Re: efi msdos puzzle

["Jonathan A. Kollasch" <jakllsch%kollasch.net@localhost>]

On Mon, Sep 09, 2019 at 04:58:01PM +0100, Patrick Welche wrote:
> gpt add -a 1024k -l efi0 -s 256M -t efi wd0   (on -current/amd64)
> 
> dk0: efi0, 524288 blocks at 2048, type: msdos
> 
> # newfs_msdos -F 32 -b 4096 /dev/rdk0
> newfs_msdos: 65404 clusters too few clusters for FAT32, need 65525
> 
> but 65525 * 8 = 524200 < 524288 ?
> 
> Where does 65404 come from?
> 

The FATs and various other FS internal data structures take space,
apparently to the tune of 132 clusters, or 1056 sectors, or 540672 bytes.

A 32-bit FAT for up to 65408 clusters is 511 sectors, FAT32 keeps a
duplicate copy of the fat, so you're up to 1022 sectors used already.

There's also the boot sector and it's backup, the file system
information block(s?), any reserved sectors, and I forget how
it worked on FAT32 (it's different than FAT16/12) but maybe also the
root directory entries.

The bit-width of the FAT is a function of the number of data clusters,
you'll just end up getting failures like this if you specify any more
than two of the partition/FS size, the cluster size, and the FAT width.

Microsoft was helpful enough to publish the FAT spec specifically for
use in the EFI context.

	Jonathan Kollasch