gistfile1.txt

$ adb reboot bootloader
$ fastboot getvar partition-size:userdata
partition-size:userdata:  0x1c94ffb000
finished. total time: 0.000s
$ python
Python 2.7.15 (default, May 16 2018, 17:50:09)
[GCC 8.1.1 20180502 (Red Hat 8.1.1-1)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> hex(0x1c94ffb000 - 0x8000)
'0x1c94ff3000'
>>> quit()
$ fastboot format:ext4:0x1c94ff3000 userdata
Warning: userdata size is        0x1c94ffb000, but 0x1c94ff3000 was requested for formating.
Creating filesystem with parameters:
    Size: 122758836224
    Block size: 4096
    Blocks per group: 32768
    Inodes per group: 8192
    Inode size: 256
    Journal blocks: 32768
    Label:
    Blocks: 29970419
    Block groups: 915
    Reserved block group size: 1024
Created filesystem with 11/7495680 inodes and 518576/29970419 blocks
target reported max download size of 536870912 bytes
erasing 'userdata'...
OKAY [  0.010s]
sending 'userdata' (145110 KB)...
OKAY [  4.553s]
writing 'userdata'...
OKAY [  1.559s]
finished. total time: 6.126s
$ fastboot reboot
rebooting...

finished. total time: 0.055s