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Re: Support for 4KB sectors size disk ?

On Tue, Jan 12, 2010 at 11:15:09PM +0900, Izumi Tsutsui wrote:
> On i386 and amd64, there is another problem around bootloader.
> Currently FFS has only 8KB free space for bootloader.
> On x86, the first sector contains PBR that loads
> bootxx_ffsvN stored in the following sectors:
> sector 0:    PBR (pbr.S in src/sys/arch/i386/stand/bootxx/)
> sector 1:    NetBSD disklabel
> sector 2-15: primary boot (bootxx.S, boot1.c, and fs specific code)
> But on 4KB/sector disk, sector 1 is at 4KB (that should have disklabel?),
> and sector 2 is at 8KB where FFS superblock should be stored.

On the 2k sector USB stick I have, the 0x55aa is still at byte offset 0x1fe.
If the bios is likely to be doing compat-mode 512 byte transfers this
will remain true.

Which may mean that the mbr treats the disk as if it has 512 byte sectors?

So bytes 0x200-0x3ff can be used for the disklabel, and bytes 0x400 on
for bootxx_xxxfs. Installboot will do this - it always does 8k read/modify/
write sequences on the start of the disk.

Note that, even if the bios does 2k/4k reads, the i386 initial boot sequence
will still work. Although the transfers will be longer than expected
they won't overwrite anything important.  The fs specific code will need
to transfer the correct number of bytes - but it has more code space and
is reading file system blocks, not disk sectors.


David Laight:

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