> On 28. märts 2017, at 18:55, Rodney W. Grimes > <free...@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 rgri...@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 _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
