Re: Disk Structures

On Sat, 13 Dec 2025 13:54:04 -0800, Doug Hardie wrote:
> I have a couple of Raspberry Pi 5s.  One of which is a production server
> for mail.  It works just fine.  However, the disk structure is quite
> confusing.  I only noticed this because of trying to bring up another
> similar server.  
> 
> If I list the dev entries for mmcsd, I get the following.
> 
> mail# ll /dev/mmcs*
> crw-r-----  1 root operator 0x49 Nov 26 15:47 /dev/mmcsd0
> crw-r-----  1 root operator 0x4a Nov 26 15:47 /dev/mmcsd0s1
> crw-r-----  1 root operator 0x4b Nov 26 15:47 /dev/mmcsd0s2
> crw-r-----  1 root operator 0x4e Nov 26 15:47 /dev/mmcsd0s2a
> crw-r-----  1 root operator 0x4f Nov 26 15:47 /dev/mmcsd0s2b
> 
> This looks like the drive was partitioned using GPT. 

No, looks like MBR. Suffixes s[12] are the "DOS primary partitions",
i. e., the slices, and s2[ab] are the BSD partitions inside the 2nd
slice (the labels, disklabels).

GPT partitions would be listed as p[1234...].



> However, gpart list shows MBR. 

Correct.



> I removed the non-relevant entries:
> 
> mail# gpart list mmcsd0
> Geom name: mmcsd0
> entries: 4
> scheme: MBR
> Providers:
> 1. Name: mmcsd0s1
>    Mediasize: 52428800 (50M)
>    type: fat32lba
> 2. Name: mmcsd0s2
>    Mediasize: 128124452864 (119G)
>    type: freebsd
> 
> There are the two partitions I expected.  A boot partition, and a data
> partition.

At least the 2nd partition is FreeBSD. The other one seems to be DOS
(FAT32), which typically indicates something related to (U)EFI.



> Looking at the df output:
> 
> mail# df -h
> Filesystem          Size    Used   Avail Capacity  Mounted on
> /dev/ufs/rootfs     108G    9.1G     90G     9%    /
> devfs               1.0K      0B    1.0K     0%    /dev
> /dev/msdosfs/EFI     50M     25M     25M    50%    /boot/efi
> tmpfs                12G     85M     12G     1%    /tmp
> /dev/da0s2           25G    1.3G     22G     6%    /mailbkup
> mail# 
> 
> The entries for / and /boot/efi are not at all what I expected.
> The /mailbkup seems normal.  How do I make sense of all this?

The partitions also have labels, and this is what you see in the
first column, instead of the device name. As I've guessed, the
/dev/mmcsd0s1 partition was labeled "EFI" and therefore became the
entry /dev/msdosfs/EFI, mounted at /boot/efi; the /dev/da0s2 seems
odd though... it should have been da0s2a because of /dev/mmcsd0s2a,
and /dev/mmcsd0s2b is a swap device (letter "b"), so it won't show
up in the mounted volume list (because it isn't mounted at all).

The letters for the labels indicate a boot partition ("a"), a swap
partition ("b"), the whole slice as a partition ("c"), and the
subsequent letters indicate further arbitrary partitions.

See this example:

	% df -h
	Filesystem      Size    Used   Avail Capacity  Mounted on
	/dev/ad10s1a    989M    438M    471M    48%    /
	devfs           1.0k    1.0k      0B   100%    /dev
	/dev/ad10s1d    989M    347M    563M    38%    /tmp
	/dev/ad10s1e    989M    407M    503M    45%    /var
	/dev/ad10s1f     48G     41G    3.3G    93%    /usr
	/dev/ad10s1g    4.9G      4G    505M    89%    /opt
	/dev/ad12       1.8T    1.7T    625M   100%    /home

As you can see, ad12 (being ad12c in fact) is a data partition that
does not have any slices (called "dedicated"). Also swap isn't on
this list (ad10s1b).



> I duplicated the microSD card but it doesn't boot the new machine.

Using which tool? Note that GPT also might store stuff at the end of
a medium, so usually only real 1:1 copies work as expected.



> Messages shows the new disk is da1.

The device name is assigned according to detection order, that's
why labels are so convenient.



> mail# mount /dev/da1s2 /mnt
> mount: /dev/da1s2: No such file or directory
> mail# mount /dev/mmcsd1s2b /mnt
> mount: /dev/mmcsd1s2b: No such file or directory

Correct. You can not mount a slice, you need to mount a partition.
You also cannot mount a swap partition (see above).

The command should probably be:

	# mount -t ufs -o ro /dev/da1s2a /mnt

or

	# mount -t ufs -o ro /dev/mmcsd1s2b /mnt

Though -t ufs is the default, you can always make it explicit to
be sure. You also could use the label names, listed in /dev/ufs
or /dev/msdos (according to the file system); GPT labels exist
(if formatted GPT) in a different directory, /dev/gpt,



> I would like to verify that the new SD card is good.

A successful boot should prove it. ;-)



-- 
Polytropon
Magdeburg, Germany
Happy FreeBSD user since 4.0
Andra moi ennepe, Mousa, ...