svn commit: r316064 - head/sys/boot/i386/boot2

[Toomas Soome]

> On 28. märts 2017, at 18:55, Rodney W. Grimes <freebsd at pdx.rh.CN85.dnsmgr.net> wrote:
> 
>> On Mon, 27 Mar 2017, Julian Elischer wrote:
>> 
>>> On Tue, 28 Mar 2017, Bruce Evans wrote:
>>> 
>>> [...]
>>> 
>>>> they have to fit below 640K and a few multiples of 64K are already
>>>> used for buffers).  The limit on 8K is mainly a historical mistake.
>>>> A limit of 7.5K simplified booting from 15-sector floppies.  18-sector
>>> 
>>> My memory says that the limit of 7.5K is becuase there was only 8k left free 
>>> at the front of UFS1 and one sector was used for the boot0 code.
>> 
>> That is only a limit if the boot code is in the ffs partition.  This causes
>> other problems.  It was the default to start the 'a' partition at offset 0,
>> but that was changed 10-15 years ago.  I can't find exactly where it is
>> changed.  I use an offset of 8192 sectors or 4M on new and repartitioned
>> hard disks.
> 
> IIRC, it was sizeof(boot0)+sizeof(boot1) had to fit in the 8K byte hole
> at the start of a ffs/ufs1 disk if and only if the disk was in dangeriously
> dedicated mode, which is the same case for a floppy.
> 
> This 8K hole, again iirc, is actually a #define.  Later someone seems
> to have though you need to offset partition a: by 16 blocks for this
> and made the installers do magic this, as far as I can see, is incorrect
> and I have manually been reseting the first partition of my bsdlabels
> to 0 and adding 16 blocks to there size.
> 
> I think we still have an 8k size limit on boot1 for ffs/(ufs1 or ufs2)
> (Proved self wrong on the 8k limit, see comments from sys/ufs/ffs/fs.h
> below)
> as it this code still lives in the start of the partition, though there
> is usually 62 (or some other similiar number that is geometry dependent)
> sectors of unused space between the mbr and the start of the bsd slice.
> 
> Here is the truth on the magic holes from sys/ufs/ffs/fs.h:
> * Depending on the architecture and the media, the superblock may
> * reside in any one of four places. For tiny media where every block
> * counts, it is placed at the very front of the partition. Historically,
> * UFS1 placed it 8K from the front to leave room for the disk label and
> * a small bootstrap. For UFS2 it got moved to 64K from the front to leave
> * room for the disk label and a bigger bootstrap, and for really piggy
> * systems we check at 256K from the front if the first three fail. In
> * all cases the size of the superblock will be SBLOCKSIZE. All values are
> * given in byte-offset form, so they do not imply a sector size. The
> * SBLOCKSEARCH specifies the order in which the locations should be searched.
> 
>> This is again affected by the existence of floppy disks.  Floppy disks are
>> usually not partitioned, and don't have space to spare for large boot
>> blocks.  Some version of the boot code has to work on small media, and
>> FreeBSD uses the same boot code for all media.  This allowed FreeBSD-1
>> to have a single boot.flp where IIRC Linux had about 100 variations.
>> Small media is not as small as it used to be.
>> 
>> Bruce
> 
> -- 
> Rod Grimes                                                 rgrimes at freebsd.org
> 


Also note that SunOS (which ufs is based on ufs1), has disk layout on sparc as sector 0 for VTOC (512B), followed by 15 sectors for bootblk, total 16 sectors, or 8KB, the setup which did allow to define slice 0 to start from the absolute sector 0, and which probably did also burn uncounted amount of DBA’s who did attempt the same for their raw databases;)

rgds,
toomas