never specified cluster size.  With 1-GB disks, I
> >> always got 3.  According to HELP BACK/INIT, I should get this since it
> >> is larger than what is otherwise used as the default.  Why in the world
> >> should I get 16?  It's not even a default in any situation.  I just had
> >> INIT/SYS/HEADERS=50000, nothing else.
>
> > In the old days my recollection is that there was a hard limit on the
> > size of bitmap.sys and, as a result, the number of allocation clusters
> > on the volume had a hard cap.  Make the volume big enough and
> > the default cluster size of 3 is inadequate to allow bitmap.sys to
> > cover
> > the whole volume.
>
> > INITIALIZE was aware of this and would bump up the cluster factor
> > for large volumes.
>
>  From HELP /INIT :
>
>       For Files-11 On-Disk Structure Level 5 (ODS-5) disks, the default
>       cluster size is 16. In this case the minimum value allowed by the
>       following equation is applied:
>
>       For Files-11 On-Disk Structure Level 2 (ODS-2) disks, the default
>       cluster size depends on the disk capacity; disks with less than
>       50,000 have a default of 1. Disks that are larger than 50,000
>       have a default of either 16 or the result of the following
>       formula,  whichever is greater:
>
>       (disk size in number of blocks)/(255 * 4096)
>
> For 1 GB that gives 16.- Hide quoted text -
>
> - Show quoted text -

The disksize/(255*4096) formula fits with a 256 block bitmap.sys

1 block for the SCB
255 blocks for the allocation bitmap
* 4096 bits per block = that many clusters available for allocation.