How to force GEOM to recalculate the free space after the disk is resized?

[Dexuan Cui]

Hi, I have a FreeBSD virtual machine (VM) running on Hyper-V and I'm testing Hyper-V's Disk Online Resizing feature. The feature can expand or shrink the (virtual) disk capacity of a VM when the VM is running.

There is an issue with gpart or GEOM: after the disk capacity is expanded (or shrunk), gpart/GEOM can detect the new bigger capacity, but the free space displayed by gpart remained the same unless I open the disk dev file for writing, e.g.,

[root at decui-bsd11 ~]# gpart create -s MBR   /dev/da1
da1 created
[root at decui-bsd11 ~]# gpart show /dev/da1
=>      63  83886017  da1  MBR  (40G)
        63  83886017       - free -  (40G)
[root at decui-bsd11 ~]# diskinfo /dev/da1
/dev/da1        512     42949672960     83886080        4096    0       5221    255     63

Now I expand the disk from 40GB to 50GB by Hyper-V management tool.

Next, I get the below, i.e., gpart/GEOM detects the new disk capacity, but the free space remains the same.
(Note: the first diskinfo failure should be expected: Hyper-V only notifies the VM of the capacity change on the VM's next read or write request, and in the VM it seems there is a race condition between the ioctl and the handling of capacity change. I'll see how this can be fixed.)

[root at decui-bsd11 ~]# diskinfo /dev/da1
diskinfo: /dev/da1: ioctl(DIOCGMEDIASIZE) failed, probably not a disk.
[root at decui-bsd11 ~]# diskinfo /dev/da1
/dev/da1        512     53687091200     104857600       4096    0       6527    255     63

[root at decui-bsd11 ~]# gpart show /dev/da1
=>      63  83886017  da1  MBR  (50G)
        63  83886017       - free -  (40G)

Now, if I run a program that only does "openat(AT_FDCWD,"/dev/da1",O_WRONLY|O_CREAT,0644);", GEOM will detect that the free space is 50GB now and GEOM will pass this info to gpart:

[root at decui-bsd11 ~]# gpart show /dev/da1
=>       63  104857537  da1  MBR  (50G)
         63  104857537       - free -  (50G)

I'm not familiar with GEOM.
Can somebody please explain the behavior?

I don't know who exactly maintains GEOM , so I just picked some names from "git log geom/" and put you to Cc.  Sorry if this mail bothers you. :-)

Thanks,
-- Dexuan