Size of EFI system partition is much smaller when it is mounted

[Yasuhiro KIMURA]

I made clean install of 11.4-RELEASE amd64 with ZFS root and UEFI boot
mode. 'Auto (ZFS)' was used at disk setup phase and disk was
partitioned as following.

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root at rolling-vm-freebsd3[80]# uname -a
FreeBSD rolling-vm-freebsd3.home.utahime.org 11.4-RELEASE-p3 FreeBSD 11.4-RELEASE-p3 #0: Tue Sep  1 08:22:33 UTC 2020     root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  amd64
root at rolling-vm-freebsd3[81]# gpart show ada0
=>       40  209715120  ada0  GPT  (100G)
         40     409600     1  efi  (200M)
     409640       2008        - free -  (1.0M)
     411648   16777216     2  freebsd-swap  (8.0G)
   17188864  192524288     3  freebsd-zfs  (92G)
  209713152       2008        - free -  (1.0M)
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According to it the size of EFI system partition is 200MB. But if I
mount it to filesystem and check its size with df(1), then it is much
smaller than 200MB.

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root at rolling-vm-freebsd3[82]# mount -t msdosfs /dev/ada0p1 /mnt
root at rolling-vm-freebsd3[83]# df -k /mnt
Filesystem  1024-blocks Used Avail Capacity  Mounted on
/dev/ada0p1         779  384   395    49%    /mnt
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And if I try to copy large file to /mnt then it fails.

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root at rolling-vm-freebsd3[94]# ls -l dummy.1MB.date
-rw-r--r--  1 root  wheel  1048576 Nov 29 10:39 dummy.1MB.date
root at rolling-vm-freebsd3[95]# cp dummy.1MB.date /mnt
cp: /mnt/dummy.1MB.date: No space left on device
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So it seems kernel(?) really regards the size of mounted EFI system
part as only 779KB.

I also tried it with 12.2-RELEASE and df(1) reported correctly.

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root at eastasia[1002]# uname -a
FreeBSD eastasia.home.utahime.org 12.2-RELEASE FreeBSD 12.2-RELEASE r366954 GENERIC  amd64
root at eastasia[1003]# gpart show ada0
=>        40  1953525088  ada0  GPT  (932G)
          40      409600     1  efi  (200M)
      409640        2008        - free -  (1.0M)
      411648    16777216     2  freebsd-swap  (8.0G)
    17188864  1936334848     3  freebsd-zfs  (923G)
  1953523712        1416        - free -  (708K)

root at eastasia[1004]# mount -t msdosfs /dev/ada0p1 /mnt
root at eastasia[1005]# df -k /mnt
Filesystem  1024-blocks Used  Avail Capacity  Mounted on
/dev/ada0p1      204696  976 203720     0%    /mnt
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Is there any bug fix about it that is merged to 12.x but not 11.x?

Best Regards.

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Yasuhiro KIMURA