Re: efi msdos puzzle

[Patrick Welche <prlw1%cam.ac.uk@localhost>]

On Mon, Sep 09, 2019 at 11:23:49AM -0500, Jonathan A. Kollasch wrote:
> 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.

Thanks - I'll experiment to find the magic number!

(https://en.wikipedia.org/wiki/File_Allocation_Table#FAT32
suggests:
Min. volume size   256 MiB-36 KiB (with 65525 clusters and 4 KiB sectors)

maybe that should be a '+' sign...)


Cheers,

Patrick