Date: Tue, 28 Mar 2017 19:07:13 +0300 From: Toomas Soome <tsoome@me.com> To: rgrimes@freebsd.org Cc: Bruce Evans <brde@optusnet.com.au>, Julian Elischer <julian@vps1.elischer.org>, Warner Losh <imp@freebsd.org>, src-committers <src-committers@freebsd.org>, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r316064 - head/sys/boot/i386/boot2 Message-ID: <8EB5A473-1218-4A57-8741-374669A9F5B8@me.com> In-Reply-To: <201703281555.v2SFtoSU005538@pdx.rh.CN85.dnsmgr.net> References: <201703281555.v2SFtoSU005538@pdx.rh.CN85.dnsmgr.net>
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> On 28. m=C3=A4rts 2017, at 18:55, Rodney W. Grimes = <freebsd@pdx.rh.CN85.dnsmgr.net> wrote: >=20 >> On Mon, 27 Mar 2017, Julian Elischer wrote: >>=20 >>> On Tue, 28 Mar 2017, Bruce Evans wrote: >>>=20 >>> [...] >>>=20 >>>> 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 >>>=20 >>> My memory says that the limit of 7.5K is becuase there was only 8k = left free=20 >>> at the front of UFS1 and one sector was used for the boot0 code. >>=20 >> 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. >=20 > 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. >=20 > 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. >=20 > 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. >=20 > 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. >=20 >> 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. >>=20 >> Bruce >=20 > --=20 > Rod Grimes = rgrimes@freebsd.org >=20 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=E2=80=99s who did attempt the same for their raw = databases;) rgds, toomas=
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