Re: FAT32 support on freebsd

Am 20.10.24 um 20:07 schrieb Andreas Hecht:
> My FAT32 formatted portable SSD "Western Digital My Passport 25F3" does not
> seem to be supported by FreeBSD. Mounting the drive works fine but I can not
> access the data stored within it:
> 
> # mount -t msdosfs /dev/da0s1 /mnt
> # cd /mnt
> # ls -al
> total 0
> 
> The 'ls -al' above should show a folder named 'data' stored in the
> top level diretory of the drive. The folder does not show up but during
> execution of 'ls -al' the following kernel message appears:
> 
> getblkx: maxsize(66560) > maxbcachebuf(65536)
> 
> I found that this particular drive uses 128 sectors per cluster. This
> multiplied with 512 bytes per sector results in 65536 bytes per sector
> which per FAT specification seems to be too much:
> (https://www.cs.fsu.edu/~cop4610t/assignments/project3/spec/fatspec.pdf)
> "Note however, that a value should never be used that results in a “bytes per
> cluster” value (BPB_BytsPerSec * BPB_SecPerClus) greater than 32K (32 * 1024).
> There is a misconception that values greater than this are OK."
> 
> What makes me wonder is that the same drive works fine with Debian 12 and
> NetBSD 10. Is FreeBSD supposed to support this drive? Shall I send a problem
> report?

Without knowing the exact details of your FAT32 partition, I do assume
that the clusters are not aligned to a 4 KB boundary.

The MSDOSFS in FreeBSD maps sectors of the FAT32 partition to pages in
such a way that the first sector of a cluster may be in the middle of
a VM page. That leads to cluster of 128 * 512 Bytes to require 17 instead
of 16 pages in the buffer cache.

The newfs_msdos command in FreeBSD has been modified to adjust the first
sector in each data cluster to a multiple of the page size, which avoids
this issue.

You can mount your FAT32 file system if you add the following to your
loader configuration file /boot/loader.conf:

vfs.maxbcachebuf=131072

In fact, setting this value to 69632 (17 * 4096) should suffice ...

I plan to remove this limit from FAT32, but setting vfs.maxbcachebuf
should allow you to work with that drive.

Beste Grüße, STefan